- Introduction
- Feature Checking Macros
- Include File Checking Macros
- Builtin Macros
- Vectors and Extended Vectors
- Half-Precision Floating Point
- Messages on
deprecatedandunavailableAttributes - Attributes on Enumerators
- 'User-Specified' System Frameworks
- Checks for Standard Language Features
- Checks for Type Trait Primitives
- Blocks
- Objective-C Features
- Initializer lists for complex numbers in C
- Builtin Functions
- Non-standard C++11 Attributes
- Target-Specific Extensions
- Extensions for Static Analysis
- Extensions for Dynamic Analysis
- Extensions for selectively disabling optimization
- Extensions for loop hint optimizations
- Extensions to specify floating-point flags
- Specifying an attribute for multiple declarations (#pragma clang attribute)
- Specifying section names for global objects (#pragma clang section)
- Specifying Linker Options on ELF Targets
- Evaluating Object Size Dynamically
.. toctree:: :hidden: ObjectiveCLiterals BlockLanguageSpec Block-ABI-Apple AutomaticReferenceCounting
This document describes the language extensions provided by Clang. In addition to the language extensions listed here, Clang aims to support a broad range of GCC extensions. Please see the GCC manual for more information on these extensions.
Language extensions can be very useful, but only if you know you can depend on them. In order to allow fine-grain features checks, we support three builtin function-like macros. This allows you to directly test for a feature in your code without having to resort to something like autoconf or fragile "compiler version checks".
This function-like macro takes a single identifier argument that is the name of a builtin function. It evaluates to 1 if the builtin is supported or 0 if not. It can be used like this:
#ifndef __has_builtin // Optional of course.
#define __has_builtin(x) 0 // Compatibility with non-clang compilers.
#endif
...
#if __has_builtin(__builtin_trap)
__builtin_trap();
#else
abort();
#endif
...These function-like macros take a single identifier argument that is the name
of a feature. __has_feature evaluates to 1 if the feature is both
supported by Clang and standardized in the current language standard or 0 if
not (but see :ref:`below <langext-has-feature-back-compat>`), while
__has_extension evaluates to 1 if the feature is supported by Clang in the
current language (either as a language extension or a standard language
feature) or 0 if not. They can be used like this:
#ifndef __has_feature // Optional of course.
#define __has_feature(x) 0 // Compatibility with non-clang compilers.
#endif
#ifndef __has_extension
#define __has_extension __has_feature // Compatibility with pre-3.0 compilers.
#endif
...
#if __has_feature(cxx_rvalue_references)
// This code will only be compiled with the -std=c++11 and -std=gnu++11
// options, because rvalue references are only standardized in C++11.
#endif
#if __has_extension(cxx_rvalue_references)
// This code will be compiled with the -std=c++11, -std=gnu++11, -std=c++98
// and -std=gnu++98 options, because rvalue references are supported as a
// language extension in C++98.
#endifFor backward compatibility, __has_feature can also be used to test
for support for non-standardized features, i.e. features not prefixed c_,
cxx_ or objc_.
Another use of __has_feature is to check for compiler features not related
to the language standard, such as e.g. :doc:`AddressSanitizer
<AddressSanitizer>`.
If the -pedantic-errors option is given, __has_extension is equivalent
to __has_feature.
The feature tag is described along with the language feature below.
The feature name or extension name can also be specified with a preceding and
following __ (double underscore) to avoid interference from a macro with
the same name. For instance, __cxx_rvalue_references__ can be used instead
of cxx_rvalue_references.
This function-like macro is available in C++2a by default, and is provided as an extension in earlier language standards. It takes a single argument that is the name of a double-square-bracket-style attribute. The argument can either be a single identifier or a scoped identifier. If the attribute is supported, a nonzero value is returned. If the attribute is a standards-based attribute, this macro returns a nonzero value based on the year and month in which the attribute was voted into the working draft. See WG21 SD-6 for the list of values returned for standards-based attributes. If the attribute is not supported by the current compliation target, this macro evaluates to 0. It can be used like this:
#ifndef __has_cpp_attribute // For backwards compatibility
#define __has_cpp_attribute(x) 0
#endif
...
#if __has_cpp_attribute(clang::fallthrough)
#define FALLTHROUGH [[clang::fallthrough]]
#else
#define FALLTHROUGH
#endif
...The attribute scope tokens clang and _Clang are interchangeable, as are
the attribute scope tokens gnu and __gnu__. Attribute tokens in either
of these namespaces can be specified with a preceding and following __
(double underscore) to avoid interference from a macro with the same name. For
instance, gnu::__const__ can be used instead of gnu::const.
This function-like macro takes a single argument that is the name of an attribute exposed with the double square-bracket syntax in C mode. The argument can either be a single identifier or a scoped identifier. If the attribute is supported, a nonzero value is returned. If the attribute is not supported by the current compilation target, this macro evaluates to 0. It can be used like this:
#ifndef __has_c_attribute // Optional of course.
#define __has_c_attribute(x) 0 // Compatibility with non-clang compilers.
#endif
...
#if __has_c_attribute(fallthrough)
#define FALLTHROUGH [[fallthrough]]
#else
#define FALLTHROUGH
#endif
...The attribute scope tokens clang and _Clang are interchangeable, as are
the attribute scope tokens gnu and __gnu__. Attribute tokens in either
of these namespaces can be specified with a preceding and following __
(double underscore) to avoid interference from a macro with the same name. For
instance, gnu::__const__ can be used instead of gnu::const.
This function-like macro takes a single identifier argument that is the name of a GNU-style attribute. It evaluates to 1 if the attribute is supported by the current compilation target, or 0 if not. It can be used like this:
#ifndef __has_attribute // Optional of course.
