# Clang 21.1.6 documentation

## Clang Language Extensions

## Introduction

This document describes the language extensions provided by Clang. In addition to the language extensions listed here, Clang aims to support a broad range of GCC extensions. Please see the GCC manual for more information on these extensions.

## Feature Checking Macros

Language extensions can be very useful, but only if you know you can depend on them. In order to allow fine-grain features checks, we support three builtin function-like macros. This allows you to directly test for a feature in your code without having to resort to something like autoconf or fragile “compiler version checks”.

### `__has_builtin`

This function-like macro takes a single identifier argument that is the name of a builtin function, a builtin pseudo-function (taking one or more type arguments), or a builtin template. It evaluates to 1 if the builtin is supported or 0 if not. It can be used like this:

    #ifndef __has_builtin         // Optional of course.
      #define __has_builtin(x) 0  // Compatibility with non-clang compilers.
    #endif

    ...
    #if __has_builtin(__builtin_trap)
      __builtin_trap();
    #else
      abort();
    #endif
    ...

Note

Prior to Clang 10, `__has_builtin` could not be used to detect most builtin pseudo-functions.

`__has_builtin` should not be used to detect support for a builtin macro; use `#ifdef` instead.

### `__has_constexpr_builtin`

This function-like macro takes a single identifier argument that is the name of a builtin function, a builtin pseudo-function (taking one or more type arguments), or a builtin template. It evaluates to 1 if the builtin is supported and can be constant evaluated or 0 if not. It can be used for writing conditionally constexpr code like this:

    #ifndef __has_constexpr_builtin         // Optional of course.
      #define __has_constexpr_builtin(x) 0  // Compatibility with non-clang compilers.
    #endif

    ...
    #if __has_constexpr_builtin(__builtin_fmax)
      constexpr
    #endif
      double money_fee(double amount) {
          return __builtin_fmax(amount * 0.03, 10.0);
      }
    ...

For example, `__has_constexpr_builtin` is used in libcxx’s implementation of the `<cmath>` header file to conditionally make a function constexpr whenever the constant evaluation of the corresponding builtin (for example, `std::fmax` calls `__builtin_fmax`) is supported in Clang.

### `__has_feature` and `__has_extension`

These function-like macros take a single identifier argument that is the name of a feature. `__has_feature` evaluates to 1 if the feature is both supported by Clang and standardized in the current language standard or 0 if not (but see below), while `__has_extension` evaluates to 1 if the feature is supported by Clang in the current language (either as a language extension or a standard language feature) or 0 if not. They can be used like this:

    #ifndef __has_feature         // Optional of course.
      #define __has_feature(x) 0  // Compatibility with non-clang compilers.
    #endif
    #ifndef __has_extension
      #define __has_extension __has_feature // Compatibility with pre-3.0 compilers.
    #endif

    ...
    #if __has_feature(cxx_rvalue_references)
    // This code will only be compiled with the -std=c++11 and -std=gnu++11
    // options, because rvalue references are only standardized in C++11.
    #endif

    #if __has_extension(cxx_rvalue_references)
    // This code will be compiled with the -std=c++11, -std=gnu++11, -std=c++98
    // and -std=gnu++98 options, because rvalue references are supported as a
    // language extension in C++98.
    #endif

For backward compatibility, `__has_feature` can also be used to test for support for non-standardized features, i.e. features not prefixed `c_`, `cxx_` or `objc_`.

Another use of `__has_feature` is to check for compiler features not related to the language standard, such as e.g. AddressSanitizer.

If the `-pedantic-errors` option is given, `__has_extension` is equivalent to `__has_feature`.

The feature tag is described along with the language feature below.

The feature name or extension name can also be specified with a preceding and following `__` (double underscore) to avoid interference from a macro with the same name. For instance, `__cxx_rvalue_references__` can be used instead of `cxx_rvalue_references`.

### `__has_cpp_attribute`

This function-like macro is available in C++20 by default, and is provided as an extension in earlier language standards. It takes a single argument that is the name of a double-square-bracket-style attribute. The argument can either be a single identifier or a scoped identifier. If the attribute is supported, a nonzero value is returned. If the attribute is a standards-based attribute, this macro returns a nonzero value based on the year and month in which the attribute was voted into the working draft. See WG21 SD-6 for the list of values returned for standards-based attributes. If the attribute is not supported by the current compilation target, this macro evaluates to 0. It can be used like this:

    #ifndef __has_cpp_attribute         // For backwards compatibility
      #define __has_cpp_attribute(x) 0
    #endif

    ...
    #if __has_cpp_attribute(clang::fallthrough)
    #define FALLTHROUGH [[clang::fallthrough]]
    #else
    #define FALLTHROUGH
    #endif
    ...

The attribute scope tokens `clang` and `_Clang` are interchangeable, as are the attribute scope tokens `gnu` and `__gnu__`. Attribute tokens in either of these namespaces can be specified with a preceding and following `__` (double underscore) to avoid interference from a macro with the same name. For instance, `gnu::__const__` can be used instead of `gnu::const`.

### `__has_c_attribute`

This function-like macro takes a single argument that is the name of an attribute exposed with the double square-bracket syntax in C mode. The argument can either be a single identifier or a scoped identifier. If the attribute is supported, a nonzero value is returned. If the attribute is not supported by the current compilation target, this macro evaluates to 0. It can be used like this:

    #ifndef __has_c_attribute         // Optional of course.
      #define __has_c_attribute(x) 0  // Compatibility with non-clang compilers.
    #endif

    ...
    #if __has_c_attribute(fallthrough)
      #define FALLTHROUGH [[fallthrough]]
    #else
      #define FALLTHROUGH
    #endif
    ...

The attribute scope tokens `clang` and `_Clang` are interchangeable, as are the attribute scope tokens `gnu` and `__gnu__`. Attribute tokens in either of these namespaces can be specified with a preceding and following `__` (double underscore) to avoid interference from a macro with the same name. For instance, `gnu::__const__` can be used instead of `gnu::const`.

### `__has_attribute`

This function-like macro takes a single identifier argument that is the name of a GNU-style attribute. It evaluates to 1 if the attribute is supported by the current compilation target, or 0 if not. It can be used like this:

    #ifndef __has_attribute         // Optional of course.
      #define __has_attribute(x) 0  // Compatibility with non-clang compilers.
    #endif

    ...
    #if __has_attribute(always_inline)
    #define ALWAYS_INLINE __attribute__((always_inline))
    #else
    #define ALWAYS_INLINE
    #endif
    ...

The attribute name can also be specified with a preceding and following `__` (double underscore) to avoid interference from a macro with the same name. For instance, `__always_inline__` can be used instead of `always_inline`.

### `__has_declspec_attribute`

This function-like macro takes a single identifier argument that is the name of an attribute implemented as a Microsoft-style `__declspec` attribute. It evaluates to 1 if the attribute is supported by the current compilation target, or 0 if not. It can be used like this:

    #ifndef __has_declspec_attribute         // Optional of course.
      #define __has_declspec_attribute(x) 0  // Compatibility with non-clang compilers.
    #endif

    ...
    #if __has_declspec_attribute(dllexport)
    #define DLLEXPORT __declspec(dllexport)
    #else
    #define DLLEXPORT
    #endif
    ...

The attribute name can also be specified with a preceding and following `__` (double underscore) to avoid interference from a macro with the same name. For instance, `__dllexport__` can be used instead of `dllexport`.

### `__is_identifier`

This function-like macro takes a single identifier argument that might be either a reserved word or a regular identifier. It evaluates to 1 if the argument is just a regular identifier and not a reserved word, in the sense that it can then be used as the name of a user-defined function or variable. Otherwise it evaluates to 0. It can be used like this:

    ...
    #ifdef __is_identifier          // Compatibility with non-clang compilers.
      #if __is_identifier(__wchar_t)
        typedef wchar_t __wchar_t;
      #endif
    #endif

    __wchar_t WideCharacter;
    ...

## Include File Checking Macros

Not all developments systems have the same include files. The __has_include and __has_include_next macros allow you to check for the existence of an include file before doing a possibly failing `#include` directive. Include file checking macros must be used as expressions in `#if` or `#elif` preprocessing directives.

### `__has_include`

This function-like macro takes a single file name string argument that is the name of an include file. It evaluates to 1 if the file can be found using the include paths, or 0 otherwise:

    // Note the two possible file name string formats.
    #if __has_include("myinclude.h") && __has_include(<stdint.h>)
    # include "myinclude.h"
    #endif

To test for this feature, use `#if defined(__has_include)`:

    // To avoid problem with non-clang compilers not having this macro.
    #if defined(__has_include)
    #if __has_include("myinclude.h")
    # include "myinclude.h"
    #endif
    #endif

### `__has_include_next`

This function-like macro takes a single file name string argument that is the name of an include file. It is like `__has_include` except that it looks for the second instance of the given file found in the include paths. It evaluates to 1 if the second instance of the file can be found using the include paths, or 0 otherwise:

    // Note the two possible file name string formats.
    #if __has_include_next("myinclude.h") && __has_include_next(<stdint.h>)
    # include_next "myinclude.h"
    #endif

    // To avoid problem with non-clang compilers not having this macro.
    #if defined(__has_include_next)
    #if __has_include_next("myinclude.h")
    # include_next "myinclude.h"
    #endif
    #endif

Note that `__has_include_next`, like the GNU extension `#include_next` directive, is intended for use in headers only, and will issue a warning if used in the top-level compilation file. A warning will also be issued if an absolute path is used in the file argument.

### `__has_warning`

This function-like macro takes a string literal that represents a command line option for a warning and returns true if that is a valid warning option.

    #if __has_warning("-Wformat")
    ...
    #endif

## Builtin Macros

`__BASE_FILE__`

  Defined to a string that contains the name of the main input file passed to Clang.

`__FILE_NAME__`

  Clang-specific extension that functions similar to `__FILE__` but only renders the last path component (the filename) instead of an invocation dependent full path to that file.

`__COUNTER__`

  Defined to an integer value that starts at zero and is incremented each time the `__COUNTER__` macro is expanded.

`__INCLUDE_LEVEL__`

  Defined to an integral value that is the include depth of the file currently being translated. For the main file, this value is zero.

`__TIMESTAMP__`

  Defined to the date and time of the last modification of the current source file.

`__clang__`

  Defined when compiling with Clang

`__clang_major__`

  Defined to the major marketing version number of Clang (e.g., the 2 in 2.0.1). Note that marketing version numbers should not be used to check for language features, as different vendors use different numbering schemes. Instead, use the Feature Checking Macros.

`__clang_minor__`

  Defined to the minor version number of Clang (e.g., the 0 in 2.0.1). Note that marketing version numbers should not be used to check for language features, as different vendors use different numbering schemes. Instead, use the Feature Checking Macros.

`__clang_patchlevel__`

  Defined to the marketing patch level of Clang (e.g., the 1 in 2.0.1).

`__clang_version__`

  Defined to a string that captures the Clang marketing version, including the Subversion tag or revision number, e.g., “`1.5 (trunk 102332)`”.

`__clang_literal_encoding__`

  Defined to a narrow string literal that represents the current encoding of narrow string literals, e.g., `"hello"`. This macro typically expands to “UTF-8” (but may change in the future if the `-fexec-charset="Encoding-Name"` option is implemented.)

`__clang_wide_literal_encoding__`

  Defined to a narrow string literal that represents the current encoding of wide string literals, e.g., `L"hello"`. This macro typically expands to “UTF-16” or “UTF-32” (but may change in the future if the `-fwide-exec-charset="Encoding-Name"` option is implemented.)

## Implementation-defined keywords

### __datasizeof

`__datasizeof` behaves like `sizeof`, except that it returns the size of the type ignoring tail padding.

### _BitInt, _ExtInt

Clang supports the C23 `_BitInt(N)` feature as an extension in older C modes and in C++. This type was previously implemented in Clang with the same semantics, but spelled `_ExtInt(N)`. This spelling has been deprecated in favor of the standard type.

Note: the ABI for `_BitInt(N)` is still in the process of being stabilized, so this type should not yet be used in interfaces that require ABI stability.

### C keywords supported in all language modes

Clang supports `_Alignas`, `_Alignof`, `_Atomic`, `_Complex`, `_Generic`, `_Imaginary`, `_Noreturn`, `_Static_assert`, `_Thread_local`, and `_Float16` in all language modes with the C semantics.

### __alignof, __alignof__

`__alignof` and `__alignof__` return, in contrast to `_Alignof` and `alignof`, the preferred alignment of a type. This may be larger than the required alignment for improved performance.

### __extension__

`__extension__` suppresses extension diagnostics in the statement it is prepended to.

### __auto_type

`__auto_type` behaves the same as `auto` in C++11 but is available in all language modes.

### __imag, __imag__

`__imag` and `__imag__` can be used to get the imaginary part of a complex value.

### __real, __real__

`__real` and `__real__` can be used to get the real part of a complex value.

### __asm, __asm__

`__asm` and `__asm__` are alternate spellings for `asm`, but available in all language modes.

### __complex, __complex__

`__complex` and `__complex__` are alternate spellings for `_Complex`.

### __const, __const__, __volatile, __volatile__, __restrict, __restrict__

These are alternate spellings for their non-underscore counterparts, but are available in all language modes.

### __decltype

`__decltype` is an alternate spelling for `decltype`, but is also available in C++ modes before C++11.

### __inline, __inline__

`__inline` and `__inline__` are alternate spellings for `inline`, but are available in all language modes.

### __nullptr

`__nullptr` is an alternate spelling for `nullptr`. It is available in all C and C++ language modes.

### __signed, __signed__

`__signed` and `__signed__` are alternate spellings for `signed`. `__unsigned` and `__unsigned__` are not supported.

### __typeof, __typeof__, __typeof_unqual, __typeof_unqual__

`__typeof` and `__typeof__` are alternate spellings for `typeof`, but are available in all language modes. These spellings result in the operand, retaining all qualifiers.

`__typeof_unqual` and `__typeof_unqual__` are alternate spellings for the C23 `typeof_unqual` type specifier, but are available in all language modes. These spellings result in the type of the operand, stripping all qualifiers.

### __char16_t, __char32_t

`__char16_t` and `__char32_t` are alternate spellings for `char16_t` and `char32_t` respectively, but are also available in C++ modes before C++11. They are only supported in C++. `__char8_t` is not available.

## Vectors and Extended Vectors

Supports the GCC, OpenCL, AltiVec, NEON and SVE vector extensions.

OpenCL vector types are created using the `ext_vector_type` attribute. It supports the `V.xyzw` syntax and other tidbits as seen in OpenCL. An example is:

    typedef float float4 __attribute__((ext_vector_type(4)));
    typedef float float2 __attribute__((ext_vector_type(2)));

    float4 foo(float2 a, float2 b) {
      float4 c;
      c.xz = a;
      c.yw = b;
      return c;
    }

Query for this feature with `__has_attribute(ext_vector_type)`.

Giving `-maltivec` option to clang enables support for AltiVec vector syntax and functions. For example:

    vector float foo(vector int a) {
      vector int b;
      b = vec_add(a, a) + a;
      return (vector float)b;
    }

NEON vector types are created using `neon_vector_type` and `neon_polyvector_type` attributes. For example:

    typedef __attribute__((neon_vector_type(8))) int8_t int8x8_t;
    typedef __attribute__((neon_polyvector_type(16))) poly8_t poly8x16_t;

    int8x8_t foo(int8x8_t a) {
      int8x8_t v;
      v = a;
      return v;
    }

GCC vector types are created using the `vector_size(N)` attribute. The argument `N` specifies the number of bytes that will be allocated for an object of this type. The size has to be multiple of the size of the vector element type. For example:

    // OK: This declares a vector type with four 'int' elements
    typedef int int4 __attribute__((vector_size(4 * sizeof(int))));

    // ERROR: '11' is not a multiple of sizeof(int)
    typedef int int_impossible __attribute__((vector_size(11)));

    int4 foo(int4 a) {
      int4 v;
      v = a;
      return v;
    }

### Boolean Vectors

Clang also supports the ext_vector_type attribute with boolean element types in C and C++. For example:

    // legal for Clang, error for GCC:
    typedef bool bool4 __attribute__((ext_vector_type(4)));
    // Objects of bool4 type hold 8 bits, sizeof(bool4) == 1

    bool4 foo(bool4 a) {
      bool4 v;
      v = a;
      return v;
    }

Boolean vectors are a Clang extension of the ext vector type. Boolean vectors are intended, though not guaranteed, to map to vector mask registers. The size parameter of a boolean vector type is the number of bits in the vector. The boolean vector is dense and each bit in the boolean vector is one vector element.

The semantics of boolean vectors borrows from C bit-fields with the following differences:

- Distinct boolean vectors are always distinct memory objects (there is no packing).

- Only the operators ?:, !, ~, |, &, ^ and comparison are allowed on boolean vectors.

- Casting a scalar bool value to a boolean vector type means broadcasting the scalar value onto all lanes (same as general ext_vector_type).

- It is not possible to access or swizzle elements of a boolean vector (different than general ext_vector_type).

The size and alignment are both the number of bits rounded up to the next power of two, but the alignment is at most the maximum vector alignment of the target.

### Vector Literals

Vector literals can be used to create vectors from a set of scalars, or vectors. Either parentheses or braces form can be used. In the parentheses form the number of literal values specified must be one, i.e. referring to a scalar value, or must match the size of the vector type being created. If a single scalar literal value is specified, the scalar literal value will be replicated to all the components of the vector type. In the brackets form any number of literals can be specified. For example:

    typedef int v4si __attribute__((__vector_size__(16)));
    typedef float float4 __attribute__((ext_vector_type(4)));
    typedef float float2 __attribute__((ext_vector_type(2)));

    v4si vsi = (v4si){1, 2, 3, 4};
    float4 vf = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
    vector int vi1 = (vector int)(1);    // vi1 will be (1, 1, 1, 1).
    vector int vi2 = (vector int){1};    // vi2 will be (1, 0, 0, 0).
    vector int vi3 = (vector int)(1, 2); // error
    vector int vi4 = (vector int){1, 2}; // vi4 will be (1, 2, 0, 0).
    vector int vi5 = (vector int)(1, 2, 3, 4);
    float4 vf = (float4)((float2)(1.0f, 2.0f), (float2)(3.0f, 4.0f));

### Vector Operations

The table below shows the support for each operation by vector extension. A dash indicates that an operation is not accepted according to a corresponding specification.

Operator | OpenCL | AltiVec | GCC | NEON | SVE
--- | --- | --- | --- | --- | ---
[] | yes | yes | yes | yes | yes
unary operators +, – | yes | yes | yes | yes | yes
++, – – | yes | yes | yes | no | no
+,–,*,/,% | yes | yes | yes | yes | yes
bitwise operators &,|,^,~ | yes | yes | yes | yes | yes
>>,<< | yes | yes | yes | yes | yes
!, &&, || | yes | – | yes | yes | yes
==, !=, >, <, >=, <= | yes | yes | yes | yes | yes
= | yes | yes | yes | yes | yes
?: [1] | yes | – | yes | yes | yes
sizeof | yes | yes | yes | yes | yes [2]
C-style cast | yes | yes | yes | no | no
reinterpret_cast | yes | no | yes | no | no
static_cast | yes | no | yes | no | no
const_cast | no | no | no | no | no
address &v[i] | no | no | no [3] | no | no

See also __builtin_shufflevector, __builtin_convertvector.

[1]

ternary operator(?:) has different behaviors depending on condition operand’s vector type. If the condition is a GNU vector (i.e. __vector_size__), a NEON vector or an SVE vector, it’s only available in C++ and uses normal bool conversions (that is, != 0). If it’s an extension (OpenCL) vector, it’s only available in C and OpenCL C. And it selects base on signedness of the condition operands (OpenCL v1.1 s6.3.9).

[2]

sizeof can only be used on vector length specific SVE types.

[3]

Clang does not allow the address of an element to be taken while GCC allows this. This is intentional for vectors with a boolean element type and not implemented otherwise.

### Vector Builtins

Note: The implementation of vector builtins is work-in-progress and incomplete.

In addition to the operators mentioned above, Clang provides a set of builtins to perform additional operations on certain scalar and vector types.

Let `T` be one of the following types:

- an integer type (as in C23 6.2.5p22), but excluding enumerated types and `bool`

- the standard floating types float or double

- a half-precision floating point type, if one is supported on the target

- a vector type.

For scalar types, consider the operation applied to a vector with a single element.

Vector Size To determine the number of elements in a vector, use `__builtin_vectorelements()`. For fixed-sized vectors, e.g., defined via `__attribute__((vector_size(N)))` or ARM NEON’s vector types (e.g., `uint16x8_t`), this returns the constant number of elements at compile-time. For scalable vectors, e.g., SVE or RISC-V V, the number of elements is not known at compile-time and is determined at runtime. This builtin can be used, e.g., to increment the loop-counter in vector-type agnostic loops.

Elementwise Builtins

Each builtin returns a vector equivalent to applying the specified operation elementwise to the input.

Unless specified otherwise operation(±0) = ±0 and operation(±infinity) = ±infinity

The integer elementwise intrinsics, including `__builtin_elementwise_popcount`, `__builtin_elementwise_bitreverse`, `__builtin_elementwise_add_sat`, `__builtin_elementwise_sub_sat` can be called in a `constexpr` context.

No implicit promotion of integer types takes place. The mixing of integer types of different sizes and signs is forbidden in binary and ternary builtins.

Name | Operation | Supported element types
--- | --- | ---
T __builtin_elementwise_abs(T x) | return the absolute value of a number x; the absolute value of the most negative integer remains the most negative integer | signed integer and floating point types
T __builtin_elementwise_fma(T x, T y, T z) | fused multiply add, (x * y) + z. | floating point types
T __builtin_elementwise_ceil(T x) | return the smallest integral value greater than or equal to x | floating point types
T __builtin_elementwise_sin(T x) | return the sine of x interpreted as an angle in radians | floating point types
T __builtin_elementwise_cos(T x) | return the cosine of x interpreted as an angle in radians | floating point types
T __builtin_elementwise_tan(T x) | return the tangent of x interpreted as an angle in radians | floating point types
T __builtin_elementwise_asin(T x) | return the arcsine of x interpreted as an angle in radians | floating point types
T __builtin_elementwise_acos(T x) | return the arccosine of x interpreted as an angle in radians | floating point types
T __builtin_elementwise_atan(T x) | return the arctangent of x interpreted as an angle in radians | floating point types
T __builtin_elementwise_atan2(T y, T x) | return the arctangent of y/x | floating point types
T __builtin_elementwise_sinh(T x) | return the hyperbolic sine of angle x in radians | floating point types
T __builtin_elementwise_cosh(T x) | return the hyperbolic cosine of angle x in radians | floating point types
T __builtin_elementwise_tanh(T x) | return the hyperbolic tangent of angle x in radians | floating point types
T __builtin_elementwise_floor(T x) | return the largest integral value less than or equal to x | floating point types
T __builtin_elementwise_log(T x) | return the natural logarithm of x | floating point types
T __builtin_elementwise_log2(T x) | return the base 2 logarithm of x | floating point types
T __builtin_elementwise_log10(T x) | return the base 10 logarithm of x | floating point types
T __builtin_elementwise_popcount(T x) | return the number of 1 bits in x | integer types
T __builtin_elementwise_pow(T x, T y) | return x raised to the power of y | floating point types
T __builtin_elementwise_bitreverse(T x) | return the integer represented after reversing the bits of x | integer types
T __builtin_elementwise_exp(T x) | returns the base-e exponential, e^x, of the specified value | floating point types
T __builtin_elementwise_exp2(T x) | returns the base-2 exponential, 2^x, of the specified value | floating point types
T __builtin_elementwise_exp10(T x) | returns the base-10 exponential, 10^x, of the specified value | floating point types
T __builtin_elementwise_sqrt(T x) | return the square root of a floating-point number | floating point types
T __builtin_elementwise_roundeven(T x) | round x to the nearest integer value in floating point format, rounding halfway cases to even (that is, to the nearest value that is an even integer), regardless of the current rounding direction. | floating point types
T __builtin_elementwise_round(T x) | round x to the nearest integer value in floating point format, rounding halfway cases away from zero, regardless of the current rounding direction. May raise floating-point exceptions. | floating point types
T __builtin_elementwise_trunc(T x) | return the integral value nearest to but no larger in magnitude than x | floating point types
T __builtin_elementwise_nearbyint(T x) | round x to the nearest integer value in floating point format, rounding according to the current rounding direction. May not raise the inexact floating-point exception. This is treated the same as `__builtin_elementwise_rint` unless FENV_ACCESS is enabled. | floating point types
T __builtin_elementwise_rint(T x) | round x to the nearest integer value in floating point format, rounding according to the current rounding direction. May raise floating-point exceptions. This is treated the same as `__builtin_elementwise_nearbyint` unless FENV_ACCESS is enabled. | floating point types
T __builtin_elementwise_canonicalize(T x) | return the platform specific canonical encoding of a floating-point number | floating point types
T __builtin_elementwise_copysign(T x, T y) | return the magnitude of x with the sign of y. | floating point types
T __builtin_elementwise_fmod(T x, T y) | return The floating-point remainder of (x/y) whose sign matches the sign of x. | floating point types
T __builtin_elementwise_max(T x, T y) | return x or y, whichever is larger For floating point types, follows semantics of maxNum in IEEE 754-2008. See LangRef for the comparison. | integer and floating point types
T __builtin_elementwise_min(T x, T y) | return x or y, whichever is smaller For floating point types, follows semantics of minNum in IEEE 754-2008. See LangRef for the comparison. | integer and floating point types
T __builtin_elementwise_maxnum(T x, T y) | return x or y, whichever is larger. Follows IEEE 754-2008 semantics (maxNum) with +0.0>-0.0. See LangRef for the comparison. | floating point types
T __builtin_elementwise_minnum(T x, T y) | return x or y, whichever is smaller. Follows IEEE 754-2008 semantics (minNum) with +0.0>-0.0. See LangRef for the comparison. | floating point types
T __builtin_elementwise_add_sat(T x, T y) | return the sum of x and y, clamped to the range of representable values for the signed/unsigned integer type. | integer types
T __builtin_elementwise_sub_sat(T x, T y) | return the difference of x and y, clamped to the range of representable values for the signed/unsigned integer type. | integer types
T __builtin_elementwise_maximum(T x, T y) | return x or y, whichever is larger. Follows IEEE 754-2019 semantics, see LangRef for the comparison. | floating point types
T __builtin_elementwise_minimum(T x, T y) | return x or y, whichever is smaller. Follows IEEE 754-2019 semantics, see LangRef for the comparison. | floating point types

Reduction Builtins

Each builtin returns a scalar equivalent to applying the specified operation(x, y) as recursive even-odd pairwise reduction to all vector elements. `operation(x, y)` is repeatedly applied to each non-overlapping even-odd element pair with indices `i * 2` and `i * 2 + 1` with `i in [0, Number of elements / 2)`. If the numbers of elements is not a power of 2, the vector is widened with neutral elements for the reduction at the end to the next power of 2.

