| ========================= |
| Clang Language Extensions |
| ========================= |
| |
| .. contents:: |
| :local: |
| :depth: 1 |
| |
| .. 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 |
| <http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html>`_ for more information on |
| these extensions. |
| |
| .. _langext-feature_check: |
| |
| 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: |
| |
| .. code-block:: c++ |
| |
| #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 |
| ... |
| |
| .. _langext-__has_feature-__has_extension: |
| |
| ``__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: |
| |
| .. code-block:: c++ |
| |
| #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 |
| |
| .. _langext-has-feature-back-compat: |
| |
| 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: |
| |
| .. code-block:: c++ |
| |
| #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: |
| |
| .. code-block:: c++ |
| |
| #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: |
| |
| .. code-block:: c++ |
| |
| #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: |
| |
| .. code-block:: c++ |
| |
| ... |
| #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. |
| |
| .. _langext-__has_include: |
| |
| ``__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: |
| |
| .. code-block:: c++ |
| |
| // 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)``: |
| |
| .. code-block:: c++ |
| |
| // 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 |
| |
| .. _langext-__has_include_next: |
| |
| ``__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: |
| |
| .. code-block:: c++ |
| |
| // 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. |
| |
| .. code-block:: c++ |
| |
| #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)``". |
| |
| .. _langext-vectors: |
| |
| 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: |
| |
| .. code-block:: c++ |
| |
| 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: |
| |
| .. code-block:: c++ |
| |
| 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: |
| |
| .. code-block:: c++ |
| |
| 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: |
| |
| .. code-block:: c++ |
| |
| 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: |
| |
| .. code-block:: c++ |
| |
| 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: |
| |
| .. code-block:: c++ |
| |
| 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: |
| |
| .. code-block:: c++ |
| |
| 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: |
| |
| .. code-block:: none |
| |
| .../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 <http://clang.llvm.org/cxx_status.html#ts>`_ 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 |
| <http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1170>`_. |
| |
| C++11 alias templates |
| ^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_alias_templates)`` or |
| ``__has_extension(cxx_alias_templates)`` to determine if support for C++11's |
| alias declarations and alias templates is enabled. |
| |
| C++11 alignment specifiers |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_alignas)`` or ``__has_extension(cxx_alignas)`` to |
| determine if support for alignment specifiers using ``alignas`` is enabled. |
| |
| Use ``__has_feature(cxx_alignof)`` or ``__has_extension(cxx_alignof)`` to |
| determine if support for the ``alignof`` keyword is enabled. |
| |
| C++11 attributes |
| ^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_attributes)`` or ``__has_extension(cxx_attributes)`` to |
| determine if support for attribute parsing with C++11's square bracket notation |
| is enabled. |
| |
| C++11 generalized constant expressions |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_constexpr)`` to determine if support for generalized |
| constant expressions (e.g., ``constexpr``) is enabled. |
| |
| C++11 ``decltype()`` |
| ^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_decltype)`` or ``__has_extension(cxx_decltype)`` to |
| determine if support for the ``decltype()`` specifier is enabled. C++11's |
| ``decltype`` does not require type-completeness of a function call expression. |
| Use ``__has_feature(cxx_decltype_incomplete_return_types)`` or |
| ``__has_extension(cxx_decltype_incomplete_return_types)`` to determine if |
| support for this feature is enabled. |
| |
| C++11 default template arguments in function templates |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_default_function_template_args)`` or |
| ``__has_extension(cxx_default_function_template_args)`` to determine if support |
| for default template arguments in function templates is enabled. |
| |
| C++11 ``default``\ ed functions |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_defaulted_functions)`` or |
| ``__has_extension(cxx_defaulted_functions)`` to determine if support for |
| defaulted function definitions (with ``= default``) is enabled. |
| |
| C++11 delegating constructors |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_delegating_constructors)`` to determine if support for |
| delegating constructors is enabled. |
| |
| C++11 ``deleted`` functions |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_deleted_functions)`` or |
| ``__has_extension(cxx_deleted_functions)`` to determine if support for deleted |
| function definitions (with ``= delete``) is enabled. |
| |
| C++11 explicit conversion functions |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_explicit_conversions)`` to determine if support for |
| ``explicit`` conversion functions is enabled. |
| |
| C++11 generalized initializers |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_generalized_initializers)`` to determine if support for |
| generalized initializers (using braced lists and ``std::initializer_list``) is |
| enabled. |
| |
| C++11 implicit move constructors/assignment operators |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_implicit_moves)`` to determine if Clang will implicitly |
| generate move constructors and move assignment operators where needed. |
| |