#define __has_attribute(x) 0 // Compatibility with non-clang compilers.
#endif
...
#if __has_attribute(always_inline)
#define ALWAYS_INLINE __attribute__((always_inline))
#else
#define ALWAYS_INLINE
#endif
...The attribute name can also be specified with a preceding and following __
(double underscore) to avoid interference from a macro with the same name. For
instance, __always_inline__ can be used instead of always_inline.
This function-like macro takes a single identifier argument that is the name of
an attribute implemented as a Microsoft-style __declspec attribute. It
evaluates to 1 if the attribute is supported by the current compilation target,
or 0 if not. It can be used like this:
#ifndef __has_declspec_attribute // Optional of course.
#define __has_declspec_attribute(x) 0 // Compatibility with non-clang compilers.
#endif
...
#if __has_declspec_attribute(dllexport)
#define DLLEXPORT __declspec(dllexport)
#else
#define DLLEXPORT
#endif
...The attribute name can also be specified with a preceding and following __
(double underscore) to avoid interference from a macro with the same name. For
instance, __dllexport__ can be used instead of dllexport.
This function-like macro takes a single identifier argument that might be either a reserved word or a regular identifier. It evaluates to 1 if the argument is just a regular identifier and not a reserved word, in the sense that it can then be used as the name of a user-defined function or variable. Otherwise it evaluates to 0. It can be used like this:
...
#ifdef __is_identifier // Compatibility with non-clang compilers.
#if __is_identifier(__wchar_t)
typedef wchar_t __wchar_t;
#endif
#endif
__wchar_t WideCharacter;
...Not all developments systems have the same include files. The
:ref:`langext-__has_include` and :ref:`langext-__has_include_next` macros allow
you to check for the existence of an include file before doing a possibly
failing #include directive. Include file checking macros must be used
as expressions in #if or #elif preprocessing directives.
This function-like macro takes a single file name string argument that is the name of an include file. It evaluates to 1 if the file can be found using the include paths, or 0 otherwise:
// Note the two possible file name string formats.
#if __has_include("myinclude.h") && __has_include(<stdint.h>)
# include "myinclude.h"
#endifTo test for this feature, use #if defined(__has_include):
// To avoid problem with non-clang compilers not having this macro.
#if defined(__has_include)
#if __has_include("myinclude.h")
# include "myinclude.h"
#endif
#endifThis function-like macro takes a single file name string argument that is the
name of an include file. It is like __has_include except that it looks for
the second instance of the given file found in the include paths. It evaluates
to 1 if the second instance of the file can be found using the include paths,
or 0 otherwise:
// Note the two possible file name string formats.
#if __has_include_next("myinclude.h") && __has_include_next(<stdint.h>)
# include_next "myinclude.h"
#endif
// To avoid problem with non-clang compilers not having this macro.
#if defined(__has_include_next)
#if __has_include_next("myinclude.h")
# include_next "myinclude.h"
#endif
#endifNote that __has_include_next, like the GNU extension #include_next
directive, is intended for use in headers only, and will issue a warning if
used in the top-level compilation file. A warning will also be issued if an
absolute path is used in the file argument.
This function-like macro takes a string literal that represents a command line option for a warning and returns true if that is a valid warning option.
#if __has_warning("-Wformat")
...
#endif__BASE_FILE__- Defined to a string that contains the name of the main input file passed to Clang.
__FILE_NAME__- Clang-specific extension that functions similar to
__FILE__but only renders the last path component (the filename) instead of an invocation dependent full path to that file. __COUNTER__- Defined to an integer value that starts at zero and is incremented each time
the
__COUNTER__macro is expanded. __INCLUDE_LEVEL__- Defined to an integral value that is the include depth of the file currently being translated. For the main file, this value is zero.
__TIMESTAMP__- Defined to the date and time of the last modification of the current source file.
__clang__- Defined when compiling with Clang
__clang_major__- Defined to the major marketing version number of Clang (e.g., the 2 in 2.0.1). Note that marketing version numbers should not be used to check for language features, as different vendors use different numbering schemes. Instead, use the :ref:`langext-feature_check`.
__clang_minor__- Defined to the minor version number of Clang (e.g., the 0 in 2.0.1). Note that marketing version numbers should not be used to check for language features, as different vendors use different numbering schemes. Instead, use the :ref:`langext-feature_check`.
__clang_patchlevel__- Defined to the marketing patch level of Clang (e.g., the 1 in 2.0.1).
__clang_version__- Defined to a string that captures the Clang marketing version, including the
Subversion tag or revision number, e.g., "
1.5 (trunk 102332)".
Supports the GCC, OpenCL, AltiVec and NEON vector extensions.
OpenCL vector types are created using ext_vector_type attribute. It
support for V.xyzw syntax and other tidbits as seen in OpenCL. An example
is:
typedef float float4 __attribute__((ext_vector_type(4)));
typedef float float2 __attribute__((ext_vector_type(2)));
float4 foo(float2 a, float2 b) {
float4 c;
c.xz = a;
c.yw = b;
return c;
}Query for this feature with __has_extension(attribute_ext_vector_type).