These reductions support both fixed-sized and scalable vector types.

The integer reduction intrinsics, including `__builtin_reduce_max`, `__builtin_reduce_min`, `__builtin_reduce_add`, `__builtin_reduce_mul`, `__builtin_reduce_and`, `__builtin_reduce_or`, and `__builtin_reduce_xor`, can be called in a `constexpr` context.

Example:

    __builtin_reduce_add([e3, e2, e1, e0]) = __builtin_reduced_add([e3 + e2, e1 + e0])
                                           = (e3 + e2) + (e1 + e0)

Let `VT` be a vector type and `ET` the element type of `VT`.

Name | Operation | Supported element types
--- | --- | ---
ET __builtin_reduce_max(VT a) | return the largest element of the vector. The floating point result will always be a number unless all elements of the vector are NaN. | integer and floating point types
ET __builtin_reduce_min(VT a) | return the smallest element of the vector. The floating point result will always be a number unless all elements of the vector are NaN. | integer and floating point types
ET __builtin_reduce_add(VT a) | + | integer types
ET __builtin_reduce_mul(VT a) | * | integer types
ET __builtin_reduce_and(VT a) | & | integer types
ET __builtin_reduce_or(VT a) | | | integer types
ET __builtin_reduce_xor(VT a) | ^ | integer types
ET __builtin_reduce_maximum(VT a) | return the largest element of the vector. Follows IEEE 754-2019 semantics, see LangRef for the comparison. | floating point types
ET __builtin_reduce_minimum(VT a) | return the smallest element of the vector. Follows IEEE 754-2019 semantics, see LangRef for the comparison. | floating point types

## Matrix Types

Clang provides an extension for matrix types, which is currently being implemented. See the draft specification for more details.

For example, the code below uses the matrix types extension to multiply two 4x4 float matrices and add the result to a third 4x4 matrix.

    typedef float m4x4_t __attribute__((matrix_type(4, 4)));

    m4x4_t f(m4x4_t a, m4x4_t b, m4x4_t c) {
      return a + b * c;
    }

The matrix type extension also supports operations on a matrix and a scalar.

    typedef float m4x4_t __attribute__((matrix_type(4, 4)));

    m4x4_t f(m4x4_t a) {
      return (a + 23) * 12;
    }

The matrix type extension supports division on a matrix and a scalar but not on a matrix and a matrix.

    typedef float m4x4_t __attribute__((matrix_type(4, 4)));

    m4x4_t f(m4x4_t a) {
      a = a / 3.0;
      return a;
    }

The matrix type extension supports compound assignments for addition, subtraction, and multiplication on matrices and on a matrix and a scalar, provided their types are consistent.

    typedef float m4x4_t __attribute__((matrix_type(4, 4)));

    m4x4_t f(m4x4_t a, m4x4_t b) {
      a += b;
      a -= b;
      a *= b;
      a += 23;
      a -= 12;
      return a;
    }

The matrix type extension supports explicit casts. Implicit type conversion between matrix types is not allowed.

    typedef int ix5x5 __attribute__((matrix_type(5, 5)));
    typedef float fx5x5 __attribute__((matrix_type(5, 5)));

    fx5x5 f1(ix5x5 i, fx5x5 f) {
      return (fx5x5) i;
    }

    template <typename X>
    using matrix_4_4 = X __attribute__((matrix_type(4, 4)));

    void f2() {
      matrix_5_5<double> d;
      matrix_5_5<int> i;
      i = (matrix_5_5<int>)d;
      i = static_cast<matrix_5_5<int>>(d);
    }

## Half-Precision Floating Point

Clang supports three half-precision (16-bit) floating point types: `__fp16`, `_Float16` and `__bf16`. These types are supported in all language modes, but their support differs between targets. A target is said to have “native support” for a type if the target processor offers instructions for directly performing basic arithmetic on that type. In the absence of native support, a type can still be supported if the compiler can emulate arithmetic on the type by promoting to `float`; see below for more information on this emulation.

- `__fp16` is supported on all targets. The special semantics of this type mean that no arithmetic is ever performed directly on `__fp16` values; see below.

- `_Float16` is supported on the following targets:

  - 32-bit ARM (natively on some architecture versions)

  - 64-bit ARM (AArch64) (natively on ARMv8.2a and above)

  - AMDGPU (natively)

  - NVPTX (natively)

  - SPIR (natively)

  - X86 (if SSE2 is available; natively if AVX512-FP16 is also available)

  - RISC-V (natively if Zfh or Zhinx is available)

  - SystemZ (emulated)

  - LoongArch (emulated)

- `__bf16` is supported on the following targets (currently never natively):

  - 32-bit ARM

  - 64-bit ARM (AArch64)

  - RISC-V

  - X86 (when SSE2 is available)

  - LoongArch

(For X86, SSE2 is available on 64-bit and all recent 32-bit processors.)

`__fp16` and `_Float16` both use the binary16 format from IEEE 754-2008, which provides a 5-bit exponent and an 11-bit significand (counting the implicit leading 1). `__bf16` uses the bfloat16 format, which provides an 8-bit exponent and an 8-bit significand; this is the same exponent range as float, just with greatly reduced precision.

`_Float16` and `__bf16` follow the usual rules for arithmetic floating-point types. Most importantly, this means that arithmetic operations on operands of these types are formally performed in the type and produce values of the type. `__fp16` does not follow those rules: most operations immediately promote operands of type `__fp16` to `float`, and so arithmetic operations are defined to be performed in `float` and so result in a value of type `float` (unless further promoted because of other operands). See below for more information on the exact specifications of these types.

When compiling arithmetic on `_Float16` and `__bf16` for a target without native support, Clang will perform the arithmetic in `float`, inserting extensions and truncations as necessary. This can be done in a way that exactly matches the operation-by-operation behavior of native support, but that can require many extra truncations and extensions. By default, when emulating `_Float16` and `__bf16` arithmetic using `float`, Clang does not truncate intermediate operands back to their true type unless the operand is the result of an explicit cast or assignment. This is generally much faster but can generate different results from strict operation-by-operation emulation. Usually the results are more precise. This is permitted by the C and C++ standards under the rules for excess precision in intermediate operands; see the discussion of evaluation formats in the C standard and [expr.pre] in the C++ standard.

The use of excess precision can be independently controlled for these two types with the `-ffloat16-excess-precision=` and `-fbfloat16-excess-precision=` options. Valid values include:

- `none`: meaning to perform strict operation-by-operation emulation

- `standard`: meaning that excess precision is permitted under the rules described in the standard, i.e. never across explicit casts or statements

- `fast`: meaning that excess precision is permitted whenever the optimizer sees an opportunity to avoid truncations; currently this has no effect beyond `standard`

The `_Float16` type is an interchange floating type specified in ISO/IEC TS 18661-3:2015 (“Floating-point extensions for C”). It will be supported on more targets as they define ABIs for it.

The `__bf16` type is a non-standard extension, but it generally follows the rules for arithmetic interchange floating types from ISO/IEC TS 18661-3:2015. In previous versions of Clang, it was a storage-only type that forbade arithmetic operations. It will be supported on more targets as they define ABIs for it.

The `__fp16` type was originally an ARM extension and is specified by the ARM C Language Extensions. Clang uses the `binary16` format from IEEE 754-2008 for `__fp16`, not the ARM alternative format. Operators that expect arithmetic operands immediately promote `__fp16` operands to `float`.

It is recommended that portable code use `_Float16` instead of `__fp16`, as it has been defined by the C standards committee and has behavior that is more familiar to most programmers.

Because `__fp16` operands are always immediately promoted to `float`, the common real type of `__fp16` and `_Float16` for the purposes of the usual arithmetic conversions is `float`.

A literal can be given `_Float16` type using the suffix `f16`. For example, `3.14f16`.

Because default argument promotion only applies to the standard floating-point types, `_Float16` values are not promoted to `double` when passed as variadic or untyped arguments. As a consequence, some caution must be taken when using certain library facilities with `_Float16`; for example, there is no `printf` format specifier for `_Float16`, and (unlike `float`) it will not be implicitly promoted to `double` when passed to `printf`, so the programmer must explicitly cast it to `double` before using it with an `%f` or similar specifier.

## Messages on `deprecated` and `unavailable` Attributes

An optional string message can be added to the `deprecated` and `unavailable` attributes. For example:

    void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));

If the deprecated or unavailable declaration is used, the message will be incorporated into the appropriate diagnostic:

    harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!!
          [-Wdeprecated-declarations]
      explode();
      ^

Query for this feature with `__has_extension(attribute_deprecated_with_message)` and `__has_extension(attribute_unavailable_with_message)`.

## Attributes on Enumerators

Clang allows attributes to be written on individual enumerators. This allows enumerators to be deprecated, made unavailable, etc. The attribute must appear after the enumerator name and before any initializer, like so:

    enum OperationMode {
      OM_Invalid,
      OM_Normal,
      OM_Terrified __attribute__((deprecated)),
      OM_AbortOnError __attribute__((deprecated)) = 4
    };

Attributes on the `enum` declaration do not apply to individual enumerators.

Query for this feature with `__has_extension(enumerator_attributes)`.

## C++11 Attributes on using-declarations

Clang allows C++-style `[[]]` attributes to be written on using-declarations. For instance:

    [[clang::using_if_exists]] using foo::bar;
    using foo::baz [[clang::using_if_exists]];

You can test for support for this extension with `__has_extension(cxx_attributes_on_using_declarations)`.

## ‘User-Specified’ System Frameworks

Clang provides a mechanism by which frameworks can be built in such a way that they will always be treated as being “system frameworks”, even if they are not present in a system framework directory. This can be useful to system framework developers who want to be able to test building other applications with development builds of their framework, including the manner in which the compiler changes warning behavior for system headers.

Framework developers can opt-in to this mechanism by creating a “`.system_framework`” file at the top-level of their framework. That is, the framework should have contents like:

    .../TestFramework.framework
    .../TestFramework.framework/.system_framework
    .../TestFramework.framework/Headers
    .../TestFramework.framework/Headers/TestFramework.h
    ...

Clang will treat the presence of this file as an indicator that the framework should be treated as a system framework, regardless of how it was found in the framework search path. For consistency, we recommend that such files never be included in installed versions of the framework.

## Checks for Standard Language Features

The `__has_feature` macro can be used to query if certain standard language features are enabled. The `__has_extension` macro can be used to query if language features are available as an extension when compiling for a standard which does not provide them. The features which can be tested are listed here.

Since Clang 3.4, the C++ SD-6 feature test macros are also supported. These are macros with names of the form `__cpp_<feature_name>`, and are intended to be a portable way to query the supported features of the compiler. See the C++ status page for information on the version of SD-6 supported by each Clang release, and the macros provided by that revision of the recommendations.

### C++98

The features listed below are part of the C++98 standard. These features are enabled by default when compiling C++ code.

#### C++ exceptions

Use `__has_feature(cxx_exceptions)` to determine if C++ exceptions have been enabled. For example, compiling code with `-fno-exceptions` disables C++ exceptions.

#### C++ RTTI

Use `__has_feature(cxx_rtti)` to determine if C++ RTTI has been enabled. For example, compiling code with `-fno-rtti` disables the use of RTTI.

### C++11

The features listed below are part of the C++11 standard. As a result, all these features are enabled with the `-std=c++11` or `-std=gnu++11` option when compiling C++ code.

#### C++11 SFINAE includes access control

Use `__has_feature(cxx_access_control_sfinae)` or `__has_extension(cxx_access_control_sfinae)` to determine whether access-control errors (e.g., calling a private constructor) are considered to be template argument deduction errors (aka SFINAE errors), per C++ DR1170.

#### C++11 alias templates

Use `__has_feature(cxx_alias_templates)` or `__has_extension(cxx_alias_templates)` to determine if support for C++11’s alias declarations and alias templates is enabled.

#### C++11 alignment specifiers

Use `__has_feature(cxx_alignas)` or `__has_extension(cxx_alignas)` to determine if support for alignment specifiers using `alignas` is enabled.

Use `__has_feature(cxx_alignof)` or `__has_extension(cxx_alignof)` to determine if support for the `alignof` keyword is enabled.

#### C++11 attributes

Use `__has_feature(cxx_attributes)` or `__has_extension(cxx_attributes)` to determine if support for attribute parsing with C++11’s square bracket notation is enabled.

#### C++11 generalized constant expressions

Use `__has_feature(cxx_constexpr)` to determine if support for generalized constant expressions (e.g., `constexpr`) is enabled.

#### C++11 `decltype()`

Use `__has_feature(cxx_decltype)` or `__has_extension(cxx_decltype)` to determine if support for the `decltype()` specifier is enabled. C++11’s `decltype` does not require type-completeness of a function call expression. Use `__has_feature(cxx_decltype_incomplete_return_types)` or `__has_extension(cxx_decltype_incomplete_return_types)` to determine if support for this feature is enabled.

#### C++11 default template arguments in function templates

Use `__has_feature(cxx_default_function_template_args)` or `__has_extension(cxx_default_function_template_args)` to determine if support for default template arguments in function templates is enabled.

#### C++11 `default`ed functions

Use `__has_feature(cxx_defaulted_functions)` or `__has_extension(cxx_defaulted_functions)` to determine if support for defaulted function definitions (with `= default`) is enabled.

#### C++11 delegating constructors

Use `__has_feature(cxx_delegating_constructors)` to determine if support for delegating constructors is enabled.

#### C++11 `deleted` functions

Use `__has_feature(cxx_deleted_functions)` or `__has_extension(cxx_deleted_functions)` to determine if support for deleted function definitions (with `= delete`) is enabled.

#### C++11 explicit conversion functions

Use `__has_feature(cxx_explicit_conversions)` to determine if support for `explicit` conversion functions is enabled.

#### C++11 generalized initializers

Use `__has_feature(cxx_generalized_initializers)` to determine if support for generalized initializers (using braced lists and `std::initializer_list`) is enabled.

#### C++11 implicit move constructors/assignment operators

Use `__has_feature(cxx_implicit_moves)` to determine if Clang will implicitly generate move constructors and move assignment operators where needed.

#### C++11 inheriting constructors

Use `__has_feature(cxx_inheriting_constructors)` to determine if support for inheriting constructors is enabled.

#### C++11 inline namespaces

Use `__has_feature(cxx_inline_namespaces)` or `__has_extension(cxx_inline_namespaces)` to determine if support for inline namespaces is enabled.

#### C++11 lambdas

Use `__has_feature(cxx_lambdas)` or `__has_extension(cxx_lambdas)` to determine if support for lambdas is enabled.

#### C++11 local and unnamed types as template arguments

Use `__has_feature(cxx_local_type_template_args)` or `__has_extension(cxx_local_type_template_args)` to determine if support for local and unnamed types as template arguments is enabled.

#### C++11 noexcept

Use `__has_feature(cxx_noexcept)` or `__has_extension(cxx_noexcept)` to determine if support for noexcept exception specifications is enabled.

#### C++11 in-class non-static data member initialization

Use `__has_feature(cxx_nonstatic_member_init)` to determine whether in-class initialization of non-static data members is enabled.

#### C++11 `nullptr`

Use `__has_feature(cxx_nullptr)` or `__has_extension(cxx_nullptr)` to determine if support for `nullptr` is enabled.

#### C++11 `override control`

Use `__has_feature(cxx_override_control)` or `__has_extension(cxx_override_control)` to determine if support for the override control keywords is enabled.

#### C++11 reference-qualified functions

Use `__has_feature(cxx_reference_qualified_functions)` or `__has_extension(cxx_reference_qualified_functions)` to determine if support for reference-qualified functions (e.g., member functions with `&` or `&&` applied to `*this`) is enabled.

#### C++11 range-based `for` loop

Use `__has_feature(cxx_range_for)` or `__has_extension(cxx_range_for)` to determine if support for the range-based for loop is enabled.

#### C++11 raw string literals

Use `__has_feature(cxx_raw_string_literals)` to determine if support for raw string literals (e.g., `R"x(foo\bar)x"`) is enabled.

#### C++11 rvalue references

Use `__has_feature(cxx_rvalue_references)` or `__has_extension(cxx_rvalue_references)` to determine if support for rvalue references is enabled.

#### C++11 `static_assert()`

Use `__has_feature(cxx_static_assert)` or `__has_extension(cxx_static_assert)` to determine if support for compile-time assertions using `static_assert` is enabled.

#### C++11 `thread_local`

Use `__has_feature(cxx_thread_local)` to determine if support for `thread_local` variables is enabled.

#### C++11 type inference

Use `__has_feature(cxx_auto_type)` or `__has_extension(cxx_auto_type)` to determine C++11 type inference is supported using the `auto` specifier. If this is disabled, `auto` will instead be a storage class specifier, as in C or C++98.

#### C++11 strongly typed enumerations

Use `__has_feature(cxx_strong_enums)` or `__has_extension(cxx_strong_enums)` to determine if support for strongly typed, scoped enumerations is enabled.

#### C++11 trailing return type

Use `__has_feature(cxx_trailing_return)` or `__has_extension(cxx_trailing_return)` to determine if support for the alternate function declaration syntax with trailing return type is enabled.

#### C++11 Unicode string literals

Use `__has_feature(cxx_unicode_literals)` to determine if support for Unicode string literals is enabled.

#### C++11 unrestricted unions

Use `__has_feature(cxx_unrestricted_unions)` to determine if support for unrestricted unions is enabled.

#### C++11 user-defined literals

Use `__has_feature(cxx_user_literals)` to determine if support for user-defined literals is enabled.

#### C++11 variadic templates

Use `__has_feature(cxx_variadic_templates)` or `__has_extension(cxx_variadic_templates)` to determine if support for variadic templates is enabled.

### C++14

The features listed below are part of the C++14 standard. As a result, all these features are enabled with the `-std=C++14` or `-std=gnu++14` option when compiling C++ code.

#### C++14 binary literals

Use `__has_feature(cxx_binary_literals)` or `__has_extension(cxx_binary_literals)` to determine whether binary literals (for instance, `0b10010`) are recognized. Clang supports this feature as an extension in all language modes.

#### C++14 contextual conversions

Use `__has_feature(cxx_contextual_conversions)` or `__has_extension(cxx_contextual_conversions)` to determine if the C++14 rules are used when performing an implicit conversion for an array bound in a new-expression, the operand of a delete-expression, an integral constant expression, or a condition in a `switch` statement.

#### C++14 decltype(auto)

Use `__has_feature(cxx_decltype_auto)` or `__has_extension(cxx_decltype_auto)` to determine if support for the `decltype(auto)` placeholder type is enabled.

#### C++14 default initializers for aggregates

Use `__has_feature(cxx_aggregate_nsdmi)` or `__has_extension(cxx_aggregate_nsdmi)` to determine if support for default initializers in aggregate members is enabled.

#### C++14 digit separators

Use `__cpp_digit_separators` to determine if support for digit separators using single quotes (for instance, `10'000`) is enabled. At this time, there is no corresponding `__has_feature` name

#### C++14 generalized lambda capture

Use `__has_feature(cxx_init_captures)` or `__has_extension(cxx_init_captures)` to determine if support for lambda captures with explicit initializers is enabled (for instance, `[n(0)] { return ++n; }`).

#### C++14 generic lambdas

Use `__has_feature(cxx_generic_lambdas)` or `__has_extension(cxx_generic_lambdas)` to determine if support for generic (polymorphic) lambdas is enabled (for instance, `[] (auto x) { return x + 1; }`).

#### C++14 relaxed constexpr

Use `__has_feature(cxx_relaxed_constexpr)` or `__has_extension(cxx_relaxed_constexpr)` to determine if variable declarations, local variable modification, and control flow constructs are permitted in `constexpr` functions.

#### C++14 return type deduction

Use `__has_feature(cxx_return_type_deduction)` or `__has_extension(cxx_return_type_deduction)` to determine if support for return type deduction for functions (using `auto` as a return type) is enabled.

#### C++14 runtime-sized arrays

Use `__has_feature(cxx_runtime_array)` or `__has_extension(cxx_runtime_array)` to determine if support for arrays of runtime bound (a restricted form of variable-length arrays) is enabled. Clang’s implementation of this feature is incomplete.

#### C++14 variable templates

Use `__has_feature(cxx_variable_templates)` or `__has_extension(cxx_variable_templates)` to determine if support for templated variable declarations is enabled.

#### C++ type aware allocators

Use `__has_extension(cxx_type_aware_allocators)` to determine the existence of support for the future C++2d type aware allocator feature. For full details see C++ Type Aware Allocators for additional details.

### C11

The features listed below are part of the C11 standard. As a result, all these features are enabled with the `-std=c11` or `-std=gnu11` option when compiling C code. Additionally, because these features are all backward-compatible, they are available as extensions in all language modes.

#### C11 alignment specifiers

Use `__has_feature(c_alignas)` or `__has_extension(c_alignas)` to determine if support for alignment specifiers using `_Alignas` is enabled.

Use `__has_feature(c_alignof)` or `__has_extension(c_alignof)` to determine if support for the `_Alignof` keyword is enabled.

#### C11 atomic operations

Use `__has_feature(c_atomic)` or `__has_extension(c_atomic)` to determine if support for atomic types using `_Atomic` is enabled. Clang also provides a set of builtins which can be used to implement the `<stdatomic.h>` operations on `_Atomic` types. Use `__has_include(<stdatomic.h>)` to determine if C11’s `<stdatomic.h>` header is available.

Clang will use the system’s `<stdatomic.h>` header when one is available, and will otherwise use its own. When using its own, implementations of the atomic operations are provided as macros. In the cases where C11 also requires a real function, this header provides only the declaration of that function (along with a shadowing macro implementation), and you must link to a library which provides a definition of the function if you use it instead of the macro.

#### C11 generic selections

Use `__has_feature(c_generic_selections)` or `__has_extension(c_generic_selections)` to determine if support for generic selections is enabled.

As an extension, the C11 generic selection expression is available in all languages supported by Clang. The syntax is the same as that given in the C11 standard.

In C, type compatibility is decided according to the rules given in the appropriate standard, but in C++, which lacks the type compatibility rules used in C, types are considered compatible only if they are equivalent.

Clang also supports an extended form of `_Generic` with a controlling type rather than a controlling expression. Unlike with a controlling expression, a controlling type argument does not undergo any conversions and thus is suitable for use when trying to match qualified types, incomplete types, or function types. Variable-length array types lack the necessary compile-time information to resolve which association they match with and thus are not allowed as a controlling type argument.

Use `__has_extension(c_generic_selection_with_controlling_type)` to determine if support for this extension is enabled.

#### C11 `_Static_assert()`

Use `__has_feature(c_static_assert)` or `__has_extension(c_static_assert)` to determine if support for compile-time assertions using `_Static_assert` is enabled.

#### C11 `_Thread_local`

Use `__has_feature(c_thread_local)` or `__has_extension(c_thread_local)` to determine if support for `_Thread_local` variables is enabled.

### C2y

The features listed below are part of the C2y standard. As a result, all these features are enabled with the `-std=c2y` or `-std=gnu2y` option when compiling C code.

#### C2y `_Countof`

Use `__has_feature(c_countof)` (in C2y or later mode) or `__has_extension(c_countof)` (in C23 or earlier mode) to determine if support for the `_Countof` operator is enabled. This feature is not available in C++ mode.

### Modules

Use `__has_feature(modules)` to determine if Modules have been enabled. For example, compiling code with `-fmodules` enables the use of Modules.

More information could be found here.

