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Clang Language Extensions

.. toctree::
   :hidden:

   ObjectiveCLiterals
   BlockLanguageSpec
   Block-ABI-Apple
   AutomaticReferenceCounting

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 :ref:`below <langext-has-feature-back-compat>`), 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. :doc:`AddressSanitizer <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 takes a single argument that is the name of a C++11-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. 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         // Optional of course.
  #define __has_cpp_attribute(x) 0  // Compatibility with non-clang compilers.
#endif

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

The attribute identifier (but not scope) can also 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 :ref:`langext-__has_include` and :ref:`langext-__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 :ref:`langext-feature_check`.
__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 :ref:`langext-feature_check`.
__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.

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 :ref:`langext-__builtin_shufflevector`, :ref:`langext-__builtin_convertvector`.

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 defaulted functions

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

C++11 delegating constructors

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

C++11 deleted functions

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

C++11 explicit conversion functions

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

C++11 generalized initializers

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

C++11 implicit move constructors/assignment operators

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

C++11 inheriting constructors

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

C++11 inline namespaces

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

C++11 lambdas

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

C++11 local and unnamed types as template arguments

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

C++11 noexcept

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

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

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

C++11 nullptr

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

C++11 override control

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

C++11 reference-qualified functions

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

C++11 range-based for loop

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

C++11 raw string literals

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

C++11 rvalue references

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

C++11 static_assert()

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

C++11 thread_local

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

C++11 type inference

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

C++11 strongly typed enumerations

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

C++11 trailing return type

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

C++11 Unicode string literals

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

C++11 unrestricted unions

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

C++11 user-defined literals

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

C++11 variadic templates

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

C++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 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++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.

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 :ref:`a set of builtins <langext-__c11_atomic>` 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_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)

Blocks

The syntax and high level language feature description is in :doc:`BlockLanguageSpec<BlockLanguageSpec>`. Implementation and ABI details for the clang implementation are in :doc:`Block-ABI-Apple<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 :doc:`automated reference counting <AutomaticReferenceCounting>` 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 :doc:`Object Literals and Subscripting <ObjectiveCLiterals>` 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, __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 :ref:`extended vector support <langext-vectors>` 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_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 <strings.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).

__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)

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 :doc:`AddressSanitizer`.

Use __has_feature(thread_sanitizer) to check if the code is being built with :doc:`ThreadSanitizer`.

Use __has_feature(memory_sanitizer) to check if the code is being built with :doc:`MemorySanitizer`.

Use __has_feature(safe_stack) to check if the code is being built with :doc:`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.