Giving -maltivec option to clang enables support for AltiVec vector syntax
and functions. For example:
vector float foo(vector int a) {
vector int b;
b = vec_add(a, a) + a;
return (vector float)b;
}NEON vector types are created using neon_vector_type and
neon_polyvector_type attributes. For example:
typedef __attribute__((neon_vector_type(8))) int8_t int8x8_t;
typedef __attribute__((neon_polyvector_type(16))) poly8_t poly8x16_t;
int8x8_t foo(int8x8_t a) {
int8x8_t v;
v = a;
return v;
}Vector literals can be used to create vectors from a set of scalars, or vectors. Either parentheses or braces form can be used. In the parentheses form the number of literal values specified must be one, i.e. referring to a scalar value, or must match the size of the vector type being created. If a single scalar literal value is specified, the scalar literal value will be replicated to all the components of the vector type. In the brackets form any number of literals can be specified. For example:
typedef int v4si __attribute__((__vector_size__(16)));
typedef float float4 __attribute__((ext_vector_type(4)));
typedef float float2 __attribute__((ext_vector_type(2)));
v4si vsi = (v4si){1, 2, 3, 4};
float4 vf = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
vector int vi1 = (vector int)(1); // vi1 will be (1, 1, 1, 1).
vector int vi2 = (vector int){1}; // vi2 will be (1, 0, 0, 0).
vector int vi3 = (vector int)(1, 2); // error
vector int vi4 = (vector int){1, 2}; // vi4 will be (1, 2, 0, 0).
vector int vi5 = (vector int)(1, 2, 3, 4);
float4 vf = (float4)((float2)(1.0f, 2.0f), (float2)(3.0f, 4.0f));The table below shows the support for each operation by vector extension. A dash indicates that an operation is not accepted according to a corresponding specification.
| Operator | OpenCL | AltiVec | GCC | NEON |
|---|---|---|---|---|
| [] | yes | yes | yes | -- |
| unary operators +, -- | yes | yes | yes | -- |
| ++, -- -- | yes | yes | yes | -- |
| +,--,*,/,% | yes | yes | yes | -- |
| bitwise operators &,|,^,~ | yes | yes | yes | -- |
| >>,<< | yes | yes | yes | -- |
| !, &&, || | yes | -- | -- | -- |
| ==, !=, >, <, >=, <= | yes | yes | -- | -- |
| = | yes | yes | yes | yes |
| :? | yes | -- | -- | -- |
| sizeof | yes | yes | yes | yes |
| C-style cast | yes | yes | yes | no |
| reinterpret_cast | yes | no | yes | no |
| static_cast | yes | no | yes | no |
| const_cast | no | no | no | no |
See also :ref:`langext-__builtin_shufflevector`, :ref:`langext-__builtin_convertvector`.
Clang supports two half-precision (16-bit) floating point types: __fp16 and
_Float16. These types are supported in all language modes.
__fp16 is supported on every target, as it is purely a storage format; see below.
_Float16 is currently only supported on the following targets, with further
targets pending ABI standardization:
- 32-bit ARM
- 64-bit ARM (AArch64)
- SPIR
_Float16 will be supported on more targets as they define ABIs for it.
__fp16 is a storage and interchange format only. This means that values of
__fp16 are immediately promoted to (at least) float when used in arithmetic
operations, so that e.g. the result of adding two __fp16 values has type float.
The behavior of __fp16 is specified by the ARM C Language Extensions (ACLE).
Clang uses the binary16 format from IEEE 754-2008 for __fp16, not the ARM
alternative format.
_Float16 is an extended floating-point type. This means that, just like arithmetic on
float or double, arithmetic on _Float16 operands is formally performed in the
_Float16 type, so that e.g. the result of adding two _Float16 values has type
_Float16. The behavior of _Float16 is specified by ISO/IEC TS 18661-3:2015
("Floating-point extensions for C"). As with __fp16, Clang uses the binary16
format from IEEE 754-2008 for _Float16.
_Float16 arithmetic will be performed using native half-precision support
when available on the target (e.g. on ARMv8.2a); otherwise it will be performed
at a higher precision (currently always float) and then truncated down to
_Float16. Note that C and C++ allow intermediate floating-point operands
of an expression to be computed with greater precision than is expressible in
their type, so Clang may avoid intermediate truncations in certain cases; this may
lead to results that are inconsistent with native arithmetic.
It is recommended that portable code use _Float16 instead of __fp16,
as it has been defined by the C standards committee and has behavior that is
more familiar to most programmers.
Because __fp16 operands are always immediately promoted to float, the
common real type of __fp16 and _Float16 for the purposes of the usual
arithmetic conversions is float.
A literal can be given _Float16 type using the suffix f16; for example:
`
3.14f16
`
Because default argument promotion only applies to the standard floating-point
types, _Float16 values are not promoted to double when passed as variadic
or untyped arguments. As a consequence, some caution must be taken when using
certain library facilities with _Float16; for example, there is no printf format
specifier for _Float16, and (unlike float) it will not be implicitly promoted to
double when passed to printf, so the programmer must explicitly cast it to
double before using it with an %f or similar specifier.
An optional string message can be added to the deprecated and
unavailable attributes. For example:
void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));If the deprecated or unavailable declaration is used, the message will be incorporated into the appropriate diagnostic:
harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!!
[-Wdeprecated-declarations]
explode();
^
Query for this feature with
__has_extension(attribute_deprecated_with_message) and
__has_extension(attribute_unavailable_with_message).
Clang allows attributes to be written on individual enumerators. This allows enumerators to be deprecated, made unavailable, etc. The attribute must appear after the enumerator name and before any initializer, like so:
enum OperationMode {
OM_Invalid,
OM_Normal,
OM_Terrified __attribute__((deprecated)),
OM_AbortOnError __attribute__((deprecated)) = 4
};Attributes on the enum declaration do not apply to individual enumerators.
Query for this feature with __has_extension(enumerator_attributes).
Clang provides a mechanism by which frameworks can be built in such a way that they will always be treated as being "system frameworks", even if they are not present in a system framework directory. This can be useful to system framework developers who want to be able to test building other applications with development builds of their framework, including the manner in which the compiler changes warning behavior for system headers.
Framework developers can opt-in to this mechanism by creating a
".system_framework" file at the top-level of their framework. That is, the
framework should have contents like:
.../TestFramework.framework
.../TestFramework.framework/.system_framework
.../TestFramework.framework/Headers
.../TestFramework.framework/Headers/TestFramework.h
...