## Language Extensions Back-ported to Previous Standards

Feature | Feature Test Macro | Introduced In | Backported To
--- | --- | --- | ---
variadic templates | __cpp_variadic_templates | C++11 | C++03
Alias templates | __cpp_alias_templates | C++11 | C++03
Non-static data member initializers | __cpp_nsdmi | C++11 | C++03
Range-based `for` loop | __cpp_range_based_for | C++11 | C++03
RValue references | __cpp_rvalue_references | C++11 | C++03
Attributes | __cpp_attributes | C++11 | C++03
Lambdas | __cpp_lambdas | C++11 | C++03
Generalized lambda captures | __cpp_init_captures | C++14 | C++03
Generic lambda expressions | __cpp_generic_lambdas | C++14 | C++03
variable templates | __cpp_variable_templates | C++14 | C++03
Binary literals | __cpp_binary_literals | C++14 | C++03
Relaxed constexpr | __cpp_constexpr | C++14 | C++11
Static assert with no message | __cpp_static_assert >= 201411L | C++17 | C++11
Pack expansion in generalized lambda-capture | __cpp_init_captures | C++17 | C++03
`if constexpr` | __cpp_if_constexpr | C++17 | C++11
fold expressions | __cpp_fold_expressions | C++17 | C++03
Lambda capture of *this by value | __cpp_capture_star_this | C++17 | C++03
Attributes on enums | __cpp_enumerator_attributes | C++17 | C++03
Guaranteed copy elision | __cpp_guaranteed_copy_elision | C++17 | C++03
Hexadecimal floating literals | __cpp_hex_float | C++17 | C++03
`inline` variables | __cpp_inline_variables | C++17 | C++03
Attributes on namespaces | __cpp_namespace_attributes | C++17 | C++11
Structured bindings | __cpp_structured_bindings | C++17 | C++03
template template arguments | __cpp_template_template_args | C++17 | C++03
Familiar template syntax for generic lambdas | __cpp_generic_lambdas | C++20 | C++03
`static operator[]` | __cpp_multidimensional_subscript | C++20 | C++03
Designated initializers | __cpp_designated_initializers | C++20 | C++03
Conditional `explicit` | __cpp_conditional_explicit | C++20 | C++03
`using enum` | __cpp_using_enum | C++20 | C++03
`if consteval` | __cpp_if_consteval | C++23 | C++20
`static operator()` | __cpp_static_call_operator | C++23 | C++03
Attributes on Lambda-Expressions |  | C++23 | C++11
Attributes on Structured Bindings | __cpp_structured_bindings | C++26 | C++03
Packs in Structured Bindings | __cpp_structured_bindings | C++26 | C++03
Structured binding declaration as a condition | __cpp_structured_bindings | C++26 | C++98
Static assert with user-generated message | __cpp_static_assert >= 202306L | C++26 | C++11
Pack Indexing | __cpp_pack_indexing | C++26 | C++03
`= delete ("should have a reason");` | __cpp_deleted_function | C++26 | C++03
Variadic Friends | __cpp_variadic_friend | C++26 | C++03
Trivial Relocatability | __cpp_trivial_relocatability | C++26 | C++03
Designated initializers (N494) |  | C99 | C89
Array & element qualification (N2607) |  | C23 | C89
Attributes (N2335) |  | C23 | C89
`#embed` (N3017) |  | C23 | C89, C++
Octal literals prefixed with `0o` or `0O` |  | C2y | C89, C++
`_Countof` (N3369, N3469) |  | C2y | C89

## Builtin type aliases

Clang provides a few builtin aliases to improve the throughput of certain metaprogramming facilities.

### __builtin_common_type

    template <template <class... Args> class BaseTemplate,
              template <class TypeMember> class HasTypeMember,
              class HasNoTypeMember,
              class... Ts>
    using __builtin_common_type = ...;

This alias is used for implementing `std::common_type`. If `std::common_type` should contain a `type` member, it is an alias to `HasTypeMember<TheCommonType>`. Otherwise it is an alias to `HasNoTypeMember`. The `BaseTemplate` is usually `std::common_type`. `Ts` are the arguments to `std::common_type`.

### __type_pack_element

    template <std::size_t Index, class... Ts>
    using __type_pack_element = ...;

This alias returns the type at `Index` in the parameter pack `Ts`.

### __make_integer_seq

    template <template <class IntSeqT, IntSeqT... Ints> class IntSeq, class T, T N>
    using __make_integer_seq = ...;

This alias returns `IntSeq` instantiated with `IntSeqT = T``and ``Ints` being the pack `0, ..., N - 1`.

## Type Trait Primitives

Type trait primitives are special builtin constant expressions that can be used by the standard C++ library to facilitate or simplify the implementation of user-facing type traits in the <type_traits> header.

They are not intended to be used directly by user code because they are implementation-defined and subject to change – as such they’re tied closely to the supported set of system headers, currently:

- LLVM’s own libc++

- GNU libstdc++

- The Microsoft standard C++ library

Clang supports the GNU C++ type traits and a subset of the Microsoft Visual C++ type traits, as well as nearly all of the Embarcadero C++ type traits.

The following type trait primitives are supported by Clang. Those traits marked (C++) provide implementations for type traits specified by the C++ standard; `__X(...)` has the same semantics and constraints as the corresponding `std::X_t<...>` or `std::X_v<...>` type trait.

- `__array_rank(type)` (Embarcadero): Returns the number of levels of array in the type `type`: `0` if `type` is not an array type, and `__array_rank(element) + 1` if `type` is an array of `element`.

- `__array_extent(type, dim)` (Embarcadero): The `dim`’th array bound in the type `type`, or `0` if `dim >= __array_rank(type)`.

- `__builtin_is_implicit_lifetime` (C++, GNU, Microsoft)

- `__builtin_is_virtual_base_of` (C++, GNU, Microsoft)

- `__can_pass_in_regs` (C++) Returns whether a class can be passed in registers under the current ABI. This type can only be applied to unqualified class types. This is not a portable type trait.

- `__has_nothrow_assign` (GNU, Microsoft, Embarcadero): Deprecated, use `__is_nothrow_assignable` instead.

- `__has_nothrow_move_assign` (GNU, Microsoft): Deprecated, use `__is_nothrow_assignable` instead.

- `__has_nothrow_copy` (GNU, Microsoft): Deprecated, use `__is_nothrow_constructible` instead.

- `__has_nothrow_constructor` (GNU, Microsoft): Deprecated, use `__is_nothrow_constructible` instead.

- `__has_trivial_assign` (GNU, Microsoft, Embarcadero): Deprecated, use `__is_trivially_assignable` instead.

- `__has_trivial_move_assign` (GNU, Microsoft): Deprecated, use `__is_trivially_assignable` instead.

- `__has_trivial_copy` (GNU, Microsoft): Deprecated, use `__is_trivially_copyable` instead.

- `__has_trivial_constructor` (GNU, Microsoft): Deprecated, use `__is_trivially_constructible` instead.

- `__has_trivial_move_constructor` (GNU, Microsoft): Deprecated, use `__is_trivially_constructible` instead.

- `__has_trivial_destructor` (GNU, Microsoft, Embarcadero): Deprecated, use `__is_trivially_destructible` instead.

- `__has_unique_object_representations` (C++, GNU)

- `__has_virtual_destructor` (C++, GNU, Microsoft, Embarcadero)

- `__is_abstract` (C++, GNU, Microsoft, Embarcadero)

- `__is_aggregate` (C++, GNU, Microsoft)

- `__is_arithmetic` (C++, Embarcadero)

- `__is_array` (C++, Embarcadero)

- `__is_assignable` (C++, MSVC 2015)

- `__is_base_of` (C++, GNU, Microsoft, Embarcadero)

- `__is_bounded_array` (C++, GNU, Microsoft, Embarcadero)

- `__is_class` (C++, GNU, Microsoft, Embarcadero)

- `__is_complete_type(type)` (Embarcadero): Return `true` if `type` is a complete type. Warning: this trait is dangerous because it can return different values at different points in the same program.

- `__is_compound` (C++, Embarcadero)

- `__is_const` (C++, Embarcadero)

- `__is_constructible` (C++, MSVC 2013)

- `__is_convertible` (C++, Embarcadero)

- `__is_nothrow_convertible` (C++, GNU)

- `__is_convertible_to` (Microsoft): Synonym for `__is_convertible`.

- `__is_destructible` (C++, MSVC 2013)

- `__is_empty` (C++, GNU, Microsoft, Embarcadero)

- `__is_enum` (C++, GNU, Microsoft, Embarcadero)

- `__is_final` (C++, GNU, Microsoft)

- `__is_floating_point` (C++, Embarcadero)

- `__is_function` (C++, Embarcadero)

- `__is_fundamental` (C++, Embarcadero)

- `__is_integral` (C++, Embarcadero)

- `__is_interface_class` (Microsoft): Returns `false`, even for types defined with `__interface`.

- `__is_layout_compatible` (C++, GNU, Microsoft)

- `__is_literal` (Clang): Synonym for `__is_literal_type`.

- `__is_literal_type` (C++, GNU, Microsoft): Note, the corresponding standard trait was deprecated in C++17 and removed in C++20.

- `__is_lvalue_reference` (C++, Embarcadero)

- `__is_member_object_pointer` (C++, Embarcadero)

- `__is_member_function_pointer` (C++, Embarcadero)

- `__is_member_pointer` (C++, Embarcadero)

- `__is_nothrow_assignable` (C++, MSVC 2013)

- `__is_nothrow_constructible` (C++, MSVC 2013)

- `__is_nothrow_destructible` (C++, MSVC 2013)

- `__is_object` (C++, Embarcadero)

- `__is_pod` (C++, GNU, Microsoft, Embarcadero): Note, the corresponding standard trait was deprecated in C++20.

- `__is_pointer` (C++, Embarcadero)

- `__is_pointer_interconvertible_base_of` (C++, GNU, Microsoft)

- `__is_polymorphic` (C++, GNU, Microsoft, Embarcadero)

- `__is_reference` (C++, Embarcadero)

- `__is_rvalue_reference` (C++, Embarcadero)

- `__is_same` (C++, Embarcadero)

- `__is_same_as` (GCC): Synonym for `__is_same`.

- `__is_scalar` (C++, Embarcadero)

- `__is_scoped_enum` (C++, GNU, Microsoft, Embarcadero)

- `__is_sealed` (Microsoft): Synonym for `__is_final`.

- `__is_signed` (C++, Embarcadero): Returns false for enumeration types, and returns true for floating-point types. Note, before Clang 10, returned true for enumeration types if the underlying type was signed, and returned false for floating-point types.

- `__is_standard_layout` (C++, GNU, Microsoft, Embarcadero)

- `__is_trivial` (C++, GNU, Microsoft, Embarcadero)

- `__is_trivially_assignable` (C++, GNU, Microsoft)

- `__is_trivially_constructible` (C++, GNU, Microsoft)

- `__is_trivially_copyable` (C++, GNU, Microsoft)

- `__is_trivially_destructible` (C++, MSVC 2013)

- `__is_trivially_relocatable` (Clang) (Deprecated, use `__builtin_is_cpp_trivially_relocatable` instead). Returns true if moving an object of the given type, and then destroying the source object, is known to be functionally equivalent to copying the underlying bytes and then dropping the source object on the floor. This is true of trivial types, C++26 relocatable types, and types which were made trivially relocatable via the `clang::trivial_abi` attribute. This trait is deprecated and should be replaced by `__builtin_is_cpp_trivially_relocatable`. Note however that it is generally unsafe to relocate a C++-relocatable type with `memcpy` or `memmove`; use `__builtin_trivially_relocate`.

- `__builtin_is_cpp_trivially_relocatable` (C++): Returns true if an object is trivially relocatable, as defined by the C++26 standard [meta.unary.prop]. Note that when relocating the caller code should ensure that if the object is polymorphic, the dynamic type is of the most derived type. Padding bytes should not be copied.

- `__builtin_is_replaceable` (C++): Returns true if an object is replaceable, as defined by the C++26 standard [meta.unary.prop].

- `__is_trivially_equality_comparable` (Clang): Returns true if comparing two objects of the provided type is known to be equivalent to comparing their object representations. Note that types containing padding bytes are never trivially equality comparable.

- `__is_unbounded_array` (C++, GNU, Microsoft, Embarcadero)

- `__is_union` (C++, GNU, Microsoft, Embarcadero)

- `__is_unsigned` (C++, Embarcadero): Returns false for enumeration types. Note, before Clang 13, returned true for enumeration types if the underlying type was unsigned.

- `__is_void` (C++, Embarcadero)

- `__is_volatile` (C++, Embarcadero)

- `__reference_binds_to_temporary(T, U)` (Clang): Determines whether a reference of type `T` bound to an expression of type `U` would bind to a materialized temporary object. If `T` is not a reference type the result is false. Note this trait will also return false when the initialization of `T` from `U` is ill-formed. Deprecated, use `__reference_constructs_from_temporary`.

- `__reference_constructs_from_temporary(T, U)` (C++) Returns true if a reference `T` can be direct-initialized from a temporary of type a non-cv-qualified `U`.

- `__reference_converts_from_temporary(T, U)` (C++)

    Returns true if a reference `T` can be copy-initialized from a temporary of type a non-cv-qualified `U`.

- `__underlying_type` (C++, GNU, Microsoft)

In addition, the following expression traits are supported:

- `__is_lvalue_expr(e)` (Embarcadero): Returns true if `e` is an lvalue expression. Deprecated, use `__is_lvalue_reference(decltype((e)))` instead.

- `__is_rvalue_expr(e)` (Embarcadero): Returns true if `e` is a prvalue expression. Deprecated, use `!__is_reference(decltype((e)))` instead.

There are multiple ways to detect support for a type trait `__X` in the compiler, depending on the oldest version of Clang you wish to support.

- From Clang 10 onwards, `__has_builtin(__X)` can be used.

- From Clang 6 onwards, `!__is_identifier(__X)` can be used.

- From Clang 3 onwards, `__has_feature(X)` can be used, but only supports the following traits:

  - `__has_nothrow_assign`

  - `__has_nothrow_copy`

  - `__has_nothrow_constructor`

  - `__has_trivial_assign`

  - `__has_trivial_copy`

  - `__has_trivial_constructor`

  - `__has_trivial_destructor`

  - `__has_virtual_destructor`

  - `__is_abstract`

  - `__is_base_of`

  - `__is_class`

  - `__is_constructible`

  - `__is_convertible_to`

  - `__is_empty`

  - `__is_enum`

  - `__is_final`

  - `__is_literal`

  - `__is_standard_layout`

  - `__is_pod`

  - `__is_polymorphic`

  - `__is_sealed`

  - `__is_trivial`

  - `__is_trivially_assignable`

  - `__is_trivially_constructible`

  - `__is_trivially_copyable`

  - `__is_union`

  - `__underlying_type`

A simplistic usage example as might be seen in standard C++ headers follows:

    #if __has_builtin(__is_convertible_to)
    template<typename From, typename To>
    struct is_convertible_to {
      static const bool value = __is_convertible_to(From, To);
    };
    #else
    // Emulate type trait for compatibility with other compilers.
    #endif

### __builtin_structured_binding_size (C++)

The `__builtin_structured_binding_size(T)` type trait returns the structured binding size ([dcl.struct.bind]) of type `T`

This is equivalent to the size of the pack `p` in `auto&& [...p] = declval<T&>();`. If the argument cannot be decomposed, `__builtin_structured_binding_size(T)` is not a valid expression (`__builtin_structured_binding_size` is SFINAE-friendly).

builtin arrays, builtin SIMD vectors, builtin complex types, tuple-like types, and decomposable class types are decomposable types.

A type is considered a valid tuple-like if `std::tuple_size_v<T>` is a valid expression, even if there is no valid `std::tuple_element` specialization or suitable `get` function for that type.

    template<std::size_t Idx, typename T>
    requires (Idx < __builtin_structured_binding_size(T))
    decltype(auto) constexpr get_binding(T&& obj) {
        auto && [...p] = std::forward<T>(obj);
        return p...[Idx];
    }
    struct S { int a = 0, b = 42; };
    static_assert(__builtin_structured_binding_size(S) == 2);
    static_assert(get_binding<1>(S{}) == 42);

## Blocks

The syntax and high level language feature description is in BlockLanguageSpec. Implementation and ABI details for the clang implementation are in Block-ABI-Apple.

Query for this feature with `__has_extension(blocks)`.

## ASM Goto with Output Constraints

Outputs may be used along any branches from the `asm goto` whether the branches are taken or not.

Query for this feature with `__has_extension(gnu_asm_goto_with_outputs)`.

Prior to clang-16, the output may only be used safely when the indirect branches are not taken. Query for this difference with `__has_extension(gnu_asm_goto_with_outputs_full)`.

When using tied-outputs (i.e. outputs that are inputs and outputs, not just outputs) with the +r constraint, there is a hidden input that’s created before the label, so numeric references to operands must account for that.

    int foo(int x) {
        // %0 and %1 both refer to x
        // %l2 refers to err
        asm goto("# %0 %1 %l2" : "+r"(x) : : : err);
        return x;
      err:
        return -1;
    }

This was changed to match GCC in clang-13; for better portability, symbolic references can be used instead of numeric references.

    int foo(int x) {
        asm goto("# %[x] %l[err]" : [x]"+r"(x) : : : err);
        return x;
      err:
        return -1;
    }

## ASM Goto versus Branch Target Enforcement

Some target architectures implement branch target enforcement, by requiring indirect (register-controlled) branch instructions to jump only to locations marked by a special instruction (such as AArch64 `bti`).

The assembler code inside an `asm goto` statement is expected not to use a branch instruction of that kind to transfer control to any of its destination labels. Therefore, using a label in an `asm goto` statement does not cause clang to put a `bti` or equivalent instruction at the label.

## Constexpr strings in GNU ASM statements

In C++11 mode (and greater), Clang supports specifying the template, constraints, and clobber strings with a parenthesized constant expression producing an object with the following member functions

    constexpr const char* data() const;
    constexpr size_t size() const;

such as `std::string`, `std::string_view`, `std::vector<char>`. This mechanism follow the same rules as `static_assert` messages in C++26, see `[dcl.pre]/p12`.

Query for this feature with `__has_extension(gnu_asm_constexpr_strings)`.

    int foo() {
       asm((std::string_view("nop")) ::: (std::string_view("memory")));
    }

## Objective-C Features

### Related result types

According to Cocoa conventions, Objective-C methods with certain names (”`init`”, “`alloc`”, etc.) always return objects that are an instance of the receiving class’s type. Such methods are said to have a “related result type”, meaning that a message send to one of these methods will have the same static type as an instance of the receiver class. For example, given the following classes:

    @interface NSObject
    + (id)alloc;
    - (id)init;
    @end

    @interface NSArray : NSObject
    @end

and this common initialization pattern

    NSArray *array = [[NSArray alloc] init];

the type of the expression `[NSArray alloc]` is `NSArray*` because `alloc` implicitly has a related result type. Similarly, the type of the expression `[[NSArray alloc] init]` is `NSArray*`, since `init` has a related result type and its receiver is known to have the type `NSArray *`. If neither `alloc` nor `init` had a related result type, the expressions would have had type `id`, as declared in the method signature.

A method with a related result type can be declared by using the type `instancetype` as its result type. `instancetype` is a contextual keyword that is only permitted in the result type of an Objective-C method, e.g.

    @interface A
    + (instancetype)constructAnA;
    @end

The related result type can also be inferred for some methods. To determine whether a method has an inferred related result type, the first word in the camel-case selector (e.g., “`init`” in “`initWithObjects`”) is considered, and the method will have a related result type if its return type is compatible with the type of its class and if:

- the first word is “`alloc`” or “`new`”, and the method is a class method, or

- the first word is “`autorelease`”, “`init`”, “`retain`”, or “`self`”, and the method is an instance method.

If a method with a related result type is overridden by a subclass method, the subclass method must also return a type that is compatible with the subclass type. For example:

    @interface NSString : NSObject
    - (NSUnrelated *)init; // incorrect usage: NSUnrelated is not NSString or a superclass of NSString
    @end

Related result types only affect the type of a message send or property access via the given method. In all other respects, a method with a related result type is treated the same way as method that returns `id`.

Use `__has_feature(objc_instancetype)` to determine whether the `instancetype` contextual keyword is available.

### Automatic reference counting

Clang provides support for automated reference counting in Objective-C, which eliminates the need for manual `retain`/`release`/`autorelease` message sends. There are three feature macros associated with automatic reference counting: `__has_feature(objc_arc)` indicates the availability of automated reference counting in general, while `__has_feature(objc_arc_weak)` indicates that automated reference counting also includes support for `__weak` pointers to Objective-C objects. `__has_feature(objc_arc_fields)` indicates that C structs are allowed to have fields that are pointers to Objective-C objects managed by automatic reference counting.

### Weak references

Clang supports ARC-style weak and unsafe references in Objective-C even outside of ARC mode. Weak references must be explicitly enabled with the `-fobjc-weak` option; use `__has_feature((objc_arc_weak))` to test whether they are enabled. Unsafe references are enabled unconditionally. ARC-style weak and unsafe references cannot be used when Objective-C garbage collection is enabled.

Except as noted below, the language rules for the `__weak` and `__unsafe_unretained` qualifiers (and the `weak` and `unsafe_unretained` property attributes) are just as laid out in the ARC specification. In particular, note that some classes do not support forming weak references to their instances, and note that special care must be taken when storing weak references in memory where initialization and deinitialization are outside the responsibility of the compiler (such as in `malloc`-ed memory).

Loading from a `__weak` variable always implicitly retains the loaded value. In non-ARC modes, this retain is normally balanced by an implicit autorelease. This autorelease can be suppressed by performing the load in the receiver position of a `-retain` message send (e.g. `[weakReference retain]`); note that this performs only a single retain (the retain done when primitively loading from the weak reference).

For the most part, `__unsafe_unretained` in non-ARC modes is just the default behavior of variables and therefore is not needed. However, it does have an effect on the semantics of block captures: normally, copying a block which captures an Objective-C object or block pointer causes the captured pointer to be retained or copied, respectively, but that behavior is suppressed when the captured variable is qualified with `__unsafe_unretained`.

Note that the `__weak` qualifier formerly meant the GC qualifier in all non-ARC modes and was silently ignored outside of GC modes. It now means the ARC-style qualifier in all non-GC modes and is no longer allowed if not enabled by either `-fobjc-arc` or `-fobjc-weak`. It is expected that `-fobjc-weak` will eventually be enabled by default in all non-GC Objective-C modes.

### Enumerations with a fixed underlying type

Clang provides support for C++11 enumerations with a fixed underlying type within Objective-C and C prior to C23. For example, one can write an enumeration type as:

    typedef enum : unsigned char { Red, Green, Blue } Color;

This specifies that the underlying type, which is used to store the enumeration value, is `unsigned char`.

Use `__has_feature(objc_fixed_enum)` to determine whether support for fixed underlying types is available in Objective-C.

Use `__has_extension(c_fixed_enum)` to determine whether support for fixed underlying types is available in C prior to C23. This will also report `true` in C23 and later modes as the functionality is available even if it’s not an extension in those modes.

Use `__has_feature(c_fixed_enum)` to determine whether support for fixed underlying types is available in C23 and later.

### Interoperability with C++11 lambdas

Clang provides interoperability between C++11 lambdas and blocks-based APIs, by permitting a lambda to be implicitly converted to a block pointer with the corresponding signature. For example, consider an API such as `NSArray`’s array-sorting method:

    - (NSArray *)sortedArrayUsingComparator:(NSComparator)cmptr;

`NSComparator` is simply a typedef for the block pointer `NSComparisonResult (^)(id, id)`, and parameters of this type are generally provided with block literals as arguments. However, one can also use a C++11 lambda so long as it provides the same signature (in this case, accepting two parameters of type `id` and returning an `NSComparisonResult`):

    NSArray *array = @[@"string 1", @"string 21", @"string 12", @"String 11",
                       @"String 02"];
    const NSStringCompareOptions comparisonOptions
      = NSCaseInsensitiveSearch | NSNumericSearch |
        NSWidthInsensitiveSearch | NSForcedOrderingSearch;
    NSLocale *currentLocale = [NSLocale currentLocale];
    NSArray *sorted
      = [array sortedArrayUsingComparator:[=](id s1, id s2) -> NSComparisonResult {
                 NSRange string1Range = NSMakeRange(0, [s1 length]);
                 return [s1 compare:s2 options:comparisonOptions
                 range:string1Range locale:currentLocale];
         }];
    NSLog(@"sorted: %@", sorted);

This code relies on an implicit conversion from the type of the lambda expression (an unnamed, local class type called the closure type) to the corresponding block pointer type. The conversion itself is expressed by a conversion operator in that closure type that produces a block pointer with the same signature as the lambda itself, e.g.,

    operator NSComparisonResult (^)(id, id)() const;

This conversion function returns a new block that simply forwards the two parameters to the lambda object (which it captures by copy), then returns the result. The returned block is first copied (with `Block_copy`) and then autoreleased. As an optimization, if a lambda expression is immediately converted to a block pointer (as in the first example, above), then the block is not copied and autoreleased: rather, it is given the same lifetime as a block literal written at that point in the program, which avoids the overhead of copying a block to the heap in the common case.

The conversion from a lambda to a block pointer is only available in Objective-C++, and not in C++ with blocks, due to its use of Objective-C memory management (autorelease).

