Clang Language Extensions — Clang 8 documentation

# Clang 8 documentation

## Clang Language Extensions

- Introduction
- Feature Checking Macros
- Include File Checking Macros
- Builtin Macros
- Vectors and Extended Vectors
- Half-Precision Floating Point
- Messages on `deprecated` and `unavailable` Attributes
- Attributes on Enumerators
- ‘User-Specified’ System Frameworks
- Checks for Standard Language Features
- Checks for Type Trait Primitives
- Blocks
- Objective-C Features
- Initializer lists for complex numbers in C
- Builtin Functions
- Non-standard C++11 Attributes
- Target-Specific Extensions
- Extensions for Static Analysis
- Extensions for Dynamic Analysis
- Extensions for selectively disabling optimization
- Extensions for loop hint optimizations
- Extensions to specify floating-point flags
- Specifying an attribute for multiple declarations (#pragma clang attribute)
- Specifying section names for global objects (#pragma clang section)
- Specifying Linker Options on ELF Targets

## 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. 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
    ...

### `__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++2a 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 compliation 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.
`__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)`”.

## Vectors and Extended Vectors

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

OpenCL vector types are created using `ext_vector_type` attribute. It support for `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_extension(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;
    }

### 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
--- | --- | --- | --- | ---
[] | yes | yes | yes | –
unary operators +, – | yes | yes | yes | –
++, – – | yes | yes | yes | –
+,–,*,/,% | yes | yes | yes | –
bitwise operators &,|,^,~ | yes | yes | yes | –
>>,<< | yes | yes | yes | –
!, &&, || | yes | – | – | –
==, !=, >, <, >=, <= | yes | yes | – | –
= | yes | yes | yes | yes
:? | yes | – | – | –
sizeof | yes | yes | yes | yes
C-style cast | yes | yes | yes | no
reinterpret_cast | yes | no | yes | no
static_cast | yes | no | yes | no
const_cast | no | no | no | no

See also __builtin_shufflevector, __builtin_convertvector.

## Half-Precision Floating Point

Clang supports two half-precision (16-bit) floating point types: `__fp16` and `_Float16`. 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) - SPIR `_Float16` will be supported on more targets as they define ABIs for it.

`__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 extended 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)`.

## ‘User-Specified’ System Frameworks

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

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

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

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

## Checks for Standard Language Features

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

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

### C++98

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

#### C++ exceptions

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

#### C++ RTTI

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

### C++11

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

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

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

#### C++11 alias templates

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

#### C++11 alignment specifiers

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

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

#### C++11 attributes

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

#### C++11 generalized constant expressions

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

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

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

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

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

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

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

#### C++11 delegating constructors

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

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

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

#### C++11 explicit conversion functions

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

#### C++11 generalized initializers

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

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

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

#### C++11 inheriting constructors

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

#### C++11 inline namespaces

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

#### C++11 lambdas

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

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

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

#### C++11 noexcept

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

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

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

#### C++11 `nullptr`

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

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

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

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

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

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

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

#### C++11 raw string literals

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

#### C++11 rvalue references

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

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

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

#### C++11 `thread_local`

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

#### C++11 type inference

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

#### C++11 strongly typed enumerations

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

#### C++11 trailing return type

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

#### C++11 Unicode string literals

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

#### C++11 unrestricted unions

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

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

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

#### C++11 variadic templates

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

### C++14

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

#### C++14 binary literals

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

#### C++14 contextual conversions

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

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

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

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

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

#### C++14 digit separators

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

#### C++14 generalized lambda capture

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

#### C++14 generic lambdas

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

#### C++14 relaxed constexpr

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

#### C++14 return type deduction

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

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

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

#### C++14 variable templates

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

### 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.

## Checks for 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.

Feature detection is supported only for some of the primitives at present. User code should not use these checks because they bear no direct relation to the actual set of type traits supported by the C++ standard library.

For type trait `__X`, `__has_extension(X)` indicates the presence of the type trait primitive in the compiler. A simplistic usage example as might be seen in standard C++ headers follows:

    #if __has_extension(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

The following type trait primitives are supported by Clang:

- `__has_nothrow_assign` (GNU, Microsoft)
- `__has_nothrow_copy` (GNU, Microsoft)
- `__has_nothrow_constructor` (GNU, Microsoft)
- `__has_trivial_assign` (GNU, Microsoft)
- `__has_trivial_copy` (GNU, Microsoft)
- `__has_trivial_constructor` (GNU, Microsoft)
- `__has_trivial_destructor` (GNU, Microsoft)
- `__has_virtual_destructor` (GNU, Microsoft)
- `__is_abstract` (GNU, Microsoft)
- `__is_aggregate` (GNU, Microsoft)
- `__is_base_of` (GNU, Microsoft)
- `__is_class` (GNU, Microsoft)
- `__is_convertible_to` (Microsoft)
- `__is_empty` (GNU, Microsoft)
- `__is_enum` (GNU, Microsoft)
- `__is_interface_class` (Microsoft)
- `__is_pod` (GNU, Microsoft)
- `__is_polymorphic` (GNU, Microsoft)
- `__is_union` (GNU, Microsoft)
- `__is_literal(type)`: Determines whether the given type is a literal type
- `__is_final`: Determines whether the given type is declared with a `final` class-virt-specifier.
- `__underlying_type(type)`: Retrieves the underlying type for a given `enum` type. This trait is required to implement the C++11 standard library.
- `__is_trivially_assignable(totype, fromtype)`: Determines whether a value of type `totype` can be assigned to from a value of type `fromtype` such that no non-trivial functions are called as part of that assignment. This trait is required to implement the C++11 standard library.
- `__is_trivially_constructible(type, argtypes...)`: Determines whether a value of type `type` can be direct-initialized with arguments of types `argtypes...` such that no non-trivial functions are called as part of that initialization. This trait is required to implement the C++11 standard library.
- `__is_destructible` (MSVC 2013)
- `__is_nothrow_destructible` (MSVC 2013)
- `__is_nothrow_assignable` (MSVC 2013, clang)
- `__is_constructible` (MSVC 2013, clang)
- `__is_nothrow_constructible` (MSVC 2013, clang)
- `__is_assignable` (MSVC 2015, clang)
- `__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.

## 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)`.

## 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.)

## 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_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_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_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`.

### `__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.

`__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.

### `__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)`.

### `__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_operator_new` and `__builtin_operator_delete`

`__builtin_operator_new` allocates memory just like a non-placement non-class new-expression. This is exactly like directly calling the normal non-placement `::operator new`, 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).

Likewise, `__builtin_operator_delete` deallocates memory just like a non-class delete-expression, and is exactly like directly calling the normal `::operator delete`, except that it permits optimizations. Only the unsized form of `__builtin_operator_delete` is currently available.

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

### 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 expressable 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 `<string.h>` header:

- `memchr`
- `memcmp`
- `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).

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

### 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`

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 http://wg21.link/P0057. The following four are intended to be used by the standard library to implement std::experimental::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)
    bool   __builtin_coro_param(void *original, void *copy)

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.

## 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

## 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(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.

## 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, unrolling and distribution. Loop hints can be specified before any loop and will be ignored if the optimization is not safe to apply.

### Vectorization and Interleaving

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_)` and 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)`.

### 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 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, only FP contraction can be controlled with the pragma. `#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 fusiong 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` flag.

## 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 accepts only one attribute regardless of the syntax used.

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`.

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_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" 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=""

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.