Clang will treat the presence of this file as an indicator that the framework should be treated as a system framework, regardless of how it was found in the framework search path. For consistency, we recommend that such files never be included in installed versions of the framework.
The __has_feature macro can be used to query if certain standard language
features are enabled. The __has_extension macro can be used to query if
language features are available as an extension when compiling for a standard
which does not provide them. The features which can be tested are listed here.
Since Clang 3.4, the C++ SD-6 feature test macros are also supported.
These are macros with names of the form __cpp_<feature_name>, and are
intended to be a portable way to query the supported features of the compiler.
See the C++ status page for
information on the version of SD-6 supported by each Clang release, and the
macros provided by that revision of the recommendations.
The features listed below are part of the C++98 standard. These features are enabled by default when compiling C++ code.
Use __has_feature(cxx_exceptions) to determine if C++ exceptions have been
enabled. For example, compiling code with -fno-exceptions disables C++
exceptions.
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.
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.
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.
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.
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.
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.
Use __has_feature(cxx_constexpr) to determine if support for generalized
constant expressions (e.g., constexpr) is enabled.
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.
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.
Use __has_feature(cxx_defaulted_functions) or
__has_extension(cxx_defaulted_functions) to determine if support for
defaulted function definitions (with = default) is enabled.
Use __has_feature(cxx_delegating_constructors) to determine if support for
delegating constructors is enabled.
Use __has_feature(cxx_deleted_functions) or
__has_extension(cxx_deleted_functions) to determine if support for deleted
function definitions (with = delete) is enabled.
Use __has_feature(cxx_explicit_conversions) to determine if support for
explicit conversion functions is enabled.
Use __has_feature(cxx_generalized_initializers) to determine if support for
generalized initializers (using braced lists and std::initializer_list) is
enabled.
Use __has_feature(cxx_implicit_moves) to determine if Clang will implicitly
generate move constructors and move assignment operators where needed.
Use __has_feature(cxx_inheriting_constructors) to determine if support for
inheriting constructors is enabled.
Use __has_feature(cxx_inline_namespaces) or
__has_extension(cxx_inline_namespaces) to determine if support for inline
namespaces is enabled.
Use __has_feature(cxx_lambdas) or __has_extension(cxx_lambdas) to
determine if support for lambdas is enabled.
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.
Use __has_feature(cxx_noexcept) or __has_extension(cxx_noexcept) to
determine if support for noexcept exception specifications is enabled.
Use __has_feature(cxx_nonstatic_member_init) to determine whether in-class
initialization of non-static data members is enabled.
Use __has_feature(cxx_nullptr) or __has_extension(cxx_nullptr) to
determine if support for nullptr is enabled.
Use __has_feature(cxx_override_control) or
__has_extension(cxx_override_control) to determine if support for the
override control keywords is enabled.
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.
Use __has_feature(cxx_range_for) or __has_extension(cxx_range_for) to
determine if support for the range-based for loop is enabled.
Use __has_feature(cxx_raw_string_literals) to determine if support for raw
string literals (e.g., R"x(foo\bar)x") is enabled.
Use __has_feature(cxx_rvalue_references) or
__has_extension(cxx_rvalue_references) to determine if support for rvalue
references is enabled.
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.
Use __has_feature(cxx_thread_local) to determine if support for
thread_local variables is enabled.
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.
Use __has_feature(cxx_strong_enums) or
__has_extension(cxx_strong_enums) to determine if support for strongly
typed, scoped enumerations is enabled.
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.
Use __has_feature(cxx_unicode_literals) to determine if support for Unicode
string literals is enabled.
Use __has_feature(cxx_unrestricted_unions) to determine if support for
unrestricted unions is enabled.
Use __has_feature(cxx_user_literals) to determine if support for
user-defined literals is enabled.
Use __has_feature(cxx_variadic_templates) or
__has_extension(cxx_variadic_templates) to determine if support for
variadic templates is enabled.
The features listed below are part of the C++14 standard. As a result, all
these features are enabled with the -std=C++14 or -std=gnu++14 option
when compiling C++ code.
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.
Use __has_feature(cxx_contextual_conversions) or
__has_extension(cxx_contextual_conversions) to determine if the C++14 rules
are used when performing an implicit conversion for an array bound in a
new-expression, the operand of a delete-expression, an integral constant
expression, or a condition in a switch statement.
Use __has_feature(cxx_decltype_auto) or
__has_extension(cxx_decltype_auto) to determine if support
for the decltype(auto) placeholder type is enabled.
Use __has_feature(cxx_aggregate_nsdmi) or
__has_extension(cxx_aggregate_nsdmi) to determine if support
for default initializers in aggregate members is enabled.
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
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; }).
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; }).
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.
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.
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.
Use __has_feature(cxx_variable_templates) or
__has_extension(cxx_variable_templates) to determine if support for
templated variable declarations is enabled.
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.
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.
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.
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.
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.
Use __has_feature(c_thread_local) or __has_extension(c_thread_local)
to determine if support for _Thread_local variables is enabled.
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.
Type trait primitives are special builtin constant expressions that can be used by the standard C++ library to facilitate or simplify the implementation of user-facing type traits in the <type_traits> header.
They are not intended to be used directly by user code because they are implementation-defined and subject to change -- as such they're tied closely to the supported set of system headers, currently:
- LLVM's own libc++
- GNU libstdc++
- The Microsoft standard C++ library
Clang supports the GNU C++ type traits and a subset of the Microsoft Visual C++ Type traits.
Feature detection is supported only for some of the primitives at present. User code should not use these checks because they bear no direct relation to the actual set of type traits supported by the C++ standard library.