### Object Literals and Subscripting

Clang provides support for Object Literals and Subscripting in Objective-C, which simplifies common Objective-C programming patterns, makes programs more concise, and improves the safety of container creation. There are several feature macros associated with object literals and subscripting: `__has_feature(objc_array_literals)` tests the availability of array literals; `__has_feature(objc_dictionary_literals)` tests the availability of dictionary literals; `__has_feature(objc_subscripting)` tests the availability of object subscripting.

### Objective-C Autosynthesis of Properties

Clang provides support for autosynthesis of declared properties. Using this feature, clang provides default synthesis of those properties not declared @dynamic and not having user provided backing getter and setter methods. `__has_feature(objc_default_synthesize_properties)` checks for availability of this feature in version of clang being used.

### Objective-C retaining behavior attributes

In Objective-C, functions and methods are generally assumed to follow the Cocoa Memory Management conventions for ownership of object arguments and return values. However, there are exceptions, and so Clang provides attributes to allow these exceptions to be documented. This are used by ARC and the static analyzer Some exceptions may be better described using the `objc_method_family` attribute instead.

Usage: The `ns_returns_retained`, `ns_returns_not_retained`, `ns_returns_autoreleased`, `cf_returns_retained`, and `cf_returns_not_retained` attributes can be placed on methods and functions that return Objective-C or CoreFoundation objects. They are commonly placed at the end of a function prototype or method declaration:

    id foo() __attribute__((ns_returns_retained));

    - (NSString *)bar:(int)x __attribute__((ns_returns_retained));

The `*_returns_retained` attributes specify that the returned object has a +1 retain count. The `*_returns_not_retained` attributes specify that the return object has a +0 retain count, even if the normal convention for its selector would be +1. `ns_returns_autoreleased` specifies that the returned object is +0, but is guaranteed to live at least as long as the next flush of an autorelease pool.

Usage: The `ns_consumed` and `cf_consumed` attributes can be placed on a parameter declaration; they specify that the argument is expected to have a +1 retain count, which will be balanced in some way by the function or method. The `ns_consumes_self` attribute can only be placed on an Objective-C method; it specifies that the method expects its `self` parameter to have a +1 retain count, which it will balance in some way.

    void foo(__attribute__((ns_consumed)) NSString *string);

    - (void) bar __attribute__((ns_consumes_self));
    - (void) baz:(id) __attribute__((ns_consumed)) x;

Further examples of these attributes are available in the static analyzer’s list of annotations for analysis.

Query for these features with `__has_attribute(ns_consumed)`, `__has_attribute(ns_returns_retained)`, etc.

### Objective-C @available

It is possible to use the newest SDK but still build a program that can run on older versions of macOS and iOS by passing `-mmacos-version-min=` / `-miphoneos-version-min=`.

Before LLVM 5.0, when calling a function that exists only in the OS that’s newer than the target OS (as determined by the minimum deployment version), programmers had to carefully check if the function exists at runtime, using null checks for weakly-linked C functions, `+class` for Objective-C classes, and `-respondsToSelector:` or `+instancesRespondToSelector:` for Objective-C methods. If such a check was missed, the program would compile fine, run fine on newer systems, but crash on older systems.

As of LLVM 5.0, `-Wunguarded-availability` uses the availability attributes together with the new `@available()` keyword to assist with this issue. When a method that’s introduced in the OS newer than the target OS is called, a -Wunguarded-availability warning is emitted if that call is not guarded:

    void my_fun(NSSomeClass* var) {
      // If fancyNewMethod was added in e.g. macOS 10.12, but the code is
      // built with -mmacos-version-min=10.11, then this unconditional call
      // will emit a -Wunguarded-availability warning:
      [var fancyNewMethod];
    }

To fix the warning and to avoid the crash on macOS 10.11, wrap it in `if(@available())`:

    void my_fun(NSSomeClass* var) {
      if (@available(macOS 10.12, *)) {
        [var fancyNewMethod];
      } else {
        // Put fallback behavior for old macOS versions (and for non-mac
        // platforms) here.
      }
    }

The `*` is required and means that platforms not explicitly listed will take the true branch, and the compiler will emit `-Wunguarded-availability` warnings for unlisted platforms based on those platform’s deployment target. More than one platform can be listed in `@available()`:

    void my_fun(NSSomeClass* var) {
      if (@available(macOS 10.12, iOS 10, *)) {
        [var fancyNewMethod];
      }
    }

If the caller of `my_fun()` already checks that `my_fun()` is only called on 10.12, then add an availability attribute to it, which will also suppress the warning and require that calls to my_fun() are checked:

    API_AVAILABLE(macos(10.12)) void my_fun(NSSomeClass* var) {
      [var fancyNewMethod];  // Now ok.
    }

`@available()` is only available in Objective-C code. To use the feature in C and C++ code, use the `__builtin_available()` spelling instead.

If existing code uses null checks or `-respondsToSelector:`, it should be changed to use `@available()` (or `__builtin_available`) instead.

`-Wunguarded-availability` is disabled by default, but `-Wunguarded-availability-new`, which only emits this warning for APIs that have been introduced in macOS >= 10.13, iOS >= 11, watchOS >= 4 and tvOS >= 11, is enabled by default.

### Objective-C++ ABI: protocol-qualifier mangling of parameters

Starting with LLVM 3.4, Clang produces a new mangling for parameters whose type is a qualified-`id` (e.g., `id<Foo>`). This mangling allows such parameters to be differentiated from those with the regular unqualified `id` type.

This was a non-backward compatible mangling change to the ABI. This change allows proper overloading, and also prevents mangling conflicts with template parameters of protocol-qualified type.

Query the presence of this new mangling with `__has_feature(objc_protocol_qualifier_mangling)`.

## Initializer lists for complex numbers in C

clang supports an extension which allows the following in C:

    #include <math.h>
    #include <complex.h>
    complex float x = { 1.0f, INFINITY }; // Init to (1, Inf)

This construct is useful because there is no way to separately initialize the real and imaginary parts of a complex variable in standard C, given that clang does not support `_Imaginary`. (Clang also supports the `__real__` and `__imag__` extensions from gcc, which help in some cases, but are not usable in static initializers.)

Note that this extension does not allow eliding the braces; the meaning of the following two lines is different:

    complex float x[] = { { 1.0f, 1.0f } }; // [0] = (1, 1)
    complex float x[] = { 1.0f, 1.0f }; // [0] = (1, 0), [1] = (1, 0)

This extension also works in C++ mode, as far as that goes, but does not apply to the C++ `std::complex`. (In C++11, list initialization allows the same syntax to be used with `std::complex` with the same meaning.)

For GCC compatibility, `__builtin_complex(re, im)` can also be used to construct a complex number from the given real and imaginary components.

## OpenCL Features

Clang supports internal OpenCL extensions documented below.

### `__cl_clang_bitfields`

With this extension it is possible to enable bitfields in structs or unions using the OpenCL extension pragma mechanism detailed in the OpenCL Extension Specification, section 1.2.

Use of bitfields in OpenCL kernels can result in reduced portability as struct layout is not guaranteed to be consistent when compiled by different compilers. If structs with bitfields are used as kernel function parameters, it can result in incorrect functionality when the layout is different between the host and device code.

Example of Use:

    #pragma OPENCL EXTENSION __cl_clang_bitfields : enable
    struct with_bitfield {
      unsigned int i : 5; // compiled - no diagnostic generated
    };

    #pragma OPENCL EXTENSION __cl_clang_bitfields : disable
    struct without_bitfield {
      unsigned int i : 5; // error - bitfields are not supported
    };

### `__cl_clang_function_pointers`

With this extension it is possible to enable various language features that are relying on function pointers using regular OpenCL extension pragma mechanism detailed in the OpenCL Extension Specification, section 1.2.

In C++ for OpenCL this also enables:

- Use of member function pointers;

- Unrestricted use of references to functions;

- Virtual member functions.

Such functionality is not conformant and does not guarantee to compile correctly in any circumstances. It can be used if:

- the kernel source does not contain call expressions to (member-) function pointers, or virtual functions. For example this extension can be used in metaprogramming algorithms to be able to specify/detect types generically.

- the generated kernel binary does not contain indirect calls because they are eliminated using compiler optimizations e.g. devirtualization.

- the selected target supports the function pointer like functionality e.g. most CPU targets.

Example of Use:

    #pragma OPENCL EXTENSION __cl_clang_function_pointers : enable
    void foo()
    {
      void (*fp)(); // compiled - no diagnostic generated
    }

    #pragma OPENCL EXTENSION __cl_clang_function_pointers : disable
    void bar()
    {
      void (*fp)(); // error - pointers to function are not allowed
    }

### `__cl_clang_variadic_functions`

With this extension it is possible to enable variadic arguments in functions using regular OpenCL extension pragma mechanism detailed in the OpenCL Extension Specification, section 1.2.

This is not conformant behavior and it can only be used portably when the functions with variadic prototypes do not get generated in binary e.g. the variadic prototype is used to specify a function type with any number of arguments in metaprogramming algorithms in C++ for OpenCL.

This extensions can also be used when the kernel code is intended for targets supporting the variadic arguments e.g. majority of CPU targets.

Example of Use:

    #pragma OPENCL EXTENSION __cl_clang_variadic_functions : enable
    void foo(int a, ...); // compiled - no diagnostic generated

    #pragma OPENCL EXTENSION __cl_clang_variadic_functions : disable
    void bar(int a, ...); // error - variadic prototype is not allowed

### `__cl_clang_non_portable_kernel_param_types`

With this extension it is possible to enable the use of some restricted types in kernel parameters specified in C++ for OpenCL v1.0 s2.4. The restrictions can be relaxed using regular OpenCL extension pragma mechanism detailed in the OpenCL Extension Specification, section 1.2.

This is not a conformant behavior and it can only be used when the kernel arguments are not accessed on the host side or the data layout/size between the host and device is known to be compatible.

Example of Use:

    // Plain Old Data type.
    struct Pod {
      int a;
      int b;
    };

    // Not POD type because of the constructor.
    // Standard layout type because there is only one access control.
    struct OnlySL {
      int a;
      int b;
      OnlySL() : a(0), b(0) {}
    };

    // Not standard layout type because of two different access controls.
    struct NotSL {
      int a;
    private:
      int b;
    };

    #pragma OPENCL EXTENSION __cl_clang_non_portable_kernel_param_types : enable
    kernel void kernel_main(
      Pod a,

      OnlySL b,
      global NotSL *c,
      global OnlySL *d
    );
    #pragma OPENCL EXTENSION __cl_clang_non_portable_kernel_param_types : disable

### Remove address space builtin function

`__remove_address_space` allows to derive types in C++ for OpenCL that have address space qualifiers removed. This utility only affects address space qualifiers, therefore, other type qualifiers such as `const` or `volatile` remain unchanged.

Example of Use:

    template<typename T>
    void foo(T *par){
      T var1; // error - local function variable with global address space
      __private T var2; // error - conflicting address space qualifiers
      __private __remove_address_space<T>::type var3; // var3 is __private int
    }

    void bar(){
      __global int* ptr;
      foo(ptr);
    }

### Legacy 1.x atomics with generic address space

Clang allows use of atomic functions from the OpenCL 1.x standards with the generic address space pointer in C++ for OpenCL mode.

This is a non-portable feature and might not be supported by all targets.

Example of Use:

    void foo(__generic volatile unsigned int* a) {
      atomic_add(a, 1);
    }

## WebAssembly Features

Clang supports the WebAssembly features documented below. For further information related to the semantics of the builtins, please refer to the WebAssembly Specification. In this section, when we refer to reference types, we are referring to WebAssembly reference types, not C++ reference types unless stated otherwise.

### `__builtin_wasm_table_set`

This builtin function stores a value in a WebAssembly table. It takes three arguments. The first argument is the table to store a value into, the second argument is the index to which to store the value into, and the third argument is a value of reference type to store in the table. It returns nothing.

    static __externref_t table[0];
    extern __externref_t JSObj;

    void store(int index) {
      __builtin_wasm_table_set(table, index, JSObj);
    }

### `__builtin_wasm_table_get`

This builtin function is the counterpart to `__builtin_wasm_table_set` and loads a value from a WebAssembly table of reference typed values. It takes 2 arguments. The first argument is a table of reference typed values and the second argument is an index from which to load the value. It returns the loaded reference typed value.

    static __externref_t table[0];

    __externref_t load(int index) {
      __externref_t Obj = __builtin_wasm_table_get(table, index);
      return Obj;
    }

### `__builtin_wasm_table_size`

This builtin function returns the size of the WebAssembly table. Takes the table as an argument and returns an unsigned integer (`size_t`) with the current table size.

    typedef void (*__funcref funcref_t)();
    static funcref_t table[0];

    size_t getSize() {
      return __builtin_wasm_table_size(table);
    }

### `__builtin_wasm_table_grow`

This builtin function grows the WebAssembly table by a certain amount. Currently, as all WebAssembly tables created in C/C++ are zero-sized, this always needs to be called to grow the table.

It takes three arguments. The first argument is the WebAssembly table to grow. The second argument is the reference typed value to store in the new table entries (the initialization value), and the third argument is the amount to grow the table by. It returns the previous table size or -1. It will return -1 if not enough space could be allocated.

    typedef void (*__funcref funcref_t)();
    static funcref_t table[0];

    // grow returns the new table size or -1 on error.
    int grow(funcref_t fn, int delta) {
      int prevSize = __builtin_wasm_table_grow(table, fn, delta);
      if (prevSize == -1)
        return -1;
      return prevSize + delta;
    }

### `__builtin_wasm_table_fill`

This builtin function sets all the entries of a WebAssembly table to a given reference typed value. It takes four arguments. The first argument is the WebAssembly table, the second argument is the index that starts the range, the third argument is the value to set in the new entries, and the fourth and the last argument is the size of the range. It returns nothing.

    static __externref_t table[0];

    // resets a table by setting all of its entries to a given value.
    void reset(__externref_t Obj) {
      int Size = __builtin_wasm_table_size(table);
      __builtin_wasm_table_fill(table, 0, Obj, Size);
    }

### `__builtin_wasm_table_copy`

This builtin function copies elements from a source WebAssembly table to a possibly overlapping destination region. It takes five arguments. The first argument is the destination WebAssembly table, and the second argument is the source WebAssembly table. The third argument is the destination index from where the copy starts, the fourth argument is the source index from there the copy starts, and the fifth and last argument is the number of elements to copy. It returns nothing.

    static __externref_t tableSrc[0];
    static __externref_t tableDst[0];

    // Copy nelem elements from [src, src + nelem - 1] in tableSrc to
    // [dst, dst + nelem - 1] in tableDst
    void copy(int dst, int src, int nelem) {
      __builtin_wasm_table_copy(tableDst, tableSrc, dst, src, nelem);
    }

## Builtin Functions

Clang supports a number of builtin library functions with the same syntax as GCC, including things like `__builtin_nan`, `__builtin_constant_p`, `__builtin_choose_expr`, `__builtin_types_compatible_p`, `__builtin_assume_aligned`, `__sync_fetch_and_add`, etc. In addition to the GCC builtins, Clang supports a number of builtins that GCC does not, which are listed here.

Please note that Clang does not and will not support all of the GCC builtins for vector operations. Instead of using builtins, you should use the functions defined in target-specific header files like `<xmmintrin.h>`, which define portable wrappers for these. Many of the Clang versions of these functions are implemented directly in terms of extended vector support instead of builtins, in order to reduce the number of builtins that we need to implement.

### `__builtin_alloca`

`__builtin_alloca` is used to dynamically allocate memory on the stack. Memory is automatically freed upon function termination.

Syntax:

    __builtin_alloca(size_t n)

Example of Use:

    void init(float* data, size_t nbelems);
    void process(float* data, size_t nbelems);
    int foo(size_t n) {
      auto mem = (float*)__builtin_alloca(n * sizeof(float));
      init(mem, n);
      process(mem, n);
      /* mem is automatically freed at this point */
    }

Description:

`__builtin_alloca` is meant to be used to allocate a dynamic amount of memory on the stack. This amount is subject to stack allocation limits.

Query for this feature with `__has_builtin(__builtin_alloca)`.

### `__builtin_alloca_with_align`

`__builtin_alloca_with_align` is used to dynamically allocate memory on the stack while controlling its alignment. Memory is automatically freed upon function termination.

Syntax:

    __builtin_alloca_with_align(size_t n, size_t align)

Example of Use:

    void init(float* data, size_t nbelems);
    void process(float* data, size_t nbelems);
    int foo(size_t n) {
      auto mem = (float*)__builtin_alloca_with_align(
                          n * sizeof(float),
                          CHAR_BIT * alignof(float));
      init(mem, n);
      process(mem, n);
      /* mem is automatically freed at this point */
    }

Description:

`__builtin_alloca_with_align` is meant to be used to allocate a dynamic amount of memory on the stack. It is similar to `__builtin_alloca` but accepts a second argument whose value is the alignment constraint, as a power of 2 in bits.

Query for this feature with `__has_builtin(__builtin_alloca_with_align)`.

### `__builtin_assume`

`__builtin_assume` is used to provide the optimizer with a boolean invariant that is defined to be true.

Syntax:

    __builtin_assume(bool)

Example of Use:

    int foo(int x) {
        __builtin_assume(x != 0);
        // The optimizer may short-circuit this check using the invariant.
        if (x == 0)
              return do_something();
        return do_something_else();
    }

Description:

The boolean argument to this function is defined to be true. The optimizer may analyze the form of the expression provided as the argument and deduce from that information used to optimize the program. If the condition is violated during execution, the behavior is undefined. The argument itself is never evaluated, so any side effects of the expression will be discarded.

Query for this feature with `__has_builtin(__builtin_assume)`.

### `__builtin_assume_separate_storage`

`__builtin_assume_separate_storage` is used to provide the optimizer with the knowledge that its two arguments point to separately allocated objects.

Syntax:

    __builtin_assume_separate_storage(const volatile void *, const volatile void *)

Example of Use:

    int foo(int *x, int *y) {
        __builtin_assume_separate_storage(x, y);
        *x = 0;
        *y = 1;
        // The optimizer may optimize this to return 0 without reloading from *x.
        return *x;
    }

Description:

The arguments to this function are assumed to point into separately allocated storage (either different variable definitions or different dynamic storage allocations). The optimizer may use this fact to aid in alias analysis. If the arguments point into the same storage, the behavior is undefined. Note that the definition of “storage” here refers to the outermost enclosing allocation of any particular object (so for example, it’s never correct to call this function passing the addresses of fields in the same struct, elements of the same array, etc.).

Query for this feature with `__has_builtin(__builtin_assume_separate_storage)`.

### `__builtin_assume_dereferenceable`

`__builtin_assume_dereferenceable` is used to provide the optimizer with the knowledge that the pointer argument P is dereferenceable up to at least the specified number of bytes.

Syntax:

    __builtin_assume_dereferenceable(const void *, size_t)

Example of Use:

    int foo(int *x, int y) {
        __builtin_assume_dereferenceable(x, sizeof(int));
        int z = 0;
        if (y == 1) {
          // The optimizer may execute the load of x unconditionally due to
          // __builtin_assume_dereferenceable guaranteeing sizeof(int) bytes can
          // be loaded speculatively without trapping.
          z = *x;
        }
        return z;
    }

Description:

The arguments to this function provide a start pointer `P` and a size `S`. `S` must be at least 1 and a constant. The optimizer may assume that `S` bytes are dereferenceable starting at `P`. Note that this does not necessarily imply that `P` is non-null as `nullptr` can be dereferenced in some cases. The assumption also does not imply that `P` is not dereferenceable past `S` bytes.

Query for this feature with `__has_builtin(__builtin_assume_dereferenceable)`.

### `__builtin_offsetof`

`__builtin_offsetof` is used to implement the `offsetof` macro, which calculates the offset (in bytes) to a given member of the given type.

Syntax:

    __builtin_offsetof(type-name, member-designator)

Example of Use:

    struct S {
      char c;
      int i;
      struct T {
        float f[2];
      } t;
    };

    const int offset_to_i = __builtin_offsetof(struct S, i);
    const int ext1 = __builtin_offsetof(struct U { int i; }, i); // C extension
    const int offset_to_subobject = __builtin_offsetof(struct S, t.f[1]);

Description:

This builtin is usable in an integer constant expression which returns a value of type `size_t`. The value returned is the offset in bytes to the subobject designated by the member-designator from the beginning of an object of type `type-name`. Clang extends the required standard functionality in the following way:

- In C language modes, the first argument may be the definition of a new type. Any type declared this way is scoped to the nearest scope containing the call to the builtin.

Query for this feature with `__has_builtin(__builtin_offsetof)`.

### `__builtin_get_vtable_pointer`

`__builtin_get_vtable_pointer` loads and authenticates the primary vtable pointer from an instance of a polymorphic C++ class. This builtin is needed for directly loading the vtable pointer when on platforms using Pointer Authentication.

Syntax:

    __builtin_get_vtable_pointer(PolymorphicClass*)

Example of Use:

    struct PolymorphicClass {
      virtual ~PolymorphicClass();
    };

    PolymorphicClass anInstance;
    const void* vtablePointer = __builtin_get_vtable_pointer(&anInstance);

Description:

The `__builtin_get_vtable_pointer` builtin loads the primary vtable pointer from a polymorphic C++ type. If the target platform authenticates vtable pointers, this builtin will perform the authentication and produce the underlying raw pointer. The object being queried must be polymorphic, and so must also be a complete type.

Query for this feature with `__has_builtin(__builtin_get_vtable_pointer)`.

### `__builtin_call_with_static_chain`

`__builtin_call_with_static_chain` is used to perform a static call while setting updating the static chain register.

Syntax:

    T __builtin_call_with_static_chain(T expr, void* ptr)

Example of Use:

    auto v = __builtin_call_with_static_chain(foo(3), foo);

Description:

This builtin returns `expr` after checking that `expr` is a non-member static call expression. The call to that expression is made while using `ptr` as a function pointer stored in a dedicated register to implement static chain calling convention, as used by some language to implement closures or nested functions.

Query for this feature with `__has_builtin(__builtin_call_with_static_chain)`.

### `__builtin_readcyclecounter`

`__builtin_readcyclecounter` is used to access the cycle counter register (or a similar low-latency, high-accuracy clock) on those targets that support it.

Syntax:

    __builtin_readcyclecounter()

Example of Use:

    unsigned long long t0 = __builtin_readcyclecounter();
    do_something();
    unsigned long long t1 = __builtin_readcyclecounter();
    unsigned long long cycles_to_do_something = t1 - t0; // assuming no overflow

Description:

The `__builtin_readcyclecounter()` builtin returns the cycle counter value, which may be either global or process/thread-specific depending on the target. As the backing counters often overflow quickly (on the order of seconds) this should only be used for timing small intervals. When not supported by the target, the return value is always zero. This builtin takes no arguments and produces an unsigned long long result.

Query for this feature with `__has_builtin(__builtin_readcyclecounter)`. Note that even if present, its use may depend on run-time privilege or other OS controlled state.

### `__builtin_readsteadycounter`

`__builtin_readsteadycounter` is used to access the fixed frequency counter register (or a similar steady-rate clock) on those targets that support it. The function is similar to `__builtin_readcyclecounter` above except that the frequency is fixed, making it suitable for measuring elapsed time.

Syntax:

    __builtin_readsteadycounter()

Example of Use:

    unsigned long long t0 = __builtin_readsteadycounter();
    do_something();
    unsigned long long t1 = __builtin_readsteadycounter();
    unsigned long long secs_to_do_something = (t1 - t0) / tick_rate;

Description:

The `__builtin_readsteadycounter()` builtin returns the frequency counter value. When not supported by the target, the return value is always zero. This builtin takes no arguments and produces an unsigned long long result. The builtin does not guarantee any particular frequency, only that it is stable. Knowledge of the counter’s true frequency will need to be provided by the user.

Query for this feature with `__has_builtin(__builtin_readsteadycounter)`.

### `__builtin_cpu_supports`

Syntax:

    int __builtin_cpu_supports(const char *features);

Example of Use::

    if (__builtin_cpu_supports("sve"))
      sve_code();

Description:

The `__builtin_cpu_supports` function detects if the run-time CPU supports features specified in string argument. It returns a positive integer if all features are supported and 0 otherwise. Feature names are target specific. On AArch64 features are combined using `+` like this `__builtin_cpu_supports("flagm+sha3+lse+rcpc2+fcma+memtag+bti+sme2")`. If a feature name is not supported, Clang will issue a warning and replace builtin by the constant 0.

Query for this feature with `__has_builtin(__builtin_cpu_supports)`.

### `__builtin_dump_struct`

Syntax:

    __builtin_dump_struct(&some_struct, some_printf_func, args...);

Examples:

    struct S {
      int x, y;
      float f;
      struct T {
        int i;
      } t;
    };

    void func(struct S *s) {
      __builtin_dump_struct(s, printf);
    }

Example output:

    struct S {
      int x = 100
      int y = 42
      float f = 3.141593
      struct T t = {
        int i = 1997
      }
    }

    #include <string>
    struct T { int a, b; };
    constexpr void constexpr_sprintf(std::string &out, const char *format,
                                     auto ...args) {
      // ...
    }
    constexpr std::string dump_struct(auto &x) {
      std::string s;
      __builtin_dump_struct(&x, constexpr_sprintf, s);
      return s;
    }
    static_assert(dump_struct(T{1, 2}) == R"(struct T {
      int a = 1
      int b = 2
    }
    )");

Description:

The `__builtin_dump_struct` function is used to print the fields of a simple structure and their values for debugging purposes. The first argument of the builtin should be a pointer to a complete record type to dump. The second argument `f` should be some callable expression, and can be a function object or an overload set. The builtin calls `f`, passing any further arguments `args...` followed by a `printf`-compatible format string and the corresponding arguments. `f` may be called more than once, and `f` and `args` will be evaluated once per call. In C++, `f` may be a template or overload set and resolve to different functions for each call.

