# Clang 3.5 documentation

## Clang Language Extensions

- Introduction
- Feature Checking Macros
- Include File Checking Macros
- Builtin Macros
- Vectors and Extended Vectors
- 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

## 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_attribute`

This function-like macro takes a single identifier argument that is the name of an 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`.

### `__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 `-faltivec` 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.

Opeator | 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 | –
!, &&, || | no | – | – | –
==, !=, >, <, >=, <= | yes | yes | – | –
= | yes | yes | yes | yes
:? | yes | – | – | –
sizeof | yes | yes | yes | yes

See also __builtin_shufflevector.

## 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.

### 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.

#### 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++1y

The features listed below are part of the committee draft for the C++1y standard. As a result, all these features are enabled with the `-std=c++1y` or `-std=gnu++1y` option when compiling C++ code.

#### C++1y 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++1y contextual conversions

Use `__has_feature(cxx_contextual_conversions)` or `__has_extension(cxx_contextual_conversions)` to determine if the C++1y 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++1y 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++1y 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++1y 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++1y 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++1y 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++1y 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++1y 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++1y 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.

#### 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.

#### 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.

## 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_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): partially implemented
- `__is_nothrow_destructible` (MSVC 2013): partially implemented
- `__is_nothrow_assignable` (MSVC 2013, clang)
- `__is_constructible` (MSVC 2013, clang)
- `__is_nothrow_constructible` (MSVC 2013, clang)

## 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 two 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.

### 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++ 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`, `__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_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) vf[0], (float) vf[1], (float) vf[2], (float) vf[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_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)`.

### `__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_umul_overflow(x, y, &result))
        return kErrorCodeHackers;
      ...
      use_multiply(result);
      ...
    }

A complete enumeration of the builtins are:

    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);

### __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 are:

- `__c11_atomic_init`
- `__c11_atomic_thread_fence`
- `__c11_atomic_signal_fence`
- `__c11_atomic_is_lock_free`
- `__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`

### 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-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 arithmatic 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 off the GS segment

Annotating a pointer with address space #256 causes it to be code generated relative to the X86 GS segment register, and address space #257 causes it to be relative to the X86 FS 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.

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

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)`.

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.