For type trait __X, __has_extension(X) indicates the presence of the
type trait primitive in the compiler. A simplistic usage example as might be
seen in standard C++ headers follows:
#if __has_extension(is_convertible_to)
template<typename From, typename To>
struct is_convertible_to {
static const bool value = __is_convertible_to(From, To);
};
#else
// Emulate type trait for compatibility with other compilers.
#endifThe following type trait primitives are supported by Clang:
__has_nothrow_assign(GNU, Microsoft)__has_nothrow_copy(GNU, Microsoft)__has_nothrow_constructor(GNU, Microsoft)__has_trivial_assign(GNU, Microsoft)__has_trivial_copy(GNU, Microsoft)__has_trivial_constructor(GNU, Microsoft)__has_trivial_destructor(GNU, Microsoft)__has_virtual_destructor(GNU, Microsoft)__is_abstract(GNU, Microsoft)__is_aggregate(GNU, Microsoft)__is_base_of(GNU, Microsoft)__is_class(GNU, Microsoft)__is_convertible_to(Microsoft)__is_empty(GNU, Microsoft)__is_enum(GNU, Microsoft)__is_interface_class(Microsoft)__is_pod(GNU, Microsoft)__is_polymorphic(GNU, Microsoft)__is_union(GNU, Microsoft)__is_literal(type): Determines whether the given type is a literal type__is_final: Determines whether the given type is declared with afinalclass-virt-specifier.__underlying_type(type): Retrieves the underlying type for a givenenumtype. This trait is required to implement the C++11 standard library.__is_trivially_assignable(totype, fromtype): Determines whether a value of typetotypecan be assigned to from a value of typefromtypesuch 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 typetypecan be direct-initialized with arguments of typesargtypes...such that no non-trivial functions are called as part of that initialization. This trait is required to implement the C++11 standard library.__is_destructible(MSVC 2013)__is_nothrow_destructible(MSVC 2013)__is_nothrow_assignable(MSVC 2013, clang)__is_constructible(MSVC 2013, clang)__is_nothrow_constructible(MSVC 2013, clang)__is_assignable(MSVC 2015, clang)__reference_binds_to_temporary(T, U)(Clang): Determines whether a reference of typeTbound to an expression of typeUwould bind to a materialized temporary object. IfTis not a reference type the result is false. Note this trait will also return false when the initialization ofTfromUis ill-formed.
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).
According to Cocoa conventions, Objective-C methods with certain names
("init", "alloc", etc.) always return objects that are an instance of
the receiving class's type. Such methods are said to have a "related result
type", meaning that a message send to one of these methods will have the same
static type as an instance of the receiver class. For example, given the
following classes:
@interface NSObject
+ (id)alloc;
- (id)init;
@end
@interface NSArray : NSObject
@endand this common initialization pattern
NSArray *array = [[NSArray alloc] init];the type of the expression [NSArray alloc] is NSArray* because
alloc implicitly has a related result type. Similarly, the type of the
expression [[NSArray alloc] init] is NSArray*, since init has a
related result type and its receiver is known to have the type NSArray *.
If neither alloc nor init had a related result type, the expressions
would have had type id, as declared in the method signature.
A method with a related result type can be declared by using the type
instancetype as its result type. instancetype is a contextual keyword
that is only permitted in the result type of an Objective-C method, e.g.
@interface A
+ (instancetype)constructAnA;
@endThe related result type can also be inferred for some methods. To determine
whether a method has an inferred related result type, the first word in the
camel-case selector (e.g., "init" in "initWithObjects") is considered,
and the method will have a related result type if its return type is compatible
with the type of its class and if:
- the first word is "
alloc" or "new", and the method is a class method, or - the first word is "
autorelease", "init", "retain", or "self", and the method is an instance method.
If a method with a related result type is overridden by a subclass method, the subclass method must also return a type that is compatible with the subclass type. For example:
@interface NSString : NSObject
- (NSUnrelated *)init; // incorrect usage: NSUnrelated is not NSString or a superclass of NSString
@endRelated 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.
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 three
feature macros associated with automatic reference counting:
__has_feature(objc_arc) indicates the availability of automated reference
counting in general, while __has_feature(objc_arc_weak) indicates that
automated reference counting also includes support for __weak pointers to
Objective-C objects. __has_feature(objc_arc_fields) indicates that C structs
are allowed to have fields that are pointers to Objective-C objects managed by
automatic reference counting.
Clang supports ARC-style weak and unsafe references in Objective-C even
outside of ARC mode. Weak references must be explicitly enabled with
the -fobjc-weak option; use __has_feature((objc_arc_weak))
to test whether they are enabled. Unsafe references are enabled
unconditionally. ARC-style weak and unsafe references cannot be used
when Objective-C garbage collection is enabled.
Except as noted below, the language rules for the __weak and
__unsafe_unretained qualifiers (and the weak and
unsafe_unretained property attributes) are just as laid out
in the :doc:`ARC specification <AutomaticReferenceCounting>`.
In particular, note that some classes do not support forming weak
references to their instances, and note that special care must be
taken when storing weak references in memory where initialization
and deinitialization are outside the responsibility of the compiler
(such as in malloc-ed memory).
Loading from a __weak variable always implicitly retains the
loaded value. In non-ARC modes, this retain is normally balanced
by an implicit autorelease. This autorelease can be suppressed
by performing the load in the receiver position of a -retain
message send (e.g. [weakReference retain]); note that this performs
only a single retain (the retain done when primitively loading from
the weak reference).
For the most part, __unsafe_unretained in non-ARC modes is just the
default behavior of variables and therefore is not needed. However,
it does have an effect on the semantics of block captures: normally,
copying a block which captures an Objective-C object or block pointer
causes the captured pointer to be retained or copied, respectively,
but that behavior is suppressed when the captured variable is qualified
with __unsafe_unretained.