In the format string, a suitable format specifier will be used for builtin types that Clang knows how to format. This includes standard builtin types, as well as aggregate structures, `void*` (printed with `%p`), and `const char*` (printed with `%s`). A `*%p` specifier will be used for a field that Clang doesn’t know how to format, and the corresponding argument will be a pointer to the field. This allows a C++ templated formatting function to detect this case and implement custom formatting. A `*` will otherwise not precede a format specifier.

This builtin does not return a value.

This builtin can be used in constant expressions.

Query for this feature with `__has_builtin(__builtin_dump_struct)`

### `__builtin_shufflevector`

`__builtin_shufflevector` is used to express generic vector permutation/shuffle/swizzle operations. This builtin is also very important for the implementation of various target-specific header files like `<xmmintrin.h>`. This builtin can be used within constant expressions.

Syntax:

    __builtin_shufflevector(vec1, vec2, index1, index2, ...)

Examples:

    // identity operation - return 4-element vector v1.
    __builtin_shufflevector(v1, v1, 0, 1, 2, 3)

    // "Splat" element 0 of V1 into a 4-element result.
    __builtin_shufflevector(V1, V1, 0, 0, 0, 0)

    // Reverse 4-element vector V1.
    __builtin_shufflevector(V1, V1, 3, 2, 1, 0)

    // Concatenate every other element of 4-element vectors V1 and V2.
    __builtin_shufflevector(V1, V2, 0, 2, 4, 6)

    // Concatenate every other element of 8-element vectors V1 and V2.
    __builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)

    // Shuffle v1 with some elements being undefined. Not allowed in constexpr.
    __builtin_shufflevector(v1, v1, 3, -1, 1, -1)

Description:

The first two arguments to `__builtin_shufflevector` are vectors that have the same element type. The remaining arguments are a list of integers that specify the elements indices of the first two vectors that should be extracted and returned in a new vector. These element indices are numbered sequentially starting with the first vector, continuing into the second vector. Thus, if `vec1` is a 4-element vector, index 5 would refer to the second element of `vec2`. An index of -1 can be used to indicate that the corresponding element in the returned vector is a don’t care and can be optimized by the backend. Values of -1 are not supported in constant expressions.

The result of `__builtin_shufflevector` is a vector with the same element type as `vec1`/`vec2` but that has an element count equal to the number of indices specified.

Query for this feature with `__has_builtin(__builtin_shufflevector)`.

### `__builtin_convertvector`

`__builtin_convertvector` is used to express generic vector type-conversion operations. The input vector and the output vector type must have the same number of elements. This builtin can be used within constant expressions.

Syntax:

    __builtin_convertvector(src_vec, dst_vec_type)

Examples:

    typedef double vector4double __attribute__((__vector_size__(32)));
    typedef float  vector4float  __attribute__((__vector_size__(16)));
    typedef short  vector4short  __attribute__((__vector_size__(8)));
    vector4float vf; vector4short vs;

    // convert from a vector of 4 floats to a vector of 4 doubles.
    __builtin_convertvector(vf, vector4double)
    // equivalent to:
    (vector4double) { (double) vf[0], (double) vf[1], (double) vf[2], (double) vf[3] }

    // convert from a vector of 4 shorts to a vector of 4 floats.
    __builtin_convertvector(vs, vector4float)
    // equivalent to:
    (vector4float) { (float) vs[0], (float) vs[1], (float) vs[2], (float) vs[3] }

Description:

The first argument to `__builtin_convertvector` is a vector, and the second argument is a vector type with the same number of elements as the first argument.

The result of `__builtin_convertvector` is a vector with the same element type as the second argument, with a value defined in terms of the action of a C-style cast applied to each element of the first argument.

Query for this feature with `__has_builtin(__builtin_convertvector)`.

### `__builtin_bitreverse`

- `__builtin_bitreverse8`

- `__builtin_bitreverse16`

- `__builtin_bitreverse32`

- `__builtin_bitreverse64`

Syntax:

    __builtin_bitreverse32(x)

Examples:

    uint8_t rev_x = __builtin_bitreverse8(x);
    uint16_t rev_x = __builtin_bitreverse16(x);
    uint32_t rev_y = __builtin_bitreverse32(y);
    uint64_t rev_z = __builtin_bitreverse64(z);

Description:

The ‘`__builtin_bitreverse`’ family of builtins is used to reverse the bitpattern of an integer value; for example `0b10110110` becomes `0b01101101`. These builtins can be used within constant expressions.

### `__builtin_rotateleft`

- `__builtin_rotateleft8`

- `__builtin_rotateleft16`

- `__builtin_rotateleft32`

- `__builtin_rotateleft64`

Syntax:

    __builtin_rotateleft32(x, y)

Examples:

    uint8_t rot_x = __builtin_rotateleft8(x, y);
    uint16_t rot_x = __builtin_rotateleft16(x, y);
    uint32_t rot_x = __builtin_rotateleft32(x, y);
    uint64_t rot_x = __builtin_rotateleft64(x, y);

Description:

The ‘`__builtin_rotateleft`’ family of builtins is used to rotate the bits in the first argument by the amount in the second argument. For example, `0b10000110` rotated left by 11 becomes `0b00110100`. The shift value is treated as an unsigned amount modulo the size of the arguments. Both arguments and the result have the bitwidth specified by the name of the builtin. These builtins can be used within constant expressions.

### `__builtin_rotateright`

- `__builtin_rotateright8`

- `__builtin_rotateright16`

- `__builtin_rotateright32`

- `__builtin_rotateright64`

Syntax:

    __builtin_rotateright32(x, y)

Examples:

    uint8_t rot_x = __builtin_rotateright8(x, y);
    uint16_t rot_x = __builtin_rotateright16(x, y);
    uint32_t rot_x = __builtin_rotateright32(x, y);
    uint64_t rot_x = __builtin_rotateright64(x, y);

Description:

The ‘`__builtin_rotateright`’ family of builtins is used to rotate the bits in the first argument by the amount in the second argument. For example, `0b10000110` rotated right by 3 becomes `0b11010000`. The shift value is treated as an unsigned amount modulo the size of the arguments. Both arguments and the result have the bitwidth specified by the name of the builtin. These builtins can be used within constant expressions.

### `__builtin_unreachable`

`__builtin_unreachable` is used to indicate that a specific point in the program cannot be reached, even if the compiler might otherwise think it can. This is useful to improve optimization and eliminates certain warnings. For example, without the `__builtin_unreachable` in the example below, the compiler assumes that the inline asm can fall through and prints a “function declared ‘`noreturn`’ should not return” warning.

Syntax:

    __builtin_unreachable()

Example of use:

    void myabort(void) __attribute__((noreturn));
    void myabort(void) {
      asm("int3");
      __builtin_unreachable();
    }

Description:

The `__builtin_unreachable()` builtin has completely undefined behavior. Since it has undefined behavior, it is a statement that it is never reached and the optimizer can take advantage of this to produce better code. This builtin takes no arguments and produces a void result.

Query for this feature with `__has_builtin(__builtin_unreachable)`.

### `__builtin_unpredictable`

`__builtin_unpredictable` is used to indicate that a branch condition is unpredictable by hardware mechanisms such as branch prediction logic.

Syntax:

    __builtin_unpredictable(long long)

Example of use:

    if (__builtin_unpredictable(x > 0)) {
       foo();
    }

Description:

The `__builtin_unpredictable()` builtin is expected to be used with control flow conditions such as in `if` and `switch` statements.

Query for this feature with `__has_builtin(__builtin_unpredictable)`.

### `__builtin_expect`

`__builtin_expect` is used to indicate that the value of an expression is anticipated to be the same as a statically known result.

Syntax:

    long __builtin_expect(long expr, long val)

Example of use:

    if (__builtin_expect(x, 0)) {
       bar();
    }

Description:

The `__builtin_expect()` builtin is typically used with control flow conditions such as in `if` and `switch` statements to help branch prediction. It means that its first argument `expr` is expected to take the value of its second argument `val`. It always returns `expr`.

Query for this feature with `__has_builtin(__builtin_expect)`.

### `__builtin_expect_with_probability`

`__builtin_expect_with_probability` is similar to `__builtin_expect` but it takes a probability as third argument.

Syntax:

    long __builtin_expect_with_probability(long expr, long val, double p)

Example of use:

    if (__builtin_expect_with_probability(x, 0, .3)) {
       bar();
    }

Description:

The `__builtin_expect_with_probability()` builtin is typically used with control flow conditions such as in `if` and `switch` statements to help branch prediction. It means that its first argument `expr` is expected to take the value of its second argument `val` with probability `p`. `p` must be within `[0.0 ; 1.0]` bounds. This builtin always returns the value of `expr`.

Query for this feature with `__has_builtin(__builtin_expect_with_probability)`.

### `__builtin_prefetch`

`__builtin_prefetch` is used to communicate with the cache handler to bring data into the cache before it gets used.

Syntax:

    void __builtin_prefetch(const void *addr, int rw=0, int locality=3)

Example of use:

    __builtin_prefetch(a + i);

Description:

The `__builtin_prefetch(addr, rw, locality)` builtin is expected to be used to avoid cache misses when the developer has a good understanding of which data are going to be used next. `addr` is the address that needs to be brought into the cache. `rw` indicates the expected access mode: `0` for read and `1` for write. In case of read write access, `1` is to be used. `locality` indicates the expected persistence of data in cache, from `0` which means that data can be discarded from cache after its next use to `3` which means that data is going to be reused a lot once in cache. `1` and `2` provide intermediate behavior between these two extremes.

Query for this feature with `__has_builtin(__builtin_prefetch)`.

### `__sync_swap`

`__sync_swap` is used to atomically swap integers or pointers in memory.

Syntax:

    type __sync_swap(type *ptr, type value, ...)

Example of Use:

    int old_value = __sync_swap(&value, new_value);

Description:

The `__sync_swap()` builtin extends the existing `__sync_*()` family of atomic intrinsics to allow code to atomically swap the current value with the new value. More importantly, it helps developers write more efficient and correct code by avoiding expensive loops around `__sync_bool_compare_and_swap()` or relying on the platform specific implementation details of `__sync_lock_test_and_set()`. The `__sync_swap()` builtin is a full barrier.

### `__builtin_addressof`

`__builtin_addressof` performs the functionality of the built-in `&` operator, ignoring any `operator&` overload. This is useful in constant expressions in C++11, where there is no other way to take the address of an object that overloads `operator&`. Clang automatically adds `[[clang::lifetimebound]]` to the parameter of `__builtin_addressof`.

Example of use:

    template<typename T> constexpr T *addressof(T &value) {
      return __builtin_addressof(value);
    }

### `__builtin_function_start`

`__builtin_function_start` returns the address of a function body.

Syntax:

    void *__builtin_function_start(function)

Example of use:

    void a() {}
    void *p = __builtin_function_start(a);

    class A {
    public:
      void a(int n);
      void a();
    };

    void A::a(int n) {}
    void A::a() {}

    void *pa1 = __builtin_function_start((void(A::*)(int)) &A::a);
    void *pa2 = __builtin_function_start((void(A::*)()) &A::a);

Description:

The `__builtin_function_start` builtin accepts an argument that can be constant-evaluated to a function, and returns the address of the function body. This builtin is not supported on all targets.

The returned pointer may differ from the normally taken function address and is not safe to call. For example, with `-fsanitize=cfi`, taking a function address produces a callable pointer to a CFI jump table, while `__builtin_function_start` returns an address that fails cfi-icall checks.

### `__builtin_operator_new` and `__builtin_operator_delete`

A call to `__builtin_operator_new(args)` is exactly the same as a call to `::operator new(args)`, except that it allows certain optimizations that the C++ standard does not permit for a direct function call to `::operator new` (in particular, removing `new` / `delete` pairs and merging allocations), and that the call is required to resolve to a replaceable global allocation function.

Likewise, `__builtin_operator_delete` is exactly the same as a call to `::operator delete(args)`, except that it permits optimizations and that the call is required to resolve to a replaceable global deallocation function.

These builtins are intended for use in the implementation of `std::allocator` and other similar allocation libraries, and are only available in C++.

Query for this feature with `__has_builtin(__builtin_operator_new)` or `__has_builtin(__builtin_operator_delete)`:

> - If the value is at least `201802L`, the builtins behave as described above.

> - If the value is non-zero, the builtins may not support calling arbitrary replaceable global (de)allocation functions, but do support calling at least `::operator new(size_t)` and `::operator delete(void*)`.

### `__builtin_trivially_relocate`

Syntax:

    T* __builtin_trivially_relocate(T* dest, T* src, size_t count)

Trivially relocates `count` objects of relocatable, complete type `T` from `src` to `dest` and returns `dest`. This builtin is used to implement `std::trivially_relocate`.

### `__builtin_invoke`

Syntax:

    template <class Callee, class... Args>
    decltype(auto) __builtin_invoke(Callee&& callee, Args&&... args);

`__builtin_invoke` is equivalent to `std::invoke`.

### `__builtin_preserve_access_index`

`__builtin_preserve_access_index` specifies a code section where array subscript access and structure/union member access are relocatable under bpf compile-once run-everywhere framework. Debuginfo (typically with `-g`) is needed, otherwise, the compiler will exit with an error. The return type for the intrinsic is the same as the type of the argument.

Syntax:

    type __builtin_preserve_access_index(type arg)

Example of Use:

    struct t {
      int i;
      int j;
      union {
        int a;
        int b;
      } c[4];
    };
    struct t *v = ...;
    int *pb =__builtin_preserve_access_index(&v->c[3].b);
    __builtin_preserve_access_index(v->j);

### `__builtin_debugtrap`

`__builtin_debugtrap` causes the program to stop its execution in such a way that a debugger can catch it.

Syntax:

    __builtin_debugtrap()

Description

`__builtin_debugtrap` is lowered to the ` `llvm.debugtrap` <https://llvm.org/docs/LangRef.html#llvm-debugtrap-intrinsic>`_ builtin. It should have the same effect as setting a breakpoint on the line where the builtin is called.

Query for this feature with `__has_builtin(__builtin_debugtrap)`.

### `__builtin_trap`

`__builtin_trap` causes the program to stop its execution abnormally.

Syntax:

    __builtin_trap()

Description

`__builtin_trap` is lowered to the ` `llvm.trap` <https://llvm.org/docs/LangRef.html#llvm-trap-intrinsic>`_ builtin.

Query for this feature with `__has_builtin(__builtin_trap)`.

### `__builtin_arm_trap`

`__builtin_arm_trap` is an AArch64 extension to `__builtin_trap` which also accepts a compile-time constant value, encoded directly into the trap instruction for later inspection.

Syntax:

    __builtin_arm_trap(const unsigned short payload)

Description

`__builtin_arm_trap` is lowered to the `llvm.aarch64.break` builtin, and then to `brk #payload`.

### `__builtin_verbose_trap`

`__builtin_verbose_trap` causes the program to stop its execution abnormally and shows a human-readable description of the reason for the termination when a debugger is attached or in a symbolicated crash log.

Syntax:

    __builtin_verbose_trap(const char *category, const char *reason)

Description

`__builtin_verbose_trap` is lowered to the ` `llvm.trap` <https://llvm.org/docs/LangRef.html#llvm-trap-intrinsic>`_ builtin. Additionally, clang emits debugging information that represents an artificial inline frame whose name encodes the category and reason strings passed to the builtin, prefixed by a “magic” prefix.

For example, consider the following code:

    void foo(int* p) {
      if (p == nullptr)
        __builtin_verbose_trap("check null", "Argument must not be null!");
    }

The debugging information would look as if it were produced for the following code:

    __attribute__((always_inline))
    inline void "__clang_trap_msg$check null$Argument must not be null!"() {
      __builtin_trap();
    }

    void foo(int* p) {
      if (p == nullptr)
        "__clang_trap_msg$check null$Argument must not be null!"();
    }

However, the generated code would not actually contain a call to the artificial function — it only exists in the debugging information.

Query for this feature with `__has_builtin(__builtin_verbose_trap)`. Note that users need to enable debug information to enable this feature. A call to this builtin is equivalent to a call to `__builtin_trap` if debug information isn’t enabled.

The optimizer can merge calls to trap with different messages, which degrades the debugging experience.

### `__builtin_allow_runtime_check`

`__builtin_allow_runtime_check` returns true if the check at the current program location should be executed. It is expected to be used to implement `assert` like checks which can be safely removed by optimizer.

Syntax:

    bool __builtin_allow_runtime_check(const char* kind)

Example of use:

    if (__builtin_allow_runtime_check("mycheck") && !ExpensiveCheck()) {
       abort();
    }

Description

`__builtin_allow_runtime_check` is lowered to the llvm.allow.runtime.check intrinsic.

The `__builtin_allow_runtime_check()` can be used within control structures like `if` to guard expensive runtime checks. The return value is determined by the following compiler options and may differ per call site:

- `-mllvm -lower-allow-check-percentile-cutoff-hot=N`: Disable checks in hot code marked by the profile summary with a hotness cutoff in the range `[0, 999999]` (a larger N disables more checks).

- `-mllvm -lower-allow-check-random-rate=P`: Keep a check with probability P, a floating point number in the range `[0.0, 1.0]`.

- If both options are specified, a check is disabled if either condition is satisfied.

- If neither is specified, all checks are allowed.

Parameter `kind`, currently unused, is a string literal specifying the check kind. Future compiler versions may use this to allow for more granular control, such as applying different hotness cutoffs to different check kinds.

Query for this feature with `__has_builtin(__builtin_allow_runtime_check)`.

### `__builtin_nondeterministic_value`

`__builtin_nondeterministic_value` returns a valid nondeterministic value of the same type as the provided argument.

Syntax:

    type __builtin_nondeterministic_value(type x)

Examples:

    int x = __builtin_nondeterministic_value(x);
    float y = __builtin_nondeterministic_value(y);
    __m256i a = __builtin_nondeterministic_value(a);

Description

Each call to `__builtin_nondeterministic_value` returns a valid value of the type given by the argument.

The types currently supported are: integer types, floating-point types, vector types.

Query for this feature with `__has_builtin(__builtin_nondeterministic_value)`.

### `__builtin_sycl_unique_stable_name`

`__builtin_sycl_unique_stable_name()` is a builtin that takes a type and produces a string literal containing a unique name for the type that is stable across split compilations, mainly to support SYCL/Data Parallel C++ language.

In cases where the split compilation needs to share a unique token for a type across the boundary (such as in an offloading situation), this name can be used for lookup purposes, such as in the SYCL Integration Header.

The value of this builtin is computed entirely at compile time, so it can be used in constant expressions. This value encodes lambda functions based on a stable numbering order in which they appear in their local declaration contexts. Once this builtin is evaluated in a constexpr context, it is erroneous to use it in an instantiation which changes its value.

In order to produce the unique name, the current implementation of the builtin uses Itanium mangling even if the host compilation uses a different name mangling scheme at runtime. The mangler marks all the lambdas required to name the SYCL kernel and emits a stable local ordering of the respective lambdas. The resulting pattern is demanglable. When non-lambda types are passed to the builtin, the mangler emits their usual pattern without any special treatment.

Syntax:

    // Computes a unique stable name for the given type.
    constexpr const char * __builtin_sycl_unique_stable_name( type-id );

### `__builtin_popcountg`

`__builtin_popcountg` returns the number of 1 bits in the argument. The argument can be of any unsigned integer type.

Syntax:

    int __builtin_popcountg(type x)

Examples:

    unsigned int x = 1;
    int x_pop = __builtin_popcountg(x);

    unsigned long y = 3;
    int y_pop = __builtin_popcountg(y);

    unsigned _BitInt(128) z = 7;
    int z_pop = __builtin_popcountg(z);

Description:

`__builtin_popcountg` is meant to be a type-generic alternative to the `__builtin_popcount{,l,ll}` builtins, with support for other integer types, such as `unsigned __int128` and C23 `unsigned _BitInt(N)`.

### `__builtin_clzg` and `__builtin_ctzg`

`__builtin_clzg` (respectively `__builtin_ctzg`) returns the number of leading (respectively trailing) 0 bits in the first argument. The first argument can be of any unsigned integer type.

If the first argument is 0 and an optional second argument of `int` type is provided, then the second argument is returned. If the first argument is 0, but only one argument is provided, then the behavior is undefined.

Syntax:

    int __builtin_clzg(type x[, int fallback])
    int __builtin_ctzg(type x[, int fallback])

Examples:

    unsigned int x = 1;
    int x_lz = __builtin_clzg(x);
    int x_tz = __builtin_ctzg(x);

    unsigned long y = 2;
    int y_lz = __builtin_clzg(y);
    int y_tz = __builtin_ctzg(y);

    unsigned _BitInt(128) z = 4;
    int z_lz = __builtin_clzg(z);
    int z_tz = __builtin_ctzg(z);

Description:

`__builtin_clzg` (respectively `__builtin_ctzg`) is meant to be a type-generic alternative to the `__builtin_clz{,l,ll}` (respectively `__builtin_ctz{,l,ll}`) builtins, with support for other integer types, such as `unsigned __int128` and C23 `unsigned _BitInt(N)`.

### `__builtin_counted_by_ref`

`__builtin_counted_by_ref` returns a pointer to the count field from the `counted_by` attribute.

The argument must be a flexible array member. If the argument isn’t a flexible array member or doesn’t have the `counted_by` attribute, the builtin returns `(void *)0`.

Syntax:

    T *__builtin_counted_by_ref(void *array)

Examples:

    #define alloc(P, FAM, COUNT) ({                                 \
       size_t __ignored_assignment;                                 \
       typeof(P) __p = NULL;                                        \
       __p = malloc(MAX(sizeof(*__p),                               \
                        sizeof(*__p) + sizeof(*__p->FAM) * COUNT)); \
                                                                    \
       *_Generic(                                                   \
         __builtin_counted_by_ref(__p->FAM),                        \
           void *: &__ignored_assignment,                           \
           default: __builtin_counted_by_ref(__p->FAM)) = COUNT;    \
                                                                    \
       __p;                                                         \
    })

Description:

The `__builtin_counted_by_ref` builtin allows the programmer to prevent a common error associated with the `counted_by` attribute. When using the `counted_by` attribute, the `count` field must be set before the flexible array member can be accessed. Otherwise, the sanitizers may view such accesses as false positives. For instance, it’s not uncommon for programmers to initialize the flexible array before setting the `count` field:

    struct s {
      int dummy;
      short count;
      long array[] __attribute__((counted_by(count)));
    };

    struct s *ptr = malloc(sizeof(struct s) + sizeof(long) * COUNT);

    for (int i = 0; i < COUNT; ++i)
      ptr->array[i] = i;

    ptr->count = COUNT;

Enforcing the rule that `ptr->count = COUNT;` must occur after every allocation of a struct with a flexible array member with the `counted_by` attribute is prone to failure in large code bases. This builtin mitigates this for allocators (like in Linux) that are implemented in a way where the counter assignment can happen automatically.

Note: The value returned by `__builtin_counted_by_ref` cannot be assigned to a variable, have its address taken, or passed into or returned from a function, because doing so violates bounds safety conventions.

