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2034 lines
73 KiB
ReStructuredText
=========================
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Clang Language Extensions
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=========================
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.. contents::
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:local:
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:depth: 1
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.. toctree::
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:hidden:
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ObjectiveCLiterals
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BlockLanguageSpec
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Block-ABI-Apple
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AutomaticReferenceCounting
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Introduction
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============
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This document describes the language extensions provided by Clang. In addition
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to the language extensions listed here, Clang aims to support a broad range of
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GCC extensions. Please see the `GCC manual
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<http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html>`_ for more information on
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these extensions.
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.. _langext-feature_check:
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Feature Checking Macros
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=======================
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Language extensions can be very useful, but only if you know you can depend on
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them. In order to allow fine-grain features checks, we support three builtin
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function-like macros. This allows you to directly test for a feature in your
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code without having to resort to something like autoconf or fragile "compiler
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version checks".
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``__has_builtin``
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-----------------
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This function-like macro takes a single identifier argument that is the name of
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a builtin function. It evaluates to 1 if the builtin is supported or 0 if not.
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It can be used like this:
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.. code-block:: c++
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#ifndef __has_builtin // Optional of course.
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#define __has_builtin(x) 0 // Compatibility with non-clang compilers.
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#endif
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...
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#if __has_builtin(__builtin_trap)
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__builtin_trap();
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#else
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abort();
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#endif
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...
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.. _langext-__has_feature-__has_extension:
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``__has_feature`` and ``__has_extension``
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-----------------------------------------
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These function-like macros take a single identifier argument that is the name
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of a feature. ``__has_feature`` evaluates to 1 if the feature is both
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supported by Clang and standardized in the current language standard or 0 if
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not (but see :ref:`below <langext-has-feature-back-compat>`), while
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``__has_extension`` evaluates to 1 if the feature is supported by Clang in the
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current language (either as a language extension or a standard language
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feature) or 0 if not. They can be used like this:
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.. code-block:: c++
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#ifndef __has_feature // Optional of course.
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#define __has_feature(x) 0 // Compatibility with non-clang compilers.
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#endif
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#ifndef __has_extension
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#define __has_extension __has_feature // Compatibility with pre-3.0 compilers.
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#endif
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...
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#if __has_feature(cxx_rvalue_references)
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// This code will only be compiled with the -std=c++11 and -std=gnu++11
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// options, because rvalue references are only standardized in C++11.
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#endif
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#if __has_extension(cxx_rvalue_references)
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// This code will be compiled with the -std=c++11, -std=gnu++11, -std=c++98
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// and -std=gnu++98 options, because rvalue references are supported as a
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// language extension in C++98.
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#endif
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.. _langext-has-feature-back-compat:
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For backward compatibility, ``__has_feature`` can also be used to test
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for support for non-standardized features, i.e. features not prefixed ``c_``,
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``cxx_`` or ``objc_``.
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Another use of ``__has_feature`` is to check for compiler features not related
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to the language standard, such as e.g. :doc:`AddressSanitizer
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<AddressSanitizer>`.
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If the ``-pedantic-errors`` option is given, ``__has_extension`` is equivalent
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to ``__has_feature``.
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The feature tag is described along with the language feature below.
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The feature name or extension name can also be specified with a preceding and
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following ``__`` (double underscore) to avoid interference from a macro with
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the same name. For instance, ``__cxx_rvalue_references__`` can be used instead
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of ``cxx_rvalue_references``.
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``__has_cpp_attribute``
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-----------------------
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This function-like macro takes a single argument that is the name of a
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C++11-style attribute. The argument can either be a single identifier, or a
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scoped identifier. If the attribute is supported, a nonzero value is returned.
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If the attribute is a standards-based attribute, this macro returns a nonzero
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value based on the year and month in which the attribute was voted into the
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working draft. If the attribute is not supported by the current compliation
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target, this macro evaluates to 0. It can be used like this:
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.. code-block:: c++
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#ifndef __has_cpp_attribute // Optional of course.
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#define __has_cpp_attribute(x) 0 // Compatibility with non-clang compilers.
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#endif
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...
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#if __has_cpp_attribute(clang::fallthrough)
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#define FALLTHROUGH [[clang::fallthrough]]
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#else
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#define FALLTHROUGH
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#endif
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...