Note that the __weak qualifier formerly meant the GC qualifier in
all non-ARC modes and was silently ignored outside of GC modes. It now
means the ARC-style qualifier in all non-GC modes and is no longer
allowed if not enabled by either -fobjc-arc or -fobjc-weak.
It is expected that -fobjc-weak will eventually be enabled by default
in all non-GC Objective-C modes.
Clang provides support for C++11 enumerations with a fixed underlying type within Objective-C. For example, one can write an enumeration type as:
typedef enum : unsigned char { Red, Green, Blue } Color;This specifies that the underlying type, which is used to store the enumeration
value, is unsigned char.
Use __has_feature(objc_fixed_enum) to determine whether support for fixed
underlying types is available in Objective-C.
Clang provides interoperability between C++11 lambdas and blocks-based APIs, by
permitting a lambda to be implicitly converted to a block pointer with the
corresponding signature. For example, consider an API such as NSArray's
array-sorting method:
- (NSArray *)sortedArrayUsingComparator:(NSComparator)cmptr;NSComparator is simply a typedef for the block pointer NSComparisonResult
(^)(id, id), and parameters of this type are generally provided with block
literals as arguments. However, one can also use a C++11 lambda so long as it
provides the same signature (in this case, accepting two parameters of type
id and returning an NSComparisonResult):
NSArray *array = @[@"string 1", @"string 21", @"string 12", @"String 11",
@"String 02"];
const NSStringCompareOptions comparisonOptions
= NSCaseInsensitiveSearch | NSNumericSearch |
NSWidthInsensitiveSearch | NSForcedOrderingSearch;
NSLocale *currentLocale = [NSLocale currentLocale];
NSArray *sorted
= [array sortedArrayUsingComparator:[=](id s1, id s2) -> NSComparisonResult {
NSRange string1Range = NSMakeRange(0, [s1 length]);
return [s1 compare:s2 options:comparisonOptions
range:string1Range locale:currentLocale];
}];
NSLog(@"sorted: %@", sorted);This code relies on an implicit conversion from the type of the lambda expression (an unnamed, local class type called the closure type) to the corresponding block pointer type. The conversion itself is expressed by a conversion operator in that closure type that produces a block pointer with the same signature as the lambda itself, e.g.,
operator NSComparisonResult (^)(id, id)() const;This conversion function returns a new block that simply forwards the two
parameters to the lambda object (which it captures by copy), then returns the
result. The returned block is first copied (with Block_copy) and then
autoreleased. As an optimization, if a lambda expression is immediately
converted to a block pointer (as in the first example, above), then the block
is not copied and autoreleased: rather, it is given the same lifetime as a
block literal written at that point in the program, which avoids the overhead
of copying a block to the heap in the common case.
The conversion from a lambda to a block pointer is only available in Objective-C++, and not in C++ with blocks, due to its use of Objective-C memory management (autorelease).
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.
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.
In Objective-C, functions and methods are generally assumed to follow the
Cocoa Memory Management
conventions for ownership of object arguments and
return values. However, there are exceptions, and so Clang provides attributes
to allow these exceptions to be documented. This are used by ARC and the
static analyzer Some exceptions may be
better described using the objc_method_family attribute instead.
Usage: The ns_returns_retained, ns_returns_not_retained,
ns_returns_autoreleased, cf_returns_retained, and
cf_returns_not_retained attributes can be placed on methods and functions
that return Objective-C or CoreFoundation objects. They are commonly placed at
the end of a function prototype or method declaration:
id foo() __attribute__((ns_returns_retained));
- (NSString *)bar:(int)x __attribute__((ns_returns_retained));The *_returns_retained attributes specify that the returned object has a +1
retain count. The *_returns_not_retained attributes specify that the return
object has a +0 retain count, even if the normal convention for its selector
would be +1. ns_returns_autoreleased specifies that the returned object is
+0, but is guaranteed to live at least as long as the next flush of an
autorelease pool.
Usage: The ns_consumed and cf_consumed attributes can be placed on
an parameter declaration; they specify that the argument is expected to have a
+1 retain count, which will be balanced in some way by the function or method.
The ns_consumes_self attribute can only be placed on an Objective-C
method; it specifies that the method expects its self parameter to have a
+1 retain count, which it will balance in some way.
void foo(__attribute__((ns_consumed)) NSString *string);
- (void) bar __attribute__((ns_consumes_self));
- (void) baz:(id) __attribute__((ns_consumed)) x;Further examples of these attributes are available in the static analyzer's list of annotations for analysis.
Query for these features with __has_attribute(ns_consumed),
__has_attribute(ns_returns_retained), etc.
It is possible to use the newest SDK but still build a program that can run on
older versions of macOS and iOS by passing -mmacosx-version-min= /
-miphoneos-version-min=.
Before LLVM 5.0, when calling a function that exists only in the OS that's
newer than the target OS (as determined by the minimum deployment version),
programmers had to carefully check if the function exists at runtime, using
null checks for weakly-linked C functions, +class for Objective-C classes,
and -respondsToSelector: or +instancesRespondToSelector: for
Objective-C methods. If such a check was missed, the program would compile
fine, run fine on newer systems, but crash on older systems.
As of LLVM 5.0, -Wunguarded-availability uses the availability attributes together
with the new @available() keyword to assist with this issue.
When a method that's introduced in the OS newer than the target OS is called, a
-Wunguarded-availability warning is emitted if that call is not guarded:
void my_fun(NSSomeClass* var) {
// If fancyNewMethod was added in e.g. macOS 10.12, but the code is
// built with -mmacosx-version-min=10.11, then this unconditional call
// will emit a -Wunguarded-availability warning:
[var fancyNewMethod];
}To fix the warning and to avoid the crash on macOS 10.11, wrap it in
if(@available()):
void my_fun(NSSomeClass* var) {
if (@available(macOS 10.12, *)) {
[var fancyNewMethod];
} else {
// Put fallback behavior for old macOS versions (and for non-mac
// platforms) here.