### Multiprecision Arithmetic Builtins

Clang provides a set of builtins which expose multiprecision arithmetic in a manner amenable to C. They all have the following form:

    unsigned x = ..., y = ..., carryin = ..., carryout;
    unsigned sum = __builtin_addc(x, y, carryin, &carryout);

Thus one can form a multiprecision addition chain in the following manner:

    unsigned *x, *y, *z, carryin=0, carryout;
    z[0] = __builtin_addc(x[0], y[0], carryin, &carryout);
    carryin = carryout;
    z[1] = __builtin_addc(x[1], y[1], carryin, &carryout);
    carryin = carryout;
    z[2] = __builtin_addc(x[2], y[2], carryin, &carryout);
    carryin = carryout;
    z[3] = __builtin_addc(x[3], y[3], carryin, &carryout);

The complete list of builtins are:

    unsigned char      __builtin_addcb (unsigned char x, unsigned char y, unsigned char carryin, unsigned char *carryout);
    unsigned short     __builtin_addcs (unsigned short x, unsigned short y, unsigned short carryin, unsigned short *carryout);
    unsigned           __builtin_addc  (unsigned x, unsigned y, unsigned carryin, unsigned *carryout);
    unsigned long      __builtin_addcl (unsigned long x, unsigned long y, unsigned long carryin, unsigned long *carryout);
    unsigned long long __builtin_addcll(unsigned long long x, unsigned long long y, unsigned long long carryin, unsigned long long *carryout);
    unsigned char      __builtin_subcb (unsigned char x, unsigned char y, unsigned char carryin, unsigned char *carryout);
    unsigned short     __builtin_subcs (unsigned short x, unsigned short y, unsigned short carryin, unsigned short *carryout);
    unsigned           __builtin_subc  (unsigned x, unsigned y, unsigned carryin, unsigned *carryout);
    unsigned long      __builtin_subcl (unsigned long x, unsigned long y, unsigned long carryin, unsigned long *carryout);
    unsigned long long __builtin_subcll(unsigned long long x, unsigned long long y, unsigned long long carryin, unsigned long long *carryout);

### Checked Arithmetic Builtins

Clang provides a set of builtins that implement checked arithmetic for security critical applications in a manner that is fast and easily expressible in C. As an example of their usage:

    errorcode_t security_critical_application(...) {
      unsigned x, y, result;
      ...
      if (__builtin_mul_overflow(x, y, &result))
        return kErrorCodeHackers;
      ...
      use_multiply(result);
      ...
    }

Clang provides the following checked arithmetic builtins:

    bool __builtin_add_overflow   (type1 x, type2 y, type3 *sum);
    bool __builtin_sub_overflow   (type1 x, type2 y, type3 *diff);
    bool __builtin_mul_overflow   (type1 x, type2 y, type3 *prod);
    bool __builtin_uadd_overflow  (unsigned x, unsigned y, unsigned *sum);
    bool __builtin_uaddl_overflow (unsigned long x, unsigned long y, unsigned long *sum);
    bool __builtin_uaddll_overflow(unsigned long long x, unsigned long long y, unsigned long long *sum);
    bool __builtin_usub_overflow  (unsigned x, unsigned y, unsigned *diff);
    bool __builtin_usubl_overflow (unsigned long x, unsigned long y, unsigned long *diff);
    bool __builtin_usubll_overflow(unsigned long long x, unsigned long long y, unsigned long long *diff);
    bool __builtin_umul_overflow  (unsigned x, unsigned y, unsigned *prod);
    bool __builtin_umull_overflow (unsigned long x, unsigned long y, unsigned long *prod);
    bool __builtin_umulll_overflow(unsigned long long x, unsigned long long y, unsigned long long *prod);
    bool __builtin_sadd_overflow  (int x, int y, int *sum);
    bool __builtin_saddl_overflow (long x, long y, long *sum);
    bool __builtin_saddll_overflow(long long x, long long y, long long *sum);
    bool __builtin_ssub_overflow  (int x, int y, int *diff);
    bool __builtin_ssubl_overflow (long x, long y, long *diff);
    bool __builtin_ssubll_overflow(long long x, long long y, long long *diff);
    bool __builtin_smul_overflow  (int x, int y, int *prod);
    bool __builtin_smull_overflow (long x, long y, long *prod);
    bool __builtin_smulll_overflow(long long x, long long y, long long *prod);

Each builtin performs the specified mathematical operation on the first two arguments and stores the result in the third argument. If possible, the result will be equal to mathematically-correct result and the builtin will return 0. Otherwise, the builtin will return 1 and the result will be equal to the unique value that is equivalent to the mathematically-correct result modulo two raised to the k power, where k is the number of bits in the result type. The behavior of these builtins is well-defined for all argument values.

The first three builtins work generically for operands of any integer type, including boolean types. The operands need not have the same type as each other, or as the result. The other builtins may implicitly promote or convert their operands before performing the operation.

Query for this feature with `__has_builtin(__builtin_add_overflow)`, etc.

### Floating point builtins

### `__builtin_isfpclass`

`__builtin_isfpclass` is used to test if the specified floating-point values fall into one of the specified floating-point classes.

Syntax:

    int __builtin_isfpclass(fp_type expr, int mask)
    int_vector __builtin_isfpclass(fp_vector expr, int mask)

Example of use:

    if (__builtin_isfpclass(x, 448)) {
       // `x` is positive finite value
           ...
    }

Description:

The `__builtin_isfpclass()` builtin is a generalization of functions `isnan`, `isinf`, `isfinite` and some others defined by the C standard. It tests if the floating-point value, specified by the first argument, falls into any of data classes, specified by the second argument. The latter is an integer constant bitmask expression, in which each data class is represented by a bit using the encoding:

Mask value | Data class | Macro
--- | --- | ---
0x0001 | Signaling NaN | __FPCLASS_SNAN
0x0002 | Quiet NaN | __FPCLASS_QNAN
0x0004 | Negative infinity | __FPCLASS_NEGINF
0x0008 | Negative normal | __FPCLASS_NEGNORMAL
0x0010 | Negative subnormal | __FPCLASS_NEGSUBNORMAL
0x0020 | Negative zero | __FPCLASS_NEGZERO
0x0040 | Positive zero | __FPCLASS_POSZERO
0x0080 | Positive subnormal | __FPCLASS_POSSUBNORMAL
0x0100 | Positive normal | __FPCLASS_POSNORMAL
0x0200 | Positive infinity | __FPCLASS_POSINF

For convenience preprocessor defines macros for these values. The function returns 1 if `expr` falls into one of the specified data classes, 0 otherwise.

In the example above the mask value 448 (0x1C0) contains the bits selecting positive zero, positive subnormal and positive normal classes. `__builtin_isfpclass(x, 448)` would return true only if `x` if of any of these data classes. Using suitable mask value, the function can implement any of the standard classification functions, for example, `__builtin_isfpclass(x, 3)` is identical to `isnan`,``__builtin_isfpclass(x, 504)`` - to `isfinite` and so on.

If the first argument is a vector, the function is equivalent to the set of scalar calls of `__builtin_isfpclass` applied to the input elementwise.

The result of `__builtin_isfpclass` is a boolean value, if the first argument is a scalar, or an integer vector with the same element count as the first argument. The element type in this vector has the same bit length as the element of the first argument type.

This function never raises floating-point exceptions and does not canonicalize its input. The floating-point argument is not promoted, its data class is determined based on its representation in its actual semantic type.

### `__builtin_canonicalize`

    double __builtin_canonicalize(double);
    float __builtin_canonicalizef(float);
    long double __builtin_canonicalizel(long double);

Returns the platform specific canonical encoding of a floating point number. This canonicalization is useful for implementing certain numeric primitives such as frexp. See LLVM canonicalize intrinsic for more information on the semantics.

### `__builtin_flt_rounds` and `__builtin_set_flt_rounds`

    int __builtin_flt_rounds();
    void __builtin_set_flt_rounds(int);

Returns and sets current floating point rounding mode. The encoding of returned values and input parameters is same as the result of FLT_ROUNDS, specified by C standard: - `0` - toward zero - `1` - to nearest, ties to even - `2` - toward positive infinity - `3` - toward negative infinity - `4` - to nearest, ties away from zero The effect of passing some other value to `__builtin_flt_rounds` is implementation-defined. `__builtin_set_flt_rounds` is currently only supported to work on x86, x86_64, powerpc, powerpc64, Arm and AArch64 targets. These builtins read and modify the floating-point environment, which is not always allowed and may have unexpected behavior. Please see the section on Accessing the floating point environment for more information.

### String builtins

Clang provides constant expression evaluation support for builtins forms of the following functions from the C standard library headers `<string.h>` and `<wchar.h>`:

- `memchr`

- `memcmp` (and its deprecated BSD / POSIX alias `bcmp`)

- `strchr`

- `strcmp`

- `strlen`

- `strncmp`

- `wcschr`

- `wcscmp`

- `wcslen`

- `wcsncmp`

- `wmemchr`

- `wmemcmp`

In each case, the builtin form has the name of the C library function prefixed by `__builtin_`. Example:

    void *p = __builtin_memchr("foobar", 'b', 5);

In addition to the above, one further builtin is provided:

    char *__builtin_char_memchr(const char *haystack, int needle, size_t size);

`__builtin_char_memchr(a, b, c)` is identical to `(char*)__builtin_memchr(a, b, c)` except that its use is permitted within constant expressions in C++11 onwards (where a cast from `void*` to `char*` is disallowed in general).

Constant evaluation support for the `__builtin_mem*` functions is provided only for arrays of `char`, `signed char`, `unsigned char`, or `char8_t`, despite these functions accepting an argument of type `const void*`.

Support for constant expression evaluation for the above builtins can be detected with `__has_feature(cxx_constexpr_string_builtins)`.

### Variadic function builtins

Clang provides several builtins for working with variadic functions from the C standard library `<stdarg.h>` header:

- `__builtin_va_list`

A predefined typedef for the target-specific `va_list` type. It is undefined behavior to use a byte-wise copy of this type produced by calling `memcpy`, `memmove`, or similar. Valid explicit copies are only produced by calling `va_copy` or `__builtin_va_copy`.

- `void __builtin_va_start(__builtin_va_list list, <parameter-name>)`

A builtin function for the target-specific `va_start` function-like macro. The `parameter-name` argument is the name of the parameter preceding the ellipsis (`...`) in the function signature. Alternatively, in C23 mode or later, it may be the integer literal `0` if there is no parameter preceding the ellipsis. This function initializes the given `__builtin_va_list` object. It is undefined behavior to call this function on an already initialized `__builtin_va_list` object.

- `void __builtin_c23_va_start(__builtin_va_list list, ...)`

A builtin function for the target-specific `va_start` function-like macro, available only in C23 and later. The builtin accepts zero or one argument for the ellipsis (`...`). If such an argument is provided, it should be the name of the parameter preceding the ellipsis, which is used for compatibility with C versions before C23. It is an error to provide two or more variadic arguments. This function initializes the given `__builtin_va_list` object. It is undefined behavior to call this function on an already initialized `__builtin_va_list` object.

- `void __builtin_va_end(__builtin_va_list list)`

A builtin function for the target-specific `va_end` function-like macro. This function finalizes the given `__builtin_va_list` object such that it is no longer usable unless re-initialized with a call to `__builtin_va_start` or `__builtin_va_copy`. It is undefined behavior to call this function with a `list` that has not been initialized by either `__builtin_va_start` or `__builtin_va_copy`.

- `<type-name> __builtin_va_arg(__builtin_va_list list, <type-name>)`

A builtin function for the target-specific `va_arg` function-like macro. This function returns the value of the next variadic argument to the call. It is undefined behavior to call this builtin when there is no next variadic argument to retrieve or if the next variadic argument does not have a type compatible with the given `type-name`. The return type of the function is the `type-name` given as the second argument. It is undefined behavior to call this function with a `list` that has not been initialized by either `__builtin_va_start` or `__builtin_va_copy`.

- `void __builtin_va_copy(__builtin_va_list dest, __builtin_va_list src)`

A builtin function for the target-specific `va_copy` function-like macro. This function initializes `dest` as a copy of `src`. It is undefined behavior to call this function with an already initialized `dest` argument.

### Memory builtins

Clang provides constant expression evaluation support for builtin forms of the following functions from the C standard library headers `<string.h>` and `<wchar.h>`:

- `memcpy`

- `memmove`

- `wmemcpy`

- `wmemmove`

In each case, the builtin form has the name of the C library function prefixed by `__builtin_`.

Constant evaluation support is only provided when the source and destination are pointers to arrays with the same trivially copyable element type, and the given size is an exact multiple of the element size that is no greater than the number of elements accessible through the source and destination operands.

#### Guaranteed inlined copy

    void __builtin_memcpy_inline(void *dst, const void *src, size_t size);

`__builtin_memcpy_inline` has been designed as a building block for efficient `memcpy` implementations. It is identical to `__builtin_memcpy` but also guarantees not to call any external functions. See LLVM IR llvm.memcpy.inline intrinsic for more information.

This is useful to implement a custom version of `memcpy`, implement a `libc` memcpy or work around the absence of a `libc`.

Note that the size argument must be a compile time constant.

Note that this intrinsic cannot yet be called in a `constexpr` context.

#### Guaranteed inlined memset

    void __builtin_memset_inline(void *dst, int value, size_t size);

`__builtin_memset_inline` has been designed as a building block for efficient `memset` implementations. It is identical to `__builtin_memset` but also guarantees not to call any external functions. See LLVM IR llvm.memset.inline intrinsic for more information.

This is useful to implement a custom version of `memset`, implement a `libc` memset or work around the absence of a `libc`.

Note that the size argument must be a compile time constant.

Note that this intrinsic cannot yet be called in a `constexpr` context.

### `__is_bitwise_cloneable`

A type trait is used to check whether a type can be safely copied by memcpy.

Syntax:

    bool __is_bitwise_cloneable(Type)

Description:

Objects of bitwise cloneable types can be bitwise copied by memcpy/memmove. The Clang compiler warrants that this behavior is well defined, and won’t be broken by compiler optimizations and sanitizers.

For implicit-lifetime types, the lifetime of the new object is implicitly started after the copy. For other types (e.g., classes with virtual methods), the lifetime isn’t started, and using the object results in undefined behavior according to the C++ Standard.

This builtin can be used in constant expressions.

### Atomic Min/Max builtins with memory ordering

There are two atomic builtins with min/max in-memory comparison and swap. The syntax and semantics are similar to GCC-compatible __atomic_* builtins.

- `__atomic_fetch_min`

- `__atomic_fetch_max`

The builtins work with signed and unsigned integers and require to specify memory ordering. The return value is the original value that was stored in memory before comparison.

Example:

    unsigned int val = __atomic_fetch_min(unsigned int *pi, unsigned int ui, __ATOMIC_RELAXED);

The third argument is one of the memory ordering specifiers `__ATOMIC_RELAXED`, `__ATOMIC_CONSUME`, `__ATOMIC_ACQUIRE`, `__ATOMIC_RELEASE`, `__ATOMIC_ACQ_REL`, or `__ATOMIC_SEQ_CST` following C++11 memory model semantics.

In terms of acquire-release ordering barriers these two operations are always considered as operations with load-store semantics, even when the original value is not actually modified after comparison.

### __c11_atomic builtins

Clang provides a set of builtins which are intended to be used to implement C11’s `<stdatomic.h>` header. These builtins provide the semantics of the `_explicit` form of the corresponding C11 operation, and are named with a `__c11_` prefix. The supported operations, and the differences from the corresponding C11 operations, are:

- `__c11_atomic_init`

- `__c11_atomic_thread_fence`

- `__c11_atomic_signal_fence`

- `__c11_atomic_is_lock_free` (The argument is the size of the `_Atomic(...)` object, instead of its address)

- `__c11_atomic_store`

- `__c11_atomic_load`

- `__c11_atomic_exchange`

- `__c11_atomic_compare_exchange_strong`

- `__c11_atomic_compare_exchange_weak`

- `__c11_atomic_fetch_add`

- `__c11_atomic_fetch_sub`

- `__c11_atomic_fetch_and`

- `__c11_atomic_fetch_or`

- `__c11_atomic_fetch_xor`

- `__c11_atomic_fetch_nand` (Nand is not presented in `<stdatomic.h>`)

- `__c11_atomic_fetch_max`

- `__c11_atomic_fetch_min`

The macros `__ATOMIC_RELAXED`, `__ATOMIC_CONSUME`, `__ATOMIC_ACQUIRE`, `__ATOMIC_RELEASE`, `__ATOMIC_ACQ_REL`, and `__ATOMIC_SEQ_CST` are provided, with values corresponding to the enumerators of C11’s `memory_order` enumeration.

(Note that Clang additionally provides GCC-compatible `__atomic_*` builtins and OpenCL 2.0 `__opencl_atomic_*` builtins. The OpenCL 2.0 atomic builtins are an explicit form of the corresponding OpenCL 2.0 builtin function, and are named with a `__opencl_` prefix. The macros `__OPENCL_MEMORY_SCOPE_WORK_ITEM`, `__OPENCL_MEMORY_SCOPE_WORK_GROUP`, `__OPENCL_MEMORY_SCOPE_DEVICE`, `__OPENCL_MEMORY_SCOPE_ALL_SVM_DEVICES`, and `__OPENCL_MEMORY_SCOPE_SUB_GROUP` are provided, with values corresponding to the enumerators of OpenCL’s `memory_scope` enumeration.)

### __scoped_atomic builtins

Clang provides a set of atomics taking a memory scope argument. These atomics are identical to the standard GNU / GCC atomic builtins but taking an extra memory scope argument. These are designed to be a generic alternative to the `__opencl_atomic_*` builtin functions for targets that support atomic memory scopes.

Atomic memory scopes are designed to assist optimizations for systems with several levels of memory hierarchy like GPUs. The following memory scopes are currently supported:

- `__MEMORY_SCOPE_SYSTEM`

- `__MEMORY_SCOPE_DEVICE`

- `__MEMORY_SCOPE_WRKGRP`

- `__MEMORY_SCOPE_WVFRNT`

- `__MEMORY_SCOPE_SINGLE`

This controls whether or not the atomic operation is ordered with respect to the whole system, the current device, an OpenCL workgroup, wavefront, or just a single thread. If these are used on a target that does not support atomic scopes, then they will behave exactly as the standard GNU atomic builtins.

### Low-level ARM exclusive memory builtins

Clang provides overloaded builtins giving direct access to the three key ARM instructions for implementing atomic operations.

    T __builtin_arm_ldrex(const volatile T *addr);
    T __builtin_arm_ldaex(const volatile T *addr);
    int __builtin_arm_strex(T val, volatile T *addr);
    int __builtin_arm_stlex(T val, volatile T *addr);
    void __builtin_arm_clrex(void);

The types `T` currently supported are:

- Integer types with width at most 64 bits (or 128 bits on AArch64).

- Floating-point types

- Pointer types.

Note that the compiler does not guarantee it will not insert stores which clear the exclusive monitor in between an `ldrex` type operation and its paired `strex`. In practice this is only usually a risk when the extra store is on the same cache line as the variable being modified and Clang will only insert stack stores on its own, so it is best not to use these operations on variables with automatic storage duration.

Also, loads and stores may be implicit in code written between the `ldrex` and `strex`. Clang will not necessarily mitigate the effects of these either, so care should be exercised.

For these reasons the higher level atomic primitives should be preferred where possible.

### Non-temporal load/store builtins

Clang provides overloaded builtins allowing generation of non-temporal memory accesses.

    T __builtin_nontemporal_load(T *addr);
    void __builtin_nontemporal_store(T value, T *addr);

The types `T` currently supported are:

- Integer types.

- Floating-point types.

- Vector types.

Note that the compiler does not guarantee that non-temporal loads or stores will be used.

### C++ Coroutines support builtins

Warning

This is a work in progress. Compatibility across Clang/LLVM releases is not guaranteed.

Clang provides experimental builtins to support C++ Coroutines as defined by https://wg21.link/P0057. The following four are intended to be used by the standard library to implement the `std::coroutine_handle` type.

Syntax:

    void  __builtin_coro_resume(void *addr);
    void  __builtin_coro_destroy(void *addr);
    bool  __builtin_coro_done(void *addr);
    void *__builtin_coro_promise(void *addr, int alignment, bool from_promise)

Example of use:

    template <> struct coroutine_handle<void> {
      void resume() const { __builtin_coro_resume(ptr); }
      void destroy() const { __builtin_coro_destroy(ptr); }
      bool done() const { return __builtin_coro_done(ptr); }
      // ...
    protected:
      void *ptr;
    };

    template <typename Promise> struct coroutine_handle : coroutine_handle<> {
      // ...
      Promise &promise() const {
        return *reinterpret_cast<Promise *>(
          __builtin_coro_promise(ptr, alignof(Promise), /*from-promise=*/false));
      }
      static coroutine_handle from_promise(Promise &promise) {
        coroutine_handle p;
        p.ptr = __builtin_coro_promise(&promise, alignof(Promise),
                                                        /*from-promise=*/true);
        return p;
      }
    };

Other coroutine builtins are either for internal clang use or for use during development of the coroutine feature. See Coroutines in LLVM for more information on their semantics. Note that builtins matching the intrinsics that take token as the first parameter (llvm.coro.begin, llvm.coro.alloc, llvm.coro.free and llvm.coro.suspend) omit the token parameter and fill it to an appropriate value during the emission.

Syntax:

    size_t __builtin_coro_size()
    void  *__builtin_coro_frame()
    void  *__builtin_coro_free(void *coro_frame)

    void  *__builtin_coro_id(int align, void *promise, void *fnaddr, void *parts)
    bool   __builtin_coro_alloc()
    void  *__builtin_coro_begin(void *memory)
    void   __builtin_coro_end(void *coro_frame, bool unwind)
    char   __builtin_coro_suspend(bool final)

Note that there is no builtin matching the llvm.coro.save intrinsic. LLVM automatically will insert one if the first argument to llvm.coro.suspend is token none. If a user calls __builtin_suspend, clang will insert token none as the first argument to the intrinsic.

### Source location builtins

Clang provides builtins to support C++ standard library implementation of `std::source_location` as specified in C++20. With the exception of `__builtin_COLUMN`, `__builtin_FILE_NAME` and `__builtin_FUNCSIG`, these builtins are also implemented by GCC.

Syntax:

    const char *__builtin_FILE();
    const char *__builtin_FILE_NAME(); // Clang only
    const char *__builtin_FUNCTION();
    const char *__builtin_FUNCSIG(); // Microsoft
    unsigned    __builtin_LINE();
    unsigned    __builtin_COLUMN(); // Clang only
    const std::source_location::__impl *__builtin_source_location();

Example of use:

    void my_assert(bool pred, int line = __builtin_LINE(), // Captures line of caller
                   const char* file = __builtin_FILE(),
                   const char* function = __builtin_FUNCTION()) {
      if (pred) return;
      printf("%s:%d assertion failed in function %s\n", file, line, function);
      std::abort();
    }

    struct MyAggregateType {
      int x;
      int line = __builtin_LINE(); // captures line where aggregate initialization occurs
    };
    static_assert(MyAggregateType{42}.line == __LINE__);

    struct MyClassType {
      int line = __builtin_LINE(); // captures line of the constructor used during initialization
      constexpr MyClassType(int) { assert(line == __LINE__); }
    };

Description:

The builtins `__builtin_LINE`, `__builtin_FUNCTION`, `__builtin_FUNCSIG`, `__builtin_FILE` and `__builtin_FILE_NAME` return the values, at the “invocation point”, for `__LINE__`, `__FUNCTION__`, `__FUNCSIG__`, `__FILE__` and `__FILE_NAME__` respectively. `__builtin_COLUMN` similarly returns the column, though there is no corresponding macro. These builtins are constant expressions.

When the builtins appear as part of a default function argument the invocation point is the location of the caller. When the builtins appear as part of a default member initializer, the invocation point is the location of the constructor or aggregate initialization used to create the object. Otherwise the invocation point is the same as the location of the builtin.

When the invocation point of `__builtin_FUNCTION` is not a function scope, the empty string is returned.

The builtin `__builtin_COLUMN` returns the offset from the start of the line, beginning from column 1. This may differ from other implementations.

The builtin `__builtin_source_location` returns a pointer to constant static data of type `std::source_location::__impl`. This type must have already been defined, and must contain exactly four fields: `const char *_M_file_name`, `const char *_M_function_name`, `<any-integral-type> _M_line`, and `<any-integral-type> _M_column`. The fields will be populated in the same manner as the above four builtins, except that `_M_function_name` is populated with `__PRETTY_FUNCTION__` rather than `__FUNCTION__`.

### Alignment builtins

Clang provides builtins to support checking and adjusting alignment of pointers and integers. These builtins can be used to avoid relying on implementation-defined behavior of arithmetic on integers derived from pointers. Additionally, these builtins retain type information and, unlike bitwise arithmetic, they can perform semantic checking on the alignment value.

Syntax:

    Type __builtin_align_up(Type value, size_t alignment);
    Type __builtin_align_down(Type value, size_t alignment);
    bool __builtin_is_aligned(Type value, size_t alignment);

Example of use:

    char* global_alloc_buffer;
    void* my_aligned_allocator(size_t alloc_size, size_t alignment) {
      char* result = __builtin_align_up(global_alloc_buffer, alignment);
      // result now contains the value of global_alloc_buffer rounded up to the
      // next multiple of alignment.
      global_alloc_buffer = result + alloc_size;
      return result;
    }

    void* get_start_of_page(void* ptr) {
      return __builtin_align_down(ptr, PAGE_SIZE);
    }

    void example(char* buffer) {
       if (__builtin_is_aligned(buffer, 64)) {
         do_fast_aligned_copy(buffer);
       } else {
         do_unaligned_copy(buffer);
       }
    }

    // In addition to pointers, the builtins can also be used on integer types
    // and are evaluatable inside constant expressions.
    static_assert(__builtin_align_up(123, 64) == 128, "");
    static_assert(__builtin_align_down(123u, 64) == 64u, "");
    static_assert(!__builtin_is_aligned(123, 64), "");

Description:

The builtins `__builtin_align_up`, `__builtin_align_down`, return their first argument aligned up/down to the next multiple of the second argument. If the value is already sufficiently aligned, it is returned unchanged. The builtin `__builtin_is_aligned` returns whether the first argument is aligned to a multiple of the second argument. All of these builtins expect the alignment to be expressed as a number of bytes.

These builtins can be used for all integer types as well as (non-function) pointer types. For pointer types, these builtins operate in terms of the integer address of the pointer and return a new pointer of the same type (including qualifiers such as `const`) with an adjusted address. When aligning pointers up or down, the resulting value must be within the same underlying allocation or one past the end (see C17 6.5.6p8, C++ [expr.add]). This means that arbitrary integer values stored in pointer-type variables must not be passed to these builtins. For those use cases, the builtins can still be used, but the operation must be performed on the pointer cast to `uintptr_t`.