| C++11 inheriting constructors |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_inheriting_constructors)`` to determine if support for |
| inheriting constructors is enabled. |
| |
| C++11 inline namespaces |
| ^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_inline_namespaces)`` or |
| ``__has_extension(cxx_inline_namespaces)`` to determine if support for inline |
| namespaces is enabled. |
| |
| C++11 lambdas |
| ^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_lambdas)`` or ``__has_extension(cxx_lambdas)`` to |
| determine if support for lambdas is enabled. |
| |
| C++11 local and unnamed types as template arguments |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_local_type_template_args)`` or |
| ``__has_extension(cxx_local_type_template_args)`` to determine if support for |
| local and unnamed types as template arguments is enabled. |
| |
| C++11 noexcept |
| ^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_noexcept)`` or ``__has_extension(cxx_noexcept)`` to |
| determine if support for noexcept exception specifications is enabled. |
| |
| C++11 in-class non-static data member initialization |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_nonstatic_member_init)`` to determine whether in-class |
| initialization of non-static data members is enabled. |
| |
| C++11 ``nullptr`` |
| ^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_nullptr)`` or ``__has_extension(cxx_nullptr)`` to |
| determine if support for ``nullptr`` is enabled. |
| |
| C++11 ``override control`` |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_override_control)`` or |
| ``__has_extension(cxx_override_control)`` to determine if support for the |
| override control keywords is enabled. |
| |
| C++11 reference-qualified functions |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_reference_qualified_functions)`` or |
| ``__has_extension(cxx_reference_qualified_functions)`` to determine if support |
| for reference-qualified functions (e.g., member functions with ``&`` or ``&&`` |
| applied to ``*this``) is enabled. |
| |
| C++11 range-based ``for`` loop |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_range_for)`` or ``__has_extension(cxx_range_for)`` to |
| determine if support for the range-based for loop is enabled. |
| |
| C++11 raw string literals |
| ^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_raw_string_literals)`` to determine if support for raw |
| string literals (e.g., ``R"x(foo\bar)x"``) is enabled. |
| |
| C++11 rvalue references |
| ^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_rvalue_references)`` or |
| ``__has_extension(cxx_rvalue_references)`` to determine if support for rvalue |
| references is enabled. |
| |
| C++11 ``static_assert()`` |
| ^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_static_assert)`` or |
| ``__has_extension(cxx_static_assert)`` to determine if support for compile-time |
| assertions using ``static_assert`` is enabled. |
| |
| C++11 ``thread_local`` |
| ^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_thread_local)`` to determine if support for |
| ``thread_local`` variables is enabled. |
| |
| C++11 type inference |
| ^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_auto_type)`` or ``__has_extension(cxx_auto_type)`` to |
| determine C++11 type inference is supported using the ``auto`` specifier. If |
| this is disabled, ``auto`` will instead be a storage class specifier, as in C |
| or C++98. |
| |
| C++11 strongly typed enumerations |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_strong_enums)`` or |
| ``__has_extension(cxx_strong_enums)`` to determine if support for strongly |
| typed, scoped enumerations is enabled. |
| |
| C++11 trailing return type |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_trailing_return)`` or |
| ``__has_extension(cxx_trailing_return)`` to determine if support for the |
| alternate function declaration syntax with trailing return type is enabled. |
| |
| C++11 Unicode string literals |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_unicode_literals)`` to determine if support for Unicode |
| string literals is enabled. |
| |
| C++11 unrestricted unions |
| ^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_unrestricted_unions)`` to determine if support for |
| unrestricted unions is enabled. |
| |
| C++11 user-defined literals |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_user_literals)`` to determine if support for |
| user-defined literals is enabled. |
| |
| C++11 variadic templates |
| ^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Use ``__has_feature(cxx_variadic_templates)`` or |
| ``__has_extension(cxx_variadic_templates)`` to determine if support for |
| variadic templates is enabled. |
| |
| C++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 <http://clang.llvm.org/docs/Modules.html>`_. |
| |
| 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 |
| <http://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html>`_ and a subset of the |
| `Microsoft Visual C++ Type traits |
| <http://msdn.microsoft.com/en-us/library/ms177194(v=VS.100).aspx>`_. |
| |
| 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: |
| |
| .. code-block:: c++ |
| |
| #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 |
| :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: |
| |
| .. code-block:: objc |
| |
| @interface NSObject |
| + (id)alloc; |
| - (id)init; |
| @end |
| |
| @interface NSArray : NSObject |
| @end |
| |
| and this common initialization pattern |
| |
| .. code-block:: objc |
| |
| 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. |
| |
| .. code-block:: objc |
| |
| @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: |
| |
| .. code-block:: objc |
| |
| @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. |
| |
| .. _objc-fixed-enum: |
| |
| 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: |
| |
| .. code-block:: c++ |
| |
| 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: |
| |
| .. code-block:: objc |
| |
| - (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``): |