}
}The * is required and means that platforms not explicitly listed will take
the true branch, and the compiler will emit -Wunguarded-availability
warnings for unlisted platforms based on those platform's deployment target.
More than one platform can be listed in @available():
void my_fun(NSSomeClass* var) {
if (@available(macOS 10.12, iOS 10, *)) {
[var fancyNewMethod];
}
}If the caller of my_fun() already checks that my_fun() is only called
on 10.12, then add an availability attribute to it,
which will also suppress the warning and require that calls to my_fun() are
checked:
API_AVAILABLE(macos(10.12)) void my_fun(NSSomeClass* var) {
[var fancyNewMethod]; // Now ok.
}@available() is only available in Objective-C code. To use the feature
in C and C++ code, use the __builtin_available() spelling instead.
If existing code uses null checks or -respondsToSelector:, it should
be changed to use @available() (or __builtin_available) instead.
-Wunguarded-availability is disabled by default, but
-Wunguarded-availability-new, which only emits this warning for APIs
that have been introduced in macOS >= 10.13, iOS >= 11, watchOS >= 4 and
tvOS >= 11, is enabled by default.
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).
clang supports an extension which allows the following in C:
#include <math.h>
#include <complex.h>
complex float x = { 1.0f, INFINITY }; // Init to (1, Inf)This construct is useful because there is no way to separately initialize the
real and imaginary parts of a complex variable in standard C, given that clang
does not support _Imaginary. (Clang also supports the __real__ and
__imag__ extensions from gcc, which help in some cases, but are not usable
in static initializers.)
Note that this extension does not allow eliding the braces; the meaning of the following two lines is different:
complex float x[] = { { 1.0f, 1.0f } }; // [0] = (1, 1)
complex float x[] = { 1.0f, 1.0f }; // [0] = (1, 0), [1] = (1, 0)This extension also works in C++ mode, as far as that goes, but does not apply
to the C++ std::complex. (In C++11, list initialization allows the same
syntax to be used with std::complex with the same meaning.)
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 is used to provide the optimizer with a boolean
invariant that is defined to be true.
Syntax:
__builtin_assume(bool)Example of Use:
int foo(int x) {
__builtin_assume(x != 0);
// The optimizer may short-circuit this check using the invariant.
if (x == 0)
return do_something();
return do_something_else();
}Description:
The boolean argument to this function is defined to be true. The optimizer may analyze the form of the expression provided as the argument and deduce from that information used to optimize the program. If the condition is violated during execution, the behavior is undefined. The argument itself is never evaluated, so any side effects of the expression will be discarded.
Query for this feature with __has_builtin(__builtin_assume).
__builtin_readcyclecounter is used to access the cycle counter register (or
a similar low-latency, high-accuracy clock) on those targets that support it.
Syntax:
__builtin_readcyclecounter()Example of Use:
unsigned long long t0 = __builtin_readcyclecounter();
do_something();
unsigned long long t1 = __builtin_readcyclecounter();
unsigned long long cycles_to_do_something = t1 - t0; // assuming no overflowDescription:
The __builtin_readcyclecounter() builtin returns the cycle counter value,
which may be either global or process/thread-specific depending on the target.
As the backing counters often overflow quickly (on the order of seconds) this
should only be used for timing small intervals. When not supported by the
target, the return value is always zero. This builtin takes no arguments and
produces an unsigned long long result.
Query for this feature with __has_builtin(__builtin_readcyclecounter). Note
that even if present, its use may depend on run-time privilege or other OS
controlled state.
__builtin_shufflevector is used to express generic vector
permutation/shuffle/swizzle operations. This builtin is also very important
for the implementation of various target-specific header files like
<xmmintrin.h>.
Syntax:
__builtin_shufflevector(vec1, vec2, index1, index2, ...)Examples:
// identity operation - return 4-element vector v1.
__builtin_shufflevector(v1, v1, 0, 1, 2, 3)
// "Splat" element 0 of V1 into a 4-element result.
__builtin_shufflevector(V1, V1, 0, 0, 0, 0)
// Reverse 4-element vector V1.
__builtin_shufflevector(V1, V1, 3, 2, 1, 0)
// Concatenate every other element of 4-element vectors V1 and V2.
__builtin_shufflevector(V1, V2, 0, 2, 4, 6)
// Concatenate every other element of 8-element vectors V1 and V2.
__builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)
// Shuffle v1 with some elements being undefined
__builtin_shufflevector(v1, v1, 3, -1, 1, -1)Description:
The first two arguments to __builtin_shufflevector are vectors that have
the same element type. The remaining arguments are a list of integers that
specify the elements indices of the first two vectors that should be extracted
and returned in a new vector. These element indices are numbered sequentially
starting with the first vector, continuing into the second vector. Thus, if
vec1 is a 4-element vector, index 5 would refer to the second element of
vec2. An index of -1 can be used to indicate that the corresponding element
in the returned vector is a don't care and can be optimized by the backend.
The result of __builtin_shufflevector is a vector with the same element
type as vec1/vec2 but that has an element count equal to the number of
indices specified.
Query for this feature with __has_builtin(__builtin_shufflevector).
__builtin_convertvector is used to express generic vector
type-conversion operations. The input vector and the output vector
type must have the same number of elements.