If Clang can determine that the alignment is not a power of two at compile time, it will result in a compilation failure. If the alignment argument is not a power of two at run time, the behavior of these builtins is undefined.

## Non-standard C++11 Attributes

Clang’s non-standard C++11 attributes live in the `clang` attribute namespace.

Clang supports GCC’s `gnu` attribute namespace. All GCC attributes which are accepted with the `__attribute__((foo))` syntax are also accepted as `[[gnu::foo]]`. This only extends to attributes which are specified by GCC (see the list of GCC function attributes, GCC variable attributes, and GCC type attributes). As with the GCC implementation, these attributes must appertain to the declarator-id in a declaration, which means they must go either at the start of the declaration or immediately after the name being declared.

For example, this applies the GNU `unused` attribute to `a` and `f`, and also applies the GNU `noreturn` attribute to `f`.

Examples: .. code-block:: c++

> [[gnu::unused]] int a, f [[gnu::noreturn]] ();

## Target-Specific Extensions

Clang supports some language features conditionally on some targets.

### AMDGPU Language Extensions

#### __builtin_amdgcn_fence

`__builtin_amdgcn_fence` emits a fence.

- `unsigned` atomic ordering, e.g. `__ATOMIC_ACQUIRE`

- `const char *` synchronization scope, e.g. `workgroup`

- Zero or more `const char *` address spaces names.

The address spaces arguments must be one of the following string literals:

- `"local"`

- `"global"`

If one or more address space name are provided, the code generator will attempt to emit potentially faster instructions that order access to at least those address spaces. Emitting such instructions may not always be possible and the compiler is free to fence more aggressively.

If no address spaces names are provided, all address spaces are fenced.

    // Fence all address spaces.
    __builtin_amdgcn_fence(__ATOMIC_SEQ_CST, "workgroup");
    __builtin_amdgcn_fence(__ATOMIC_ACQUIRE, "agent");

    // Fence only requested address spaces.
    __builtin_amdgcn_fence(__ATOMIC_SEQ_CST, "workgroup", "local")
    __builtin_amdgcn_fence(__ATOMIC_SEQ_CST, "workgroup", "local", "global")

### ARM/AArch64 Language Extensions

#### Memory Barrier Intrinsics

Clang implements the `__dmb`, `__dsb` and `__isb` intrinsics as defined in the Arm C Language Extensions. Note that these intrinsics are implemented as motion barriers that block reordering of memory accesses and side effect instructions. Other instructions like simple arithmetic may be reordered around the intrinsic. If you expect to have no reordering at all, use inline assembly instead.

#### Pointer Authentication

See Pointer Authentication.

### X86/X86-64 Language Extensions

The X86 backend has these language extensions:

#### Memory references to specified segments

Annotating a pointer with address space #256 causes it to be code generated relative to the X86 GS segment register, address space #257 causes it to be relative to the X86 FS segment, and address space #258 causes it to be relative to the X86 SS segment. Note that this is a very very low-level feature that should only be used if you know what you’re doing (for example in an OS kernel).

Here is an example:

    #define GS_RELATIVE __attribute__((address_space(256)))
    int foo(int GS_RELATIVE *P) {
      return *P;
    }

Which compiles to (on X86-32):

    _foo:
            movl    4(%esp), %eax
            movl    %gs:(%eax), %eax
            ret

You can also use the GCC compatibility macros `__seg_fs` and `__seg_gs` for the same purpose. The preprocessor symbols `__SEG_FS` and `__SEG_GS` indicate their support.

### PowerPC Language Extensions

#### Set the Floating Point Rounding Mode

PowerPC64/PowerPC64le supports the builtin function `__builtin_setrnd` to set the floating point rounding mode. This function will use the least significant two bits of integer argument to set the floating point rounding mode.

    double __builtin_setrnd(int mode);

The effective values for mode are:

> - 0 - round to nearest

> - 1 - round to zero

> - 2 - round to +infinity

> - 3 - round to -infinity

Note that the mode argument will modulo 4, so if the integer argument is greater than 3, it will only use the least significant two bits of the mode. Namely, `__builtin_setrnd(102))` is equal to `__builtin_setrnd(2)`.

#### PowerPC cache builtins

The PowerPC architecture specifies instructions implementing cache operations. Clang provides builtins that give direct programmer access to these cache instructions.

Currently the following builtins are implemented in clang:

`__builtin_dcbf` copies the contents of a modified block from the data cache to main memory and flushes the copy from the data cache.

Syntax:

    void __dcbf(const void* addr); /* Data Cache Block Flush */

Example of Use:

    int a = 1;
    __builtin_dcbf (&a);

## Extensions for Static Analysis

Clang supports additional attributes that are useful for documenting program invariants and rules for static analysis tools, such as the Clang Static Analyzer. These attributes are documented in the analyzer’s list of annotations for analysis.

## Extensions for Dynamic Analysis

Use `__has_feature(address_sanitizer)` to check if the code is being built with AddressSanitizer.

Use `__has_feature(thread_sanitizer)` to check if the code is being built with ThreadSanitizer.

Use `__has_feature(memory_sanitizer)` to check if the code is being built with MemorySanitizer.

Use `__has_feature(dataflow_sanitizer)` to check if the code is being built with DataFlowSanitizer.

Use `__has_feature(safe_stack)` to check if the code is being built with SafeStack.

## Extensions for selectively disabling optimization

Clang provides a mechanism for selectively disabling optimizations in functions and methods.

To disable optimizations in a single function definition, the GNU-style or C++11 non-standard attribute `optnone` can be used.

    // The following functions will not be optimized.
    // GNU-style attribute
    __attribute__((optnone)) int foo() {
      // ... code
    }
    // C++11 attribute
    [[clang::optnone]] int bar() {
      // ... code
    }

To facilitate disabling optimization for a range of function definitions, a range-based pragma is provided. Its syntax is `#pragma clang optimize` followed by `off` or `on`.

All function definitions in the region between an `off` and the following `on` will be decorated with the `optnone` attribute unless doing so would conflict with explicit attributes already present on the function (e.g. the ones that control inlining).

    #pragma clang optimize off
    // This function will be decorated with optnone.
    int foo() {
      // ... code
    }

    // optnone conflicts with always_inline, so bar() will not be decorated.
    __attribute__((always_inline)) int bar() {
      // ... code
    }
    #pragma clang optimize on

If no `on` is found to close an `off` region, the end of the region is the end of the compilation unit.

Note that a stray `#pragma clang optimize on` does not selectively enable additional optimizations when compiling at low optimization levels. This feature can only be used to selectively disable optimizations.

The pragma has an effect on functions only at the point of their definition; for function templates, this means that the state of the pragma at the point of an instantiation is not necessarily relevant. Consider the following example:

    template<typename T> T twice(T t) {
      return 2 * t;
    }

    #pragma clang optimize off
    template<typename T> T thrice(T t) {
      return 3 * t;
    }

    int container(int a, int b) {
      return twice(a) + thrice(b);
    }
    #pragma clang optimize on

In this example, the definition of the template function `twice` is outside the pragma region, whereas the definition of `thrice` is inside the region. The `container` function is also in the region and will not be optimized, but it causes the instantiation of `twice` and `thrice` with an `int` type; of these two instantiations, `twice` will be optimized (because its definition was outside the region) and `thrice` will not be optimized.

Clang also implements MSVC’s range-based pragma, `#pragma optimize("[optimization-list]", on | off)`. At the moment, Clang only supports an empty optimization list, whereas MSVC supports the arguments, `s`, `g`, `t`, and `y`. Currently, the implementation of `pragma optimize` behaves the same as `#pragma clang optimize`. All functions between `off` and `on` will be decorated with the `optnone` attribute.

    #pragma optimize("", off)
    // This function will be decorated with optnone.
    void f1() {}

    #pragma optimize("", on)
    // This function will be optimized with whatever was specified on
    // the commandline.
    void f2() {}

    // This will warn with Clang's current implementation.
    #pragma optimize("g", on)
    void f3() {}

For MSVC, an empty optimization list and `off` parameter will turn off all optimizations, `s`, `g`, `t`, and `y`. An empty optimization and `on` parameter will reset the optimizations to the ones specified on the commandline.

Parameters (unsupported by Clang)
Parameter | Type of optimization
g | Deprecated
s or t | Short or fast sequences of machine code
y | Enable frame pointers

## Extensions for loop hint optimizations

The `#pragma clang loop` directive is used to specify hints for optimizing the subsequent for, while, do-while, or c++11 range-based for loop. The directive provides options for vectorization, interleaving, predication, unrolling and distribution. Loop hints can be specified before any loop and will be ignored if the optimization is not safe to apply.

There are loop hints that control transformations (e.g. vectorization, loop unrolling) and there are loop hints that set transformation options (e.g. `vectorize_width`, `unroll_count`). Pragmas setting transformation options imply the transformation is enabled, as if it was enabled via the corresponding transformation pragma (e.g. `vectorize(enable)`). If the transformation is disabled (e.g. `vectorize(disable)`), that takes precedence over transformations option pragmas implying that transformation.

### Vectorization, Interleaving, and Predication

A vectorized loop performs multiple iterations of the original loop in parallel using vector instructions. The instruction set of the target processor determines which vector instructions are available and their vector widths. This restricts the types of loops that can be vectorized. The vectorizer automatically determines if the loop is safe and profitable to vectorize. A vector instruction cost model is used to select the vector width.

Interleaving multiple loop iterations allows modern processors to further improve instruction-level parallelism (ILP) using advanced hardware features, such as multiple execution units and out-of-order execution. The vectorizer uses a cost model that depends on the register pressure and generated code size to select the interleaving count.

Vectorization is enabled by `vectorize(enable)` and interleaving is enabled by `interleave(enable)`. This is useful when compiling with `-Os` to manually enable vectorization or interleaving.

    #pragma clang loop vectorize(enable)
    #pragma clang loop interleave(enable)
    for(...) {
      ...
    }

The vector width is specified by `vectorize_width(_value_[, fixed|scalable])`, where _value_ is a positive integer and the type of vectorization can be specified with an optional second parameter. The default for the second parameter is ‘fixed’ and refers to fixed width vectorization, whereas ‘scalable’ indicates the compiler should use scalable vectors instead. Another use of vectorize_width is `vectorize_width(fixed|scalable)` where the user can hint at the type of vectorization to use without specifying the exact width. In both variants of the pragma the vectorizer may decide to fall back on fixed width vectorization if the target does not support scalable vectors.

The interleave count is specified by `interleave_count(_value_)`, where _value_ is a positive integer. This is useful for specifying the optimal width/count of the set of target architectures supported by your application.

    #pragma clang loop vectorize_width(2)
    #pragma clang loop interleave_count(2)
    for(...) {
      ...
    }

Specifying a width/count of 1 disables the optimization, and is equivalent to `vectorize(disable)` or `interleave(disable)`.

Vector predication is enabled by `vectorize_predicate(enable)`, for example:

    #pragma clang loop vectorize(enable)
    #pragma clang loop vectorize_predicate(enable)
    for(...) {
      ...
    }

This predicates (masks) all instructions in the loop, which allows the scalar remainder loop (the tail) to be folded into the main vectorized loop. This might be more efficient when vector predication is efficiently supported by the target platform.

### Loop Unrolling

Unrolling a loop reduces the loop control overhead and exposes more opportunities for ILP. Loops can be fully or partially unrolled. Full unrolling eliminates the loop and replaces it with an enumerated sequence of loop iterations. Full unrolling is only possible if the loop trip count is known at compile time. Partial unrolling replicates the loop body within the loop and reduces the trip count.

If `unroll(enable)` is specified the unroller will attempt to fully unroll the loop if the trip count is known at compile time. If the fully unrolled code size is greater than an internal limit the loop will be partially unrolled up to this limit. If the trip count is not known at compile time the loop will be partially unrolled with a heuristically chosen unroll factor.

    #pragma clang loop unroll(enable)
    for(...) {
      ...
    }

If `unroll(full)` is specified the unroller will attempt to fully unroll the loop if the trip count is known at compile time identically to `unroll(enable)`. However, with `unroll(full)` the loop will not be unrolled if the loop count is not known at compile time.

    #pragma clang loop unroll(full)
    for(...) {
      ...
    }

The unroll count can be specified explicitly with `unroll_count(_value_)` where _value_ is a positive integer. If this value is greater than the trip count the loop will be fully unrolled. Otherwise the loop is partially unrolled subject to the same code size limit as with `unroll(enable)`.

    #pragma clang loop unroll_count(8)
    for(...) {
      ...
    }

Unrolling of a loop can be prevented by specifying `unroll(disable)`.

Loop unroll parameters can be controlled by options -mllvm -unroll-count=n and -mllvm -pragma-unroll-threshold=n.

### Loop Distribution

Loop Distribution allows splitting a loop into multiple loops. This is beneficial for example when the entire loop cannot be vectorized but some of the resulting loops can.

If `distribute(enable))` is specified and the loop has memory dependencies that inhibit vectorization, the compiler will attempt to isolate the offending operations into a new loop. This optimization is not enabled by default, only loops marked with the pragma are considered.

    #pragma clang loop distribute(enable)
    for (i = 0; i < N; ++i) {
      S1: A[i + 1] = A[i] + B[i];
      S2: C[i] = D[i] * E[i];
    }

This loop will be split into two loops between statements S1 and S2. The second loop containing S2 will be vectorized.

Loop Distribution is currently not enabled by default in the optimizer because it can hurt performance in some cases. For example, instruction-level parallelism could be reduced by sequentializing the execution of the statements S1 and S2 above.

If Loop Distribution is turned on globally with `-mllvm -enable-loop-distribution`, specifying `distribute(disable)` can be used the disable it on a per-loop basis.

### Additional Information

For convenience multiple loop hints can be specified on a single line.

    #pragma clang loop vectorize_width(4) interleave_count(8)
    for(...) {
      ...
    }

If an optimization cannot be applied any hints that apply to it will be ignored. For example, the hint `vectorize_width(4)` is ignored if the loop is not proven safe to vectorize. To identify and diagnose optimization issues use -Rpass, -Rpass-missed, and -Rpass-analysis command line options. See the user guide for details.

## Extensions to specify floating-point flags

The `#pragma clang fp` pragma allows floating-point options to be specified for a section of the source code. This pragma can only appear at file scope or at the start of a compound statement (excluding comments). When using within a compound statement, the pragma is active within the scope of the compound statement.

Currently, the following settings can be controlled with this pragma:

`#pragma clang fp reassociate` allows control over the reassociation of floating point expressions. When enabled, this pragma allows the expression `x + (y + z)` to be reassociated as `(x + y) + z`. Reassociation can also occur across multiple statements. This pragma can be used to disable reassociation when it is otherwise enabled for the translation unit with the `-fassociative-math` flag. The pragma can take two values: `on` and `off`.

    float f(float x, float y, float z)
    {
      // Enable floating point reassociation across statements
      #pragma clang fp reassociate(on)
      float t = x + y;
      float v = t + z;
    }

`#pragma clang fp reciprocal` allows control over using reciprocal approximations in floating point expressions. When enabled, this pragma allows the expression `x / y` to be approximated as `x * (1.0 / y)`. This pragma can be used to disable reciprocal approximation when it is otherwise enabled for the translation unit with the `-freciprocal-math` flag or other fast-math options. The pragma can take two values: `on` and `off`.

    float f(float x, float y)
    {
      // Enable floating point reciprocal approximation
      #pragma clang fp reciprocal(on)
      return x / y;
    }

`#pragma clang fp contract` specifies whether the compiler should contract a multiply and an addition (or subtraction) into a fused FMA operation when supported by the target.

The pragma can take three values: `on`, `fast` and `off`. The `on` option is identical to using `#pragma STDC FP_CONTRACT(ON)` and it allows fusion as specified the language standard. The `fast` option allows fusion in cases when the language standard does not make this possible (e.g. across statements in C).

    for(...) {
      #pragma clang fp contract(fast)
      a = b[i] * c[i];
      d[i] += a;
    }

The pragma can also be used with `off` which turns FP contraction off for a section of the code. This can be useful when fast contraction is otherwise enabled for the translation unit with the `-ffp-contract=fast-honor-pragmas` flag. Note that `-ffp-contract=fast` will override pragmas to fuse multiply and addition across statements regardless of any controlling pragmas.

`#pragma clang fp exceptions` specifies floating point exception behavior. It may take one of the values: `ignore`, `maytrap` or `strict`. Meaning of these values is same as for constrained floating point intrinsics.

    {
      // Preserve floating point exceptions
      #pragma clang fp exceptions(strict)
      z = x + y;
      if (fetestexcept(FE_OVERFLOW))
        ...
    }

A `#pragma clang fp` pragma may contain any number of options:

    void func(float *dest, float a, float b) {
      #pragma clang fp exceptions(maytrap) contract(fast) reassociate(on)
      ...
    }

`#pragma clang fp eval_method` allows floating-point behavior to be specified for a section of the source code. This pragma can appear at file or namespace scope, or at the start of a compound statement (excluding comments). The pragma is active within the scope of the compound statement.

When `pragma clang fp eval_method(source)` is enabled, the section of code governed by the pragma behaves as though the command-line option `-ffp-eval-method=source` is enabled. Rounds intermediate results to source-defined precision.

When `pragma clang fp eval_method(double)` is enabled, the section of code governed by the pragma behaves as though the command-line option `-ffp-eval-method=double` is enabled. Rounds intermediate results to `double` precision.

When `pragma clang fp eval_method(extended)` is enabled, the section of code governed by the pragma behaves as though the command-line option `-ffp-eval-method=extended` is enabled. Rounds intermediate results to target-dependent `long double` precision. In Win32 programming, for instance, the long double data type maps to the double, 64-bit precision data type.

The full syntax this pragma supports is `#pragma clang fp eval_method(source|double|extended)`.

    for(...) {
      // The compiler will use long double as the floating-point evaluation
      // method.
      #pragma clang fp eval_method(extended)
      a = b[i] * c[i] + e;
    }

Note: `math.h` defines the typedefs `float_t` and `double_t` based on the active evaluation method at the point where the header is included, not where the typedefs are used. Because of this, it is unwise to combine these typedefs with `#pragma clang fp eval_method`. To catch obvious bugs, Clang will emit an error for any references to these typedefs within the scope of this pragma; however, this is not a fool-proof protection, and programmers must take care.

The `#pragma float_control` pragma allows precise floating-point semantics and floating-point exception behavior to be specified for a section of the source code. This pragma can only appear at file or namespace scope, within a language linkage specification or at the start of a compound statement (excluding comments). When used within a compound statement, the pragma is active within the scope of the compound statement. This pragma is modeled after a Microsoft pragma with the same spelling and syntax. For pragmas specified at file or namespace scope, or within a language linkage specification, a stack is supported so that the `pragma float_control` settings can be pushed or popped.

When `pragma float_control(precise, on)` is enabled, the section of code governed by the pragma uses precise floating point semantics, effectively `-ffast-math` is disabled and `-ffp-contract=on` (fused multiply add) is enabled. This pragma enables `-fmath-errno`.

When `pragma float_control(precise, off)` is enabled, unsafe-floating point optimizations are enabled in the section of code governed by the pragma. Effectively `-ffast-math` is enabled and `-ffp-contract=fast`. This pragma disables `-fmath-errno`.

When `pragma float_control(except, on)` is enabled, the section of code governed by the pragma behaves as though the command-line option `-ffp-exception-behavior=strict` is enabled, when `pragma float_control(except, off)` is enabled, the section of code governed by the pragma behaves as though the command-line option `-ffp-exception-behavior=ignore` is enabled.

The full syntax this pragma supports is `float_control(except|precise, on|off [, push])` and `float_control(push|pop)`. The `push` and `pop` forms, including using `push` as the optional third argument, can only occur at file scope.

    for(...) {
      // This block will be compiled with -fno-fast-math and -ffp-contract=on
      #pragma float_control(precise, on)
      a = b[i] * c[i] + e;
    }

## Extensions for controlling atomic code generation

The `[[clang::atomic]]` statement attribute enables users to control how atomic operations are lowered in LLVM IR by conveying additional metadata to the backend. The primary goal is to allow users to specify certain options, like whether the affected atomic operations might be used with specific types of memory or whether to ignore denormal mode correctness in floating-point operations, without affecting the correctness of code that does not rely on these properties.

In LLVM, lowering of atomic operations (e.g., `atomicrmw`) can differ based on the target’s capabilities. Some backends support native atomic instructions only for certain operation types or alignments, or only in specific memory regions. Likewise, floating-point atomic instructions may or may not respect IEEE denormal requirements. When the user is unconcerned about denormal-mode compliance (for performance reasons) or knows that certain atomic operations will not be performed on a particular type of memory, extra hints are needed to tell the backend how to proceed.

A classic example is an architecture where floating-point atomic add does not fully conform to IEEE denormal-mode handling. If the user does not mind ignoring that aspect, they would prefer to emit a faster hardware atomic instruction, rather than a fallback or CAS loop. Conversely, on certain GPUs (e.g., AMDGPU), memory accessed via PCIe may only support a subset of atomic operations. To ensure correct and efficient lowering, the compiler must know whether the user needs the atomic operations to work with that type of memory.

The allowed atomic attribute values are now `remote_memory`, `fine_grained_memory`, and `ignore_denormal_mode`, each optionally prefixed with `no_`. The meanings are as follows:

- `remote_memory` means atomic operations may be performed on remote memory, i.e. memory accessed through off-chip interconnects (e.g., PCIe). On ROCm platforms using HIP, remote memory refers to memory accessed via PCIe and is subject to specific atomic operation support. See ROCm PCIe Atomics for further details. Prefixing with `no_remote_memory` indicates that atomic operations should not be performed on remote memory.

- `fine_grained_memory` means atomic operations may be performed on fine-grained memory, i.e. memory regions that support fine-grained coherence, where updates to memory are visible to other parts of the system even while modifications are ongoing. For example, in HIP, fine-grained coherence ensures that host and device share up-to-date data without explicit synchronization (see HIP Definition). Similarly, OpenCL 2.0 provides fine-grained synchronization in shared virtual memory allocations, allowing concurrent modifications by host and device (see OpenCL 2.0 Overview). Prefixing with `no_fine_grained_memory` indicates that atomic operations should not be performed on fine-grained memory.

- `ignore_denormal_mode` means that atomic operations are allowed to ignore correctness for denormal mode in floating-point operations, potentially improving performance on architectures that handle denormals inefficiently. The negated form, if specified as `no_ignore_denormal_mode`, would enforce strict denormal mode correctness.

Any unspecified option is inherited from the global defaults, which can be set by a compiler flag or the target’s built-in defaults.

Within the same atomic attribute, duplicate and conflicting values are accepted, and the last of any conflicting values wins. Multiple atomic attributes are allowed for the same compound statement, and the last atomic attribute wins.

Without any atomic metadata, LLVM IR defaults to conservative settings for correctness: atomic operations enforce denormal mode correctness and are assumed to potentially use remote and fine-grained memory (i.e., the equivalent of `remote_memory`, `fine_grained_memory`, and `no_ignore_denormal_mode`).

The attribute may be applied only to a compound statement and looks like:

    [[clang::atomic(remote_memory, fine_grained_memory, ignore_denormal_mode)]]
    {
        // Atomic instructions in this block carry extra metadata reflecting
        // these user-specified options.
    }

A new compiler option now globally sets the defaults for these atomic-lowering options. The command-line format has changed to:

    $ clang -fatomic-remote-memory -fno-atomic-fine-grained-memory -fatomic-ignore-denormal-mode file.cpp

Each option has a corresponding flag: `-fatomic-remote-memory` / `-fno-atomic-remote-memory`, `-fatomic-fine-grained-memory` / `-fno-atomic-fine-grained-memory`, and `-fatomic-ignore-denormal-mode` / `-fno-atomic-ignore-denormal-mode`.

Code using the `[[clang::atomic]]` attribute can then selectively override the command-line defaults on a per-block basis. For instance:

    // Suppose the global defaults assume:
    //   remote_memory, fine_grained_memory, and no_ignore_denormal_mode
    // (for conservative correctness)

    void example() {
        // Locally override the settings: disable remote_memory and enable
        // fine_grained_memory.
        [[clang::atomic(no_remote_memory, fine_grained_memory)]]
        {
            // In this block:
            //   - Atomic operations are not performed on remote memory.
            //   - Atomic operations are performed on fine-grained memory.
            //   - The setting for denormal mode remains as the global default
            //     (typically no_ignore_denormal_mode, enforcing strict denormal mode correctness).
            // ...
        }
    }

Function bodies do not accept statement attributes, so this will not work:

    void func() [[clang::atomic(remote_memory)]] {  // Wrong: applies to function type
    }

Use the attribute on a compound statement within the function:

    void func() {
        [[clang::atomic(remote_memory)]]
        {
            // Atomic operations in this block carry the specified metadata.
        }
    }

The `[[clang::atomic]]` attribute affects only the code generation of atomic instructions within the annotated compound statement. Clang attaches target-specific metadata to those atomic instructions in the emitted LLVM IR to guide backend lowering. This metadata is fixed at the Clang code generation phase and is not modified by later LLVM passes (such as function inlining).