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The attribute identifier (but not scope) can also be specified with a preceding
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and following ``__`` (double underscore) to avoid interference from a macro with
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the same name. For instance, ``gnu::__const__`` can be used instead of
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``gnu::const``.
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``__has_attribute``
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-------------------
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This function-like macro takes a single identifier argument that is the name of
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a GNU-style attribute. It evaluates to 1 if the attribute is supported by the
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current compilation target, or 0 if not. It can be used like this:
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.. code-block:: c++
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#ifndef __has_attribute // Optional of course.
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#define __has_attribute(x) 0 // Compatibility with non-clang compilers.
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#endif
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...
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#if __has_attribute(always_inline)
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#define ALWAYS_INLINE __attribute__((always_inline))
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#else
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#define ALWAYS_INLINE
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#endif
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...
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The attribute name can also be specified with a preceding and following ``__``
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(double underscore) to avoid interference from a macro with the same name. For
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instance, ``__always_inline__`` can be used instead of ``always_inline``.
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``__has_declspec_attribute``
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----------------------------
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This function-like macro takes a single identifier argument that is the name of
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an attribute implemented as a Microsoft-style ``__declspec`` attribute. It
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evaluates to 1 if the attribute is supported by the current compilation target,
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or 0 if not. It can be used like this:
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.. code-block:: c++
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#ifndef __has_declspec_attribute // Optional of course.
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#define __has_declspec_attribute(x) 0 // Compatibility with non-clang compilers.
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#endif
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...
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#if __has_declspec_attribute(dllexport)
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#define DLLEXPORT __declspec(dllexport)
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#else
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#define DLLEXPORT
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#endif
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...
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The attribute name can also be specified with a preceding and following ``__``
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(double underscore) to avoid interference from a macro with the same name. For
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instance, ``__dllexport__`` can be used instead of ``dllexport``.
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``__is_identifier``
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-------------------
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This function-like macro takes a single identifier argument that might be either
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a reserved word or a regular identifier. It evaluates to 1 if the argument is just
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a regular identifier and not a reserved word, in the sense that it can then be
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used as the name of a user-defined function or variable. Otherwise it evaluates
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to 0. It can be used like this:
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.. code-block:: c++
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...
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#ifdef __is_identifier // Compatibility with non-clang compilers.
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#if __is_identifier(__wchar_t)
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typedef wchar_t __wchar_t;
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#endif
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#endif
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__wchar_t WideCharacter;
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...
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Include File Checking Macros
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============================
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Not all developments systems have the same include files. The
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:ref:`langext-__has_include` and :ref:`langext-__has_include_next` macros allow
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you to check for the existence of an include file before doing a possibly
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failing ``#include`` directive. Include file checking macros must be used
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as expressions in ``#if`` or ``#elif`` preprocessing directives.
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.. _langext-__has_include:
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``__has_include``
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-----------------
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This function-like macro takes a single file name string argument that is the
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name of an include file. It evaluates to 1 if the file can be found using the
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include paths, or 0 otherwise:
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.. code-block:: c++
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// Note the two possible file name string formats.
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#if __has_include("myinclude.h") && __has_include(<stdint.h>)
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# include "myinclude.h"
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#endif
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To test for this feature, use ``#if defined(__has_include)``:
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.. code-block:: c++
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// To avoid problem with non-clang compilers not having this macro.
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#if defined(__has_include)
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#if __has_include("myinclude.h")
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# include "myinclude.h"
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#endif
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#endif
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.. _langext-__has_include_next:
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``__has_include_next``
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----------------------
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This function-like macro takes a single file name string argument that is the
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name of an include file. It is like ``__has_include`` except that it looks for
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the second instance of the given file found in the include paths. It evaluates
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to 1 if the second instance of the file can be found using the include paths,
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or 0 otherwise:
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.. code-block:: c++
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// Note the two possible file name string formats.
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#if __has_include_next("myinclude.h") && __has_include_next(<stdint.h>)
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# include_next "myinclude.h"
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#endif
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// To avoid problem with non-clang compilers not having this macro.