| |
| .. code-block:: objc |
| |
| 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., |
| |
| .. code-block:: objc |
| |
| 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. |
| |
| .. _langext-objc-retain-release: |
| |
| Objective-C retaining behavior attributes |
| ----------------------------------------- |
| |
| In Objective-C, functions and methods are generally assumed to follow the |
| `Cocoa Memory Management |
| <http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html>`_ |
| 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 <http://clang-analyzer.llvm.org>`_ 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: |
| |
| .. code-block:: objc |
| |
| 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. |
| |
| .. code-block:: objc |
| |
| 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 |
| <http://clang-analyzer.llvm.org/annotations.html#cocoa_mem>`_. |
| |
| 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)``. |
| |
| .. _langext-overloading: |
| |
| Initializer lists for complex numbers in C |
| ========================================== |
| |
| clang supports an extension which allows the following in C: |
| |
| .. code-block:: 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: |
| |
| .. code-block:: c++ |
| |
| 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**: |
| |
| .. code-block:: c++ |
| |
| __builtin_assume(bool) |
| |
| **Example of Use**: |
| |
| .. code-block:: c++ |
| |
| 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**: |
| |
| .. code-block:: c++ |
| |
| __builtin_readcyclecounter() |
| |
| **Example of Use**: |
| |
| .. code-block:: c++ |
| |
| 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. |
| |
| .. _langext-__builtin_shufflevector: |
| |
| ``__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**: |
| |
| .. code-block:: c++ |
| |
| __builtin_shufflevector(vec1, vec2, index1, index2, ...) |
| |
| **Examples**: |
| |
| .. code-block:: c++ |
| |
| // 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)``. |
| |
| .. _langext-__builtin_convertvector: |
| |
| ``__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**: |
| |
| .. code-block:: c++ |
| |
| __builtin_convertvector(src_vec, dst_vec_type) |
| |
| **Examples**: |
| |
| .. code-block:: c++ |
| |
| 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_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**: |
| |
| .. code-block:: c++ |
| |
| __builtin_unreachable() |
| |
| **Example of use**: |
| |
| .. code-block:: c++ |
| |
| 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**: |
| |
| .. code-block:: c++ |
| |
| __builtin_unpredictable(long long) |
| |
| **Example of use**: |
| |
| .. code-block:: c++ |
| |
| 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**: |
| |
| .. code-block:: c++ |
| |
| type __sync_swap(type *ptr, type value, ...) |
| |
| **Example of Use**: |
| |
| .. code-block:: c++ |
| |
| 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**: |
| |
| .. code-block:: c++ |
| |
| 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: |
| |
| .. code-block:: c |
| |
| 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: |
| |
| .. code-block:: c |
| |
| 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: |
| |
| .. code-block:: c |
| |
| 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: |
| |
| .. code-block:: c |
| |
| 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: |
| |
| .. code-block:: c |
| |
| 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. |
| |
| .. _langext-__c11_atomic: |
| |
| __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. |
| |
| .. code-block:: c |
| |
| 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. |
| |
| .. code-block:: c |
| |
| 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. |
| |
| 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 |
| <http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_, `GCC variable |
| attributes <http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html>`_, and |
| `GCC type attributes |
| <http://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html>`_). 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``. |
| |
| .. code-block:: c++ |
| |
| [[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 |
| <http://infocenter.arm.com/help/topic/com.arm.doc.ihi0053c/IHI0053C_acle_2_0.pdf>`_. |
| 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: |
| |
| .. code-block:: c++ |
| |
| #define GS_RELATIVE __attribute__((address_space(256))) |
| int foo(int GS_RELATIVE *P) { |
| return *P; |
| } |
| |
| Which compiles to (on X86-32): |
| |
| .. code-block:: gas |
| |
| _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 <http://clang-analyzer.llvm.org/>`_. These attributes are documented |
| in the analyzer's `list of source-level annotations |
| <http://clang-analyzer.llvm.org/annotations.html>`_. |
| |
| |
| 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. |
| |
| .. code-block:: c++ |
| |
| // 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). |
| |
| .. code-block:: c++ |
| |
| #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: |
| |
| .. code-block:: c++ |
| |
| 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, and unrolling. 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. |
| |
| .. code-block:: c++ |
| |
| #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. |
| |
| .. code-block:: c++ |
| |
| #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. |
| |
| .. code-block:: c++ |
| |
| #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. |
| |
| .. code-block:: c++ |
| |
| #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)``. |
| |
| .. code-block:: c++ |
| |
| #pragma clang loop unroll_count(8) |
| for(...) { |
| ... |
| } |
| |
| Unrolling of a loop can be prevented by specifying ``unroll(disable)``. |
| |
| Additional Information |
| ---------------------- |
| |
| For convenience multiple loop hints can be specified on a single line. |
| |
| .. code-block:: c++ |
| |
| #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. |