For example, consider:

    inline void func() {
      [[clang::atomic(remote_memory)]]
      {
        // Atomic instructions lowered with metadata.
      }
    }

    void foo() {
      [[clang::atomic(no_remote_memory)]]
      {
        func(); // Inlined by LLVM, but the metadata from 'func()' remains unchanged.
      }
    }

Although current usage focuses on AMDGPU, the mechanism is general. Other backends can ignore or implement their own responses to these flags if desired. If a target does not understand or enforce these hints, the IR remains valid, and the resulting program is still correct (although potentially less optimized for that user’s needs).

## Specifying an attribute for multiple declarations (#pragma clang attribute)

The `#pragma clang attribute` directive can be used to apply an attribute to multiple declarations. The `#pragma clang attribute push` variation of the directive pushes a new “scope” of `#pragma clang attribute` that attributes can be added to. The `#pragma clang attribute (...)` variation adds an attribute to that scope, and the `#pragma clang attribute pop` variation pops the scope. You can also use `#pragma clang attribute push (...)`, which is a shorthand for when you want to add one attribute to a new scope. Multiple push directives can be nested inside each other.

The attributes that are used in the `#pragma clang attribute` directives can be written using the GNU-style syntax:

    #pragma clang attribute push (__attribute__((annotate("custom"))), apply_to = function)

    void function(); // The function now has the annotate("custom") attribute

    #pragma clang attribute pop

The attributes can also be written using the C++11 style syntax:

    #pragma clang attribute push ([[noreturn]], apply_to = function)

    void function(); // The function now has the [[noreturn]] attribute

    #pragma clang attribute pop

The `__declspec` style syntax is also supported:

    #pragma clang attribute push (__declspec(dllexport), apply_to = function)

    void function(); // The function now has the __declspec(dllexport) attribute

    #pragma clang attribute pop

A single push directive can contain multiple attributes, however, only one syntax style can be used within a single directive:

    #pragma clang attribute push ([[noreturn, noinline]], apply_to = function)

    void function1(); // The function now has the [[noreturn]] and [[noinline]] attributes

    #pragma clang attribute pop

    #pragma clang attribute push (__attribute((noreturn, noinline)), apply_to = function)

    void function2(); // The function now has the __attribute((noreturn)) and __attribute((noinline)) attributes

    #pragma clang attribute pop

Because multiple push directives can be nested, if you’re writing a macro that expands to `_Pragma("clang attribute")` it’s good hygiene (though not required) to add a namespace to your push/pop directives. A pop directive with a namespace will pop the innermost push that has that same namespace. This will ensure that another macro’s `pop` won’t inadvertently pop your attribute. Note that an `pop` without a namespace will pop the innermost `push` without a namespace. `push``es with a namespace can only be popped by ``pop` with the same namespace. For instance:

    #define ASSUME_NORETURN_BEGIN _Pragma("clang attribute AssumeNoreturn.push ([[noreturn]], apply_to = function)")
    #define ASSUME_NORETURN_END   _Pragma("clang attribute AssumeNoreturn.pop")

    #define ASSUME_UNAVAILABLE_BEGIN _Pragma("clang attribute Unavailable.push (__attribute__((unavailable)), apply_to=function)")
    #define ASSUME_UNAVAILABLE_END   _Pragma("clang attribute Unavailable.pop")

    ASSUME_NORETURN_BEGIN
    ASSUME_UNAVAILABLE_BEGIN
    void function(); // function has [[noreturn]] and __attribute__((unavailable))
    ASSUME_NORETURN_END
    void other_function(); // function has __attribute__((unavailable))
    ASSUME_UNAVAILABLE_END

Without the namespaces on the macros, `other_function` will be annotated with `[[noreturn]]` instead of `__attribute__((unavailable))`. This may seem like a contrived example, but its very possible for this kind of situation to appear in real code if the pragmas are spread out across a large file. You can test if your version of clang supports namespaces on `#pragma clang attribute` with `__has_extension(pragma_clang_attribute_namespaces)`.

### Subject Match Rules

The set of declarations that receive a single attribute from the attribute stack depends on the subject match rules that were specified in the pragma. Subject match rules are specified after the attribute. The compiler expects an identifier that corresponds to the subject set specifier. The `apply_to` specifier is currently the only supported subject set specifier. It allows you to specify match rules that form a subset of the attribute’s allowed subject set, i.e. the compiler doesn’t require all of the attribute’s subjects. For example, an attribute like `[[nodiscard]]` whose subject set includes `enum`, `record` and `hasType(functionType)`, requires the presence of at least one of these rules after `apply_to`:

    #pragma clang attribute push([[nodiscard]], apply_to = enum)

    enum Enum1 { A1, B1 }; // The enum will receive [[nodiscard]]

    struct Record1 { }; // The struct will *not* receive [[nodiscard]]

    #pragma clang attribute pop

    #pragma clang attribute push([[nodiscard]], apply_to = any(record, enum))

    enum Enum2 { A2, B2 }; // The enum will receive [[nodiscard]]

    struct Record2 { }; // The struct *will* receive [[nodiscard]]

    #pragma clang attribute pop

    // This is an error, since [[nodiscard]] can't be applied to namespaces:
    #pragma clang attribute push([[nodiscard]], apply_to = any(record, namespace))

    #pragma clang attribute pop

Multiple match rules can be specified using the `any` match rule, as shown in the example above. The `any` rule applies attributes to all declarations that are matched by at least one of the rules in the `any`. It doesn’t nest and can’t be used inside the other match rules. Redundant match rules or rules that conflict with one another should not be used inside of `any`. Failing to specify a rule within the `any` rule results in an error.

Clang supports the following match rules:

- `function`: Can be used to apply attributes to functions. This includes C++ member functions, static functions, operators, and constructors/destructors.

- `function(is_member)`: Can be used to apply attributes to C++ member functions. This includes members like static functions, operators, and constructors/destructors.

- `hasType(functionType)`: Can be used to apply attributes to functions, C++ member functions, and variables/fields whose type is a function pointer. It does not apply attributes to Objective-C methods or blocks.

- `type_alias`: Can be used to apply attributes to `typedef` declarations and C++11 type aliases.

- `record`: Can be used to apply attributes to `struct`, `class`, and `union` declarations.

- `record(unless(is_union))`: Can be used to apply attributes only to `struct` and `class` declarations.

- `enum`: Can be used to apply attributes to enumeration declarations.

- `enum_constant`: Can be used to apply attributes to enumerators.

- `variable`: Can be used to apply attributes to variables, including local variables, parameters, global variables, and static member variables. It does not apply attributes to instance member variables or Objective-C ivars.

- `variable(is_thread_local)`: Can be used to apply attributes to thread-local variables only.

- `variable(is_global)`: Can be used to apply attributes to global variables only.

- `variable(is_local)`: Can be used to apply attributes to local variables only.

- `variable(is_parameter)`: Can be used to apply attributes to parameters only.

- `variable(unless(is_parameter))`: Can be used to apply attributes to all the variables that are not parameters.

- `field`: Can be used to apply attributes to non-static member variables in a record. This includes Objective-C ivars.

- `namespace`: Can be used to apply attributes to `namespace` declarations.

- `objc_interface`: Can be used to apply attributes to `@interface` declarations.

- `objc_protocol`: Can be used to apply attributes to `@protocol` declarations.

- `objc_category`: Can be used to apply attributes to category declarations, including class extensions.

- `objc_method`: Can be used to apply attributes to Objective-C methods, including instance and class methods. Implicit methods like implicit property getters and setters do not receive the attribute.

- `objc_method(is_instance)`: Can be used to apply attributes to Objective-C instance methods.

- `objc_property`: Can be used to apply attributes to `@property` declarations.

- `block`: Can be used to apply attributes to block declarations. This does not include variables/fields of block pointer type.

The use of `unless` in match rules is currently restricted to a strict set of sub-rules that are used by the supported attributes. That means that even though `variable(unless(is_parameter))` is a valid match rule, `variable(unless(is_thread_local))` is not.

### Supported Attributes

Not all attributes can be used with the `#pragma clang attribute` directive. Notably, statement attributes like `[[fallthrough]]` or type attributes like `address_space` aren’t supported by this directive. You can determine whether or not an attribute is supported by the pragma by referring to the individual documentation for that attribute.

The attributes are applied to all matching declarations individually, even when the attribute is semantically incorrect. The attributes that aren’t applied to any declaration are not verified semantically.

## Specifying section names for global objects (#pragma clang section)

The `#pragma clang section` directive provides a means to assign section-names to global variables, functions and static variables.

The section names can be specified as:

    #pragma clang section bss="myBSS" data="myData" rodata="myRodata" relro="myRelro" text="myText"

The section names can be reverted back to default name by supplying an empty string to the section kind, for example:

    #pragma clang section bss="" data="" text="" rodata="" relro=""

The `#pragma clang section` directive obeys the following rules:

- The pragma applies to all global variable, statics and function declarations from the pragma to the end of the translation unit.

- The pragma clang section is enabled automatically, without need of any flags.

- This feature is only defined to work sensibly for ELF, Mach-O and COFF targets.

- If section name is specified through _attribute_((section(“myname”))), then the attribute name gains precedence.

- Global variables that are initialized to zero will be placed in the named bss section, if one is present.

- The `#pragma clang section` directive does not try to infer section-kind from the name. For example, naming a section “`.bss.mySec`” does NOT mean it will be a bss section name.

- The decision about which section-kind applies to each global is taken in the back-end. Once the section-kind is known, appropriate section name, as specified by the user using `#pragma clang section` directive, is applied to that global.

## Specifying Linker Options on ELF Targets

The `#pragma comment(lib, ...)` directive is supported on all ELF targets. The second parameter is the library name (without the traditional Unix prefix of `lib`). This allows you to provide an implicit link of dependent libraries.

## Evaluating Object Size

Clang supports the builtins `__builtin_object_size` and `__builtin_dynamic_object_size`. The semantics are compatible with GCC’s builtins of the same names, but the details are slightly different.

    size_t __builtin_[dynamic_]object_size(const void *ptr, int type)

Returns the number of accessible bytes `n` past `ptr`. The value returned depends on `type`, which is required to be an integer constant between 0 and 3:

- If `type & 2 == 0`, the least `n` is returned such that accesses to `(const char*)ptr + n` and beyond are known to be out of bounds. This is `(size_t)-1` if no better bound is known.

- If `type & 2 == 2`, the greatest `n` is returned such that accesses to `(const char*)ptr + i` are known to be in bounds, for 0 <= `i` < `n`. This is `(size_t)0` if no better bound is known.

    char small[10], large[100];
    bool cond;
    // Returns 100: writes of more than 100 bytes are known to be out of bounds.
    int n100 = __builtin_object_size(cond ? small : large, 0);
    // Returns 10: writes of 10 or fewer bytes are known to be in bounds.
    int n10 = __builtin_object_size(cond ? small : large, 2);

- If `type & 1 == 0`, pointers are considered to be in bounds if they point into the same storage as `ptr` – that is, the same stack object, global variable, or heap allocation.

- If `type & 1 == 1`, pointers are considered to be in bounds if they point to the same subobject that `ptr` points to. If `ptr` points to an array element, other elements of the same array, but not of enclosing arrays, are considered in bounds.

    struct X { char a, b, c; } x;
    static_assert(__builtin_object_size(&x, 0) == 3);
    static_assert(__builtin_object_size(&x.b, 0) == 2);
    static_assert(__builtin_object_size(&x.b, 1) == 1);

    char a[10][10][10];
    static_assert(__builtin_object_size(&a, 1) == 1000);
    static_assert(__builtin_object_size(&a[1], 1) == 900);
    static_assert(__builtin_object_size(&a[1][1], 1) == 90);
    static_assert(__builtin_object_size(&a[1][1][1], 1) == 9);

The values returned by this builtin are a best effort conservative approximation of the correct answers. When `type & 2 == 0`, the true value is less than or equal to the value returned by the builtin, and when `type & 2 == 1`, the true value is greater than or equal to the value returned by the builtin.

For `__builtin_object_size`, the value is determined entirely at compile time. With optimization enabled, better results will be produced, especially when the call to `__builtin_object_size` is in a different function from the formation of the pointer. Unlike in GCC, enabling optimization in Clang does not allow more information about subobjects to be determined, so the `type & 1 == 1` case will often give imprecise results when used across a function call boundary even when optimization is enabled.

The pass_object_size and pass_dynamic_object_size attributes can be used to invisibly pass the object size for a pointer parameter alongside the pointer in a function call. This allows more precise object sizes to be determined both when building without optimizations and in the `type & 1 == 1` case.

For `__builtin_dynamic_object_size`, the result is not limited to being a compile time constant. Instead, a small amount of runtime evaluation is permitted to determine the size of the object, in order to give a more precise result. `__builtin_dynamic_object_size` is meant to be used as a drop-in replacement for `__builtin_object_size` in libraries that support it. For instance, here is a program that `__builtin_dynamic_object_size` will make safer:

    void copy_into_buffer(size_t size) {
      char* buffer = malloc(size);
      strlcpy(buffer, "some string", strlen("some string"));
      // Previous line preprocesses to:
      // __builtin___strlcpy_chk(buffer, "some string", strlen("some string"), __builtin_object_size(buffer, 0))
    }

Since the size of `buffer` can’t be known at compile time, Clang will fold `__builtin_object_size(buffer, 0)` into `-1`. However, if this was written as `__builtin_dynamic_object_size(buffer, 0)`, Clang will fold it into `size`, providing some extra runtime safety.

## Deprecating Macros

Clang supports the pragma `#pragma clang deprecated`, which can be used to provide deprecation warnings for macro uses. For example:

    #define MIN(x, y) x < y ? x : y
    #pragma clang deprecated(MIN, "use std::min instead")

    int min(int a, int b) {
      return MIN(a, b); // warning: MIN is deprecated: use std::min instead
    }

`#pragma clang deprecated` should be preferred for this purpose over `#pragma GCC warning` because the warning can be controlled with `-Wdeprecated`.

## Restricted Expansion Macros

Clang supports the pragma `#pragma clang restrict_expansion`, which can be used restrict macro expansion in headers. This can be valuable when providing headers with ABI stability requirements. Any expansion of the annotated macro processed by the preprocessor after the `#pragma` annotation will log a warning. Redefining the macro or undefining the macro will not be diagnosed, nor will expansion of the macro within the main source file. For example:

    #define TARGET_ARM 1
    #pragma clang restrict_expansion(TARGET_ARM, "<reason>")

    /// Foo.h
    struct Foo {
    #if TARGET_ARM // warning: TARGET_ARM is marked unsafe in headers: <reason>
      uint32_t X;
    #else
      uint64_t X;
    #endif
    };

    /// main.c
    #include "foo.h"
    #if TARGET_ARM // No warning in main source file
    X_TYPE uint32_t
    #else
    X_TYPE uint64_t
    #endif

This warning is controlled by `-Wpedantic-macros`.

## Final Macros

Clang supports the pragma `#pragma clang final`, which can be used to mark macros as final, meaning they cannot be undef’d or re-defined. For example:

    #define FINAL_MACRO 1
    #pragma clang final(FINAL_MACRO)

    #define FINAL_MACRO // warning: FINAL_MACRO is marked final and should not be redefined
    #undef FINAL_MACRO  // warning: FINAL_MACRO is marked final and should not be undefined

This is useful for enforcing system-provided macros that should not be altered in user headers or code. This is controlled by `-Wpedantic-macros`. Final macros will always warn on redefinition, including situations with identical bodies and in system headers.

## Line Control

Clang supports an extension for source line control, which takes the form of a preprocessor directive starting with an unsigned integral constant. In addition to the standard `#line` directive, this form allows control of an include stack and header file type, which is used in issuing diagnostics. These lines are emitted in preprocessed output.

    # <line:number> <filename:string> <header-type:numbers>

The filename is optional, and if unspecified indicates no change in source filename. The header-type is an optional, whitespace-delimited, sequence of magic numbers as follows.

- `1:` Push the current source file name onto the include stack and enter a new file.

- `2`: Pop the include stack and return to the specified file. If the filename is `""`, the name popped from the include stack is used. Otherwise there is no requirement that the specified filename matches the current source when originally pushed.

- `3`: Enter a system-header region. System headers often contain implementation-specific source that would normally emit a diagnostic.

- `4`: Enter an implicit `extern "C"` region. This is not required on modern systems where system headers are C++-aware.

At most a single `1` or `2` can be present, and values must be in ascending order.

Examples are:

    # 57 // Advance (or return) to line 57 of the current source file
    # 57 "frob" // Set to line 57 of "frob"
    # 1 "foo.h" 1 // Enter "foo.h" at line 1
    # 59 "main.c" 2 // Leave current include and return to "main.c"
    # 1 "/usr/include/stdio.h" 1 3 // Enter a system header
    # 60 "" 2 // return to "main.c"
    # 1 "/usr/ancient/header.h" 1 4 // Enter an implicit extern "C" header

## Intrinsics Support within Constant Expressions

The following builtin intrinsics can be used in constant expressions:

- `__builtin_addcb`

- `__builtin_addcs`

- `__builtin_addc`

- `__builtin_addcl`

- `__builtin_addcll`

- `__builtin_bitreverse8`

- `__builtin_bitreverse16`

- `__builtin_bitreverse32`

- `__builtin_bitreverse64`

- `__builtin_bswap16`

- `__builtin_bswap32`

- `__builtin_bswap64`

- `__builtin_clrsb`

- `__builtin_clrsbl`

- `__builtin_clrsbll`

- `__builtin_clz`

- `__builtin_clzl`

- `__builtin_clzll`

- `__builtin_clzs`

- `__builtin_clzg`

- `__builtin_ctz`

- `__builtin_ctzl`

- `__builtin_ctzll`

- `__builtin_ctzs`

- `__builtin_ctzg`

- `__builtin_ffs`

- `__builtin_ffsl`

- `__builtin_ffsll`

- `__builtin_fmax`

- `__builtin_fmin`

- `__builtin_fpclassify`

- `__builtin_inf`

- `__builtin_isinf`

- `__builtin_isinf_sign`

- `__builtin_isfinite`

- `__builtin_isnan`

- `__builtin_isnormal`

- `__builtin_nan`

- `__builtin_nans`

- `__builtin_parity`

- `__builtin_parityl`

- `__builtin_parityll`

- `__builtin_popcount`

- `__builtin_popcountl`

- `__builtin_popcountll`

- `__builtin_popcountg`

- `__builtin_rotateleft8`

- `__builtin_rotateleft16`

- `__builtin_rotateleft32`

- `__builtin_rotateleft64`

- `__builtin_rotateright8`

- `__builtin_rotateright16`

- `__builtin_rotateright32`

- `__builtin_rotateright64`

- `__builtin_subcb`

- `__builtin_subcs`

- `__builtin_subc`

- `__builtin_subcl`

- `__builtin_subcll`

The following x86-specific intrinsics can be used in constant expressions:

- `_addcarry_u32`

- `_addcarry_u64`

- `_bit_scan_forward`

- `_bit_scan_reverse`

- `__bsfd`

- `__bsfq`

- `__bsrd`

- `__bsrq`

- `__bswap`

- `__bswapd`

- `__bswap64`

- `__bswapq`

- `_castf32_u32`

- `_castf64_u64`

- `_castu32_f32`

- `_castu64_f64`

- `__lzcnt16`

- `__lzcnt`

- `__lzcnt64`

- `_mm_popcnt_u32`

- `_mm_popcnt_u64`

- `_popcnt32`

- `_popcnt64`

- `__popcntd`

- `__popcntq`

- `__popcnt16`

- `__popcnt`

- `__popcnt64`

- `__rolb`

- `__rolw`

- `__rold`

- `__rolq`

- `__rorb`

- `__rorw`

- `__rord`

- `__rorq`

- `_rotl`

- `_rotr`

- `_rotwl`

- `_rotwr`

- `_lrotl`

- `_lrotr`

- `_subborrow_u32`

- `_subborrow_u64`

## Debugging the Compiler

Clang supports a number of pragma directives that help debugging the compiler itself. Syntax is the following: #pragma clang __debug <command> <arguments>. Note, all of debugging pragmas are subject to change.

### dump

Accepts either a single identifier or an expression. When a single identifier is passed, the lookup results for the identifier are printed to stderr. When an expression is passed, the AST for the expression is printed to stderr. The expression is an unevaluated operand, so things like overload resolution and template instantiations are performed, but the expression has no runtime effects. Type- and value-dependent expressions are not supported yet.

This facility is designed to aid with testing name lookup machinery.

## Predefined Macros

### __GCC_DESTRUCTIVE_SIZE and __GCC_CONSTRUCTIVE_SIZE

Specify the mimum offset between two objects to avoid false sharing and the maximum size of contiguous memory to promote true sharing, respectively. These macros are predefined in all C and C++ language modes, but can be redefined on the command line with `-D` to specify different values as needed or can be undefined on the command line with `-U` to disable support for the feature.

Note: the values the macros expand to are not guaranteed to be stable. They are are affected by architectures and CPU tuning flags, can change between releases of Clang and will not match the values defined by other compilers such as GCC.

Compiling different TUs depending on these flags (including use of `std::hardware_constructive_interference` or `std::hardware_destructive_interference`) with different compilers, macro definitions, or architecture flags will lead to ODR violations and should be avoided.

## `#embed` Parameters

### `clang::offset`

The `clang::offset` embed parameter may appear zero or one time in the embed parameter sequence. Its preprocessor argument clause shall be present and have the form:

..code-block: text

> ( constant-expression )

and shall be an integer constant expression. The integer constant expression shall not evaluate to a value less than 0. The token `defined` shall not appear within the constant expression.

The offset will be used when reading the contents of the embedded resource to specify the starting offset to begin embedding from. The resources is treated as being empty if the specified offset is larger than the number of bytes in the resource. The offset will be applied before any `limit` parameters are applied.

## Union and aggregate initialization in C

In C23 (N2900), when an object is initialized from initializer `= {}`, all elements of arrays, all members of structs, and the first members of unions are empty-initialized recursively. In addition, all padding bits are initialized to zero.

Clang guarantees the following behaviors:

- `1:` Clang supports initializer `= {}` mentioned above in all C standards.

- `2:` When unions are initialized from initializer `= {}`, bytes outside of the first members of unions are also initialized to zero.

- `3:` When unions, structures and arrays are initialized from initializer `= { initializer-list }`, all members not explicitly initialized in the initializer list are empty-initialized recursively. In addition, all padding bits are initialized to zero.

Currently, the above extension only applies to C source code, not C++.

## Empty Objects in C

The declaration of a structure or union type which has no named members is undefined behavior (C23 and earlier) or implementation-defined behavior (C2y). Clang allows the declaration of a structure or union type with no named members in all C language modes. sizeof for such a type returns 0, which is different behavior than in C++ (where the size of such an object is typically 1).

## Qualified function types in C

Declaring a function with a qualified type in C is undefined behavior (C23 and earlier) or implementation-defined behavior (C2y). Clang allows a function type to be specified with the `const` and `volatile` qualifiers, but ignores the qualifications.

    typedef int f(void);
    const volatile f func; // Qualifier on function type has no effect.

Note, Clang does not allow an `_Atomic` function type because of explicit constraints against atomically qualified (arrays and) function types.

## Underspecified Object Declarations in C

C23 introduced the notion of underspecified object declarations (note, the final standards text is different from WG14 N3006 due to changes during national body comment review). When an object is declared with the `constexpr` storage class specifier or has a deduced type (with the `auto` specifier), it is said to be “underspecified”. Underspecified declarations have different requirements than non-underspecified declarations. In particular, the identifier being declared cannot be used in its initialization. e.g.,

    auto x = x; // Invalid
    constexpr int y = y; // Invalid

The standard leaves it implementation-defined whether an underspecified declaration may introduce additional identifiers as part of the declaration.

Clang allows additional identifiers to be declared in the following cases:

- A compound literal may introduce a new type. e.g.,

    auto x = (struct S { int x, y; }){ 1, 2 };      // Accepted by Clang
    constexpr int i = (struct T { int x; }){ 1 }.x; // Accepted by Clang

- The type specifier for a `constexpr` declaration may define a new type. e.g.,

    constexpr struct S { int x; } s = { 1 }; // Accepted by Clang

- A function declarator may be declared with parameters, including parameters which introduce a new type. e.g.,

    constexpr int (*fp)(int x) = nullptr;              // Accepted by Clang
    auto f = (void (*)(struct S { int x; } s))nullptr; // Accepted by Clang

- The initializer may contain a GNU statement expression which defines new types or objects. e.g.,

    constexpr int i = ({                              // Accepted by Clang
      constexpr int x = 12;
      constexpr struct S { int x; } s = { x };
      s.x;
    });
    auto x = ({ struct S { int x; } s = { 0 }; s; }); // Accepted by Clang

Clang intentionally does not implement the changed scoping rules from C23 for underspecified declarations. Doing so would significantly complicate the implementation in order to get reasonable diagnostic behavior and also means Clang fails to reject some code that should be rejected. e.g.,

    // This should be rejected because 'x' is not in scope within the initializer
    // of an underspecified declaration. Clang accepts because it treats the scope
    // of the identifier as beginning immediately after the declarator, same as with
    // a non-underspecified declaration.
    constexpr int x = sizeof(x);

    // Clang rejects this code with a diagnostic about using the variable within its
    // own initializer rather than rejecting the code with an undeclared identifier
    // diagnostic.
    auto x = x;