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#if defined(__has_include_next)
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#if __has_include_next("myinclude.h")
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# include_next "myinclude.h"
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#endif
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#endif
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Note that ``__has_include_next``, like the GNU extension ``#include_next``
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directive, is intended for use in headers only, and will issue a warning if
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used in the top-level compilation file. A warning will also be issued if an
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absolute path is used in the file argument.
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``__has_warning``
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-----------------
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This function-like macro takes a string literal that represents a command line
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option for a warning and returns true if that is a valid warning option.
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.. code-block:: c++
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#if __has_warning("-Wformat")
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...
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#endif
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Builtin Macros
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==============
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``__BASE_FILE__``
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Defined to a string that contains the name of the main input file passed to
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Clang.
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``__COUNTER__``
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Defined to an integer value that starts at zero and is incremented each time
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the ``__COUNTER__`` macro is expanded.
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``__INCLUDE_LEVEL__``
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Defined to an integral value that is the include depth of the file currently
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being translated. For the main file, this value is zero.
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``__TIMESTAMP__``
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Defined to the date and time of the last modification of the current source
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file.
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``__clang__``
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Defined when compiling with Clang
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``__clang_major__``
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Defined to the major marketing version number of Clang (e.g., the 2 in
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2.0.1). Note that marketing version numbers should not be used to check for
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language features, as different vendors use different numbering schemes.
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Instead, use the :ref:`langext-feature_check`.
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``__clang_minor__``
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Defined to the minor version number of Clang (e.g., the 0 in 2.0.1). Note
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that marketing version numbers should not be used to check for language
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features, as different vendors use different numbering schemes. Instead, use
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the :ref:`langext-feature_check`.
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``__clang_patchlevel__``
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Defined to the marketing patch level of Clang (e.g., the 1 in 2.0.1).
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``__clang_version__``
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Defined to a string that captures the Clang marketing version, including the
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Subversion tag or revision number, e.g., "``1.5 (trunk 102332)``".
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.. _langext-vectors:
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Vectors and Extended Vectors
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============================
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Supports the GCC, OpenCL, AltiVec and NEON vector extensions.
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OpenCL vector types are created using ``ext_vector_type`` attribute. It
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support for ``V.xyzw`` syntax and other tidbits as seen in OpenCL. An example
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is:
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.. code-block:: c++
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typedef float float4 __attribute__((ext_vector_type(4)));
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typedef float float2 __attribute__((ext_vector_type(2)));
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float4 foo(float2 a, float2 b) {
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float4 c;
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c.xz = a;
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c.yw = b;
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return c;
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}
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Query for this feature with ``__has_extension(attribute_ext_vector_type)``.
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Giving ``-faltivec`` option to clang enables support for AltiVec vector syntax
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and functions. For example:
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.. code-block:: c++
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vector float foo(vector int a) {
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vector int b;
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b = vec_add(a, a) + a;
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return (vector float)b;
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}
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NEON vector types are created using ``neon_vector_type`` and
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``neon_polyvector_type`` attributes. For example:
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.. code-block:: c++
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typedef __attribute__((neon_vector_type(8))) int8_t int8x8_t;
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typedef __attribute__((neon_polyvector_type(16))) poly8_t poly8x16_t;
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int8x8_t foo(int8x8_t a) {
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int8x8_t v;
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v = a;
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return v;
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}
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Vector Literals
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---------------
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Vector literals can be used to create vectors from a set of scalars, or
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vectors. Either parentheses or braces form can be used. In the parentheses
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form the number of literal values specified must be one, i.e. referring to a
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scalar value, or must match the size of the vector type being created. If a
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single scalar literal value is specified, the scalar literal value will be
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replicated to all the components of the vector type. In the brackets form any
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number of literals can be specified. For example:
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.. code-block:: c++
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typedef int v4si __attribute__((__vector_size__(16)));
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typedef float float4 __attribute__((ext_vector_type(4)));
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typedef float float2 __attribute__((ext_vector_type(2)));
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v4si vsi = (v4si){1, 2, 3, 4};
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float4 vf = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
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vector int vi1 = (vector int)(1); // vi1 will be (1, 1, 1, 1).
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vector int vi2 = (vector int){1}; // vi2 will be (1, 0, 0, 0).
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vector int vi3 = (vector int)(1, 2); // error
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vector int vi4 = (vector int){1, 2}; // vi4 will be (1, 2, 0, 0).
