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_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.)

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 C2x 6.2.5p19), 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.

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

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_ceil(T x)	return the smallest integral value greater than or equal to x	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_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_trunc(T x)	return the integral value nearest to but no larger in magnitude than x	floating point types
T __builtin_elementwise_max(T x, T y)	return x or y, whichever is larger	integer and floating point types
T __builtin_elementwise_min(T x, T y)	return x or y, whichever is smaller	integer and 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

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.

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 x or y, whichever is larger; If exactly one argument is a NaN, return the other argument. If both arguments are NaNs, fmax() return a NaN.	integer and floating point types
ET __builtin_reduce_min(VT a)	return x or y, whichever is smaller; If exactly one argument is a NaN, return the other argument. If both arguments are NaNs, fmax() return a NaN.	integer and floating point types
ET __builtin_reduce_add(VT a)	+	integer and floating point types
ET __builtin_reduce_mul(VT a)	*	integer and floating point 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

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.

__fp16 is supported on every target, as it is purely a storage format; see below. _Float16 is currently only supported on the following targets, with further targets pending ABI standardization:

• 32-bit ARM

• 64-bit ARM (AArch64)

• AMDGPU

• SPIR

• X86 (see below)

On X86 targets, _Float16 is supported as long as SSE2 is available, which includes all 64-bit and all recent 32-bit processors. When the target supports AVX512-FP16, _Float16 arithmetic is performed using that native support. Otherwise, _Float16 arithmetic is performed by promoting to float, performing the operation, and then truncating to _Float16.

_Float16 will be supported on more targets as they define ABIs for it.

__bf16 is purely a storage format; it is currently only supported on the following targets: * 32-bit ARM * 64-bit ARM (AArch64) * X86 (see below)

On X86 targets, __bf16 is supported as long as SSE2 is available, which includes all 64-bit and all recent 32-bit processors.

The __bf16 type is only available when supported in hardware.

__fp16 is a storage and interchange format only. This means that values of __fp16 are immediately promoted to (at least) float when used in arithmetic operations, so that e.g. the result of adding two __fp16 values has type float. The behavior of __fp16 is specified by the ARM C Language Extensions (ACLE). Clang uses the binary16 format from IEEE 754-2008 for __fp16, not the ARM alternative format.

_Float16 is an interchange floating-point type. This means that, just like arithmetic on float or double, arithmetic on _Float16 operands is formally performed in the _Float16 type, so that e.g. the result of adding two _Float16 values has type _Float16. The behavior of _Float16 is specified by ISO/IEC TS 18661-3:2015 (“Floating-point extensions for C”). As with __fp16, Clang uses the binary16 format from IEEE 754-2008 for _Float16.

_Float16 arithmetic will be performed using native half-precision support when available on the target (e.g. on ARMv8.2a); otherwise it will be performed at a higher precision (currently always float) and then truncated down to _Float16. Note that C and C++ allow intermediate floating-point operands of an expression to be computed with greater precision than is expressible in their type, so Clang may avoid intermediate truncations in certain cases; this may lead to results that are inconsistent with native arithmetic.

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 defaulted 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.

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.

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.

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.

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).

• __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_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_convertible_to (Microsoft): Synonym for __is_convertible.

• __is_destructible (C++, MSVC 2013): Only available in -fms-extensions mode.

• __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_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) Only available in -fms-extensions mode.

• __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_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_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): 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 and types which were made trivially relocatable via the clang::trivial_abi attribute.

• __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.

• __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

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

In addition to the functionality provided by GCC’s extended assembly, clang supports output constraints with the goto form.

The goto form of GCC’s extended assembly allows the programmer to branch to a C label from within an inline assembly block. Clang extends this behavior by allowing the programmer to use output constraints:

int foo(int x) {
    int y;
    asm goto("# %0 %1 %l2" : "=r"(y) : "r"(x) : : err);
    return y;
  err:
    return -1;
}

It’s important to note that outputs are valid only on the “fallthrough” branch. Using outputs on an indirect branch may result in undefined behavior. For example, in the function above, use of the value assigned to y in the err block is undefined behavior.

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;
}

Query for this feature with __has_extension(gnu_asm_goto_with_outputs).

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. 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.

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 an 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 -mmacosx-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 -mmacosx-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;
  NotPod() : a(0), b(0) {}
};

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

kernel void kernel_main(
  Pod a,
#pragma OPENCL EXTENSION __cl_clang_non_portable_kernel_param_types : enable
  OnlySL b,
  global NotSL *c,
#pragma OPENCL EXTENSION __cl_clang_non_portable_kernel_param_types : disable
  global OnlySL *d,
);

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);
}

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_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_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 the struct 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 corresopnding 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>.

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
__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.

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.

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 developper 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 persistance 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&.

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_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_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 bultin 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 );

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_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.

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).

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.

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 or aquire-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.)

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 __builin_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, these builtins are also implemented by GCC.

Syntax:

const char *__builtin_FILE();
const char *__builtin_FUNCTION();
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, and __builtin_FILE return the values, at the “invocation point”, for __LINE__, __FUNCTION__, and __FILE__ 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_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.

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

Target-Specific Extensions

Clang supports some language features conditionally on some targets.

ARM/AArch64 Language Extensions

Memory Barrier Intrinsics

Clang implements the __dmb, __dsb and __isb intrinsics as defined in the ARM C Language Extensions Release 2.0. 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.

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 source-level annotations.

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 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;
}

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.

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;
}

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 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 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 does 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 Dynamically

Clang supports the builtin __builtin_dynamic_object_size, the semantics are the same as GCC’s __builtin_object_size (which Clang also supports), but __builtin_dynamic_object_size can evaluate the object’s size at runtime. __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")

void 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

Extended Integer Types

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.

Intrinsics Support within Constant Expressions

The following builtin intrinsics can be used in constant expressions:

• __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_ctz

• __builtin_ctzl

• __builtin_ctzll

• __builtin_ctzs

• __builtin_ffs

• __builtin_ffsl

• __builtin_ffsll

• __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_rotateleft8

• __builtin_rotateleft16

• __builtin_rotateleft32

• __builtin_rotateleft64

• __builtin_rotateright8

• __builtin_rotateright16

• __builtin_rotateright32

• __builtin_rotateright64

The following x86-specific intrinsics can be used in constant expressions:

• _bit_scan_forward

• _bit_scan_reverse

• __bsfd

• __bsfq

• __bsrd

• __bsrq

• __bswap

• __bswapd

• __bswap64

• __bswapq

• _castf32_u32

• _castf64_u64

• _castu32_f32

• _castu64_f64

• _mm_popcnt_u32

• _mm_popcnt_u64

• _popcnt32

• _popcnt64

• __popcntd

• __popcntq

• __rolb

• __rolw

• __rold

• __rolq

• __rorb

• __rorw

• __rord

• __rorq

• _rotl

• _rotr

• _rotwl

• _rotwr

• _lrotl

• _lrotr