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vector int vi5 = (vector int)(1, 2, 3, 4);
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float4 vf = (float4)((float2)(1.0f, 2.0f), (float2)(3.0f, 4.0f));
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Vector Operations
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-----------------
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The table below shows the support for each operation by vector extension. A
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dash indicates that an operation is not accepted according to a corresponding
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specification.
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============================== ======= ======= ======= =======
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Opeator OpenCL AltiVec GCC NEON
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============================== ======= ======= ======= =======
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[] yes yes yes --
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unary operators +, -- yes yes yes --
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++, -- -- yes yes yes --
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+,--,*,/,% yes yes yes --
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bitwise operators &,|,^,~ yes yes yes --
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>>,<< yes yes yes --
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!, &&, || yes -- -- --
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==, !=, >, <, >=, <= yes yes -- --
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= yes yes yes yes
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:? yes -- -- --
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sizeof yes yes yes yes
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C-style cast yes yes yes no
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reinterpret_cast yes no yes no
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static_cast yes no yes no
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const_cast no no no no
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============================== ======= ======= ======= =======
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See also :ref:`langext-__builtin_shufflevector`, :ref:`langext-__builtin_convertvector`.
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Messages on ``deprecated`` and ``unavailable`` Attributes
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=========================================================
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An optional string message can be added to the ``deprecated`` and
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``unavailable`` attributes. For example:
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.. code-block:: c++
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void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));
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If the deprecated or unavailable declaration is used, the message will be
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incorporated into the appropriate diagnostic:
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.. code-block:: c++
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harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!!
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[-Wdeprecated-declarations]
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explode();
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^
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Query for this feature with
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``__has_extension(attribute_deprecated_with_message)`` and
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``__has_extension(attribute_unavailable_with_message)``.
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Attributes on Enumerators
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=========================
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Clang allows attributes to be written on individual enumerators. This allows
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enumerators to be deprecated, made unavailable, etc. The attribute must appear
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after the enumerator name and before any initializer, like so:
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.. code-block:: c++
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enum OperationMode {
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OM_Invalid,
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OM_Normal,
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OM_Terrified __attribute__((deprecated)),
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OM_AbortOnError __attribute__((deprecated)) = 4
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};
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Attributes on the ``enum`` declaration do not apply to individual enumerators.
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Query for this feature with ``__has_extension(enumerator_attributes)``.
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'User-Specified' System Frameworks
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==================================
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Clang provides a mechanism by which frameworks can be built in such a way that
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they will always be treated as being "system frameworks", even if they are not
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present in a system framework directory. This can be useful to system
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framework developers who want to be able to test building other applications
|
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with development builds of their framework, including the manner in which the
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compiler changes warning behavior for system headers.
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Framework developers can opt-in to this mechanism by creating a
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"``.system_framework``" file at the top-level of their framework. That is, the
|
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framework should have contents like:
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.. code-block:: none
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.../TestFramework.framework
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.../TestFramework.framework/.system_framework
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.../TestFramework.framework/Headers
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.../TestFramework.framework/Headers/TestFramework.h
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...
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Clang will treat the presence of this file as an indicator that the framework
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should be treated as a system framework, regardless of how it was found in the
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framework search path. For consistency, we recommend that such files never be
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included in installed versions of the framework.
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|
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Checks for Standard Language Features
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=====================================
|
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|
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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.
|
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|
|
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)``.
|
|
|
|
``__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_umul_overflow(x, y, &result))
|
|
return kErrorCodeHackers;
|
|
...
|
|
use_multiply(result);
|
|
...
|
|
}
|
|
|
|
A complete enumeration of the builtins are:
|
|
|
|
.. code-block:: c
|
|
|
|
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);
|
|
|
|
|
|
.. _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.
|
|
|
|
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-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`.
|
|
|
|
|
|
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(full)`` is specified the unroller will attempt to fully unroll the
|
|
loop if the trip count is known at compile time. If the loop count is not known
|
|
or the fully unrolled code size is greater than the limit specified by the
|
|
`-pragma-unroll-threshold` command line option the loop will be partially
|
|
unrolled subject to the same limit.
|
|
|
|
.. 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 `-pragma-unroll-threshold` limit.
|
|
|
|
.. 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.
|