Syntax:
__builtin_convertvector(src_vec, dst_vec_type)Examples:
typedef double vector4double __attribute__((__vector_size__(32)));
typedef float vector4float __attribute__((__vector_size__(16)));
typedef short vector4short __attribute__((__vector_size__(8)));
vector4float vf; vector4short vs;
// convert from a vector of 4 floats to a vector of 4 doubles.
__builtin_convertvector(vf, vector4double)
// equivalent to:
(vector4double) { (double) vf[0], (double) vf[1], (double) vf[2], (double) vf[3] }
// convert from a vector of 4 shorts to a vector of 4 floats.
__builtin_convertvector(vs, vector4float)
// equivalent to:
(vector4float) { (float) vs[0], (float) vs[1], (float) vs[2], (float) vs[3] }Description:
The first argument to __builtin_convertvector is a vector, and the second
argument is a vector type with the same number of elements as the first
argument.
The result of __builtin_convertvector is a vector with the same element
type as the second argument, with a value defined in terms of the action of a
C-style cast applied to each element of the first argument.
Query for this feature with __has_builtin(__builtin_convertvector).
__builtin_bitreverse8__builtin_bitreverse16__builtin_bitreverse32__builtin_bitreverse64
Syntax:
__builtin_bitreverse32(x)Examples:
uint8_t rev_x = __builtin_bitreverse8(x);
uint16_t rev_x = __builtin_bitreverse16(x);
uint32_t rev_y = __builtin_bitreverse32(y);
uint64_t rev_z = __builtin_bitreverse64(z);Description:
The '__builtin_bitreverse' family of builtins is used to reverse
the bitpattern of an integer value; for example 0b10110110 becomes
0b01101101.
__builtin_rotateleft8__builtin_rotateleft16__builtin_rotateleft32__builtin_rotateleft64
Syntax:
__builtin_rotateleft32(x, y)Examples:
uint8_t rot_x = __builtin_rotateleft8(x, y);
uint16_t rot_x = __builtin_rotateleft16(x, y);
uint32_t rot_x = __builtin_rotateleft32(x, y);
uint64_t rot_x = __builtin_rotateleft64(x, y);Description:
The '__builtin_rotateleft' family of builtins is used to rotate
the bits in the first argument by the amount in the second argument.
For example, 0b10000110 rotated left by 11 becomes 0b00110100.
The shift value is treated as an unsigned amount modulo the size of
the arguments. Both arguments and the result have the bitwidth specified
by the name of the builtin.
__builtin_rotateright8__builtin_rotateright16__builtin_rotateright32__builtin_rotateright64
Syntax:
__builtin_rotateright32(x, y)Examples:
uint8_t rot_x = __builtin_rotateright8(x, y);
uint16_t rot_x = __builtin_rotateright16(x, y);
uint32_t rot_x = __builtin_rotateright32(x, y);
uint64_t rot_x = __builtin_rotateright64(x, y);Description:
The '__builtin_rotateright' family of builtins is used to rotate
the bits in the first argument by the amount in the second argument.
For example, 0b10000110 rotated right by 3 becomes 0b11010000.
The shift value is treated as an unsigned amount modulo the size of
the arguments. Both arguments and the result have the bitwidth specified
by the name of the builtin.
__builtin_unreachable is used to indicate that a specific point in the
program cannot be reached, even if the compiler might otherwise think it can.
This is useful to improve optimization and eliminates certain warnings. For
example, without the __builtin_unreachable in the example below, the
compiler assumes that the inline asm can fall through and prints a "function
declared 'noreturn' should not return" warning.
Syntax:
__builtin_unreachable()Example of use:
void myabort(void) __attribute__((noreturn));
void myabort(void) {
asm("int3");
__builtin_unreachable();
}Description:
The __builtin_unreachable() builtin has completely undefined behavior.
Since it has undefined behavior, it is a statement that it is never reached and
the optimizer can take advantage of this to produce better code. This builtin
takes no arguments and produces a void result.
Query for this feature with __has_builtin(__builtin_unreachable).
__builtin_unpredictable is used to indicate that a branch condition is
unpredictable by hardware mechanisms such as branch prediction logic.
Syntax:
__builtin_unpredictable(long long)Example of use:
if (__builtin_unpredictable(x > 0)) {
foo();
}Description:
The __builtin_unpredictable() builtin is expected to be used with control
flow conditions such as in if and switch statements.
Query for this feature with __has_builtin(__builtin_unpredictable).
__sync_swap is used to atomically swap integers or pointers in memory.
Syntax:
type __sync_swap(type *ptr, type value, ...)Example of Use:
int old_value = __sync_swap(&value, new_value);Description:
The __sync_swap() builtin extends the existing __sync_*() family of
atomic intrinsics to allow code to atomically swap the current value with the
new value. More importantly, it helps developers write more efficient and
correct code by avoiding expensive loops around
__sync_bool_compare_and_swap() or relying on the platform specific
implementation details of __sync_lock_test_and_set(). The
__sync_swap() builtin is a full barrier.
__builtin_addressof performs the functionality of the built-in &
operator, ignoring any operator& overload. This is useful in constant
expressions in C++11, where there is no other way to take the address of an
object that overloads operator&.
Example of use:
template<typename T> constexpr T *addressof(T &value) {
return __builtin_addressof(value);
}__builtin_operator_new 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++.
Clang provides a set of builtins which expose multiprecision arithmetic in a manner amenable to C. They all have the following form:
unsigned x = ..., y = ..., carryin = ..., carryout;
unsigned sum = __builtin_addc(x, y, carryin, &carryout);Thus one can form a multiprecision addition chain in the following manner:
unsigned *x, *y, *z, carryin=0, carryout;
z[0] = __builtin_addc(x[0], y[0], carryin, &carryout);
carryin = carryout;
z[1] = __builtin_addc(x[1], y[1], carryin, &carryout);
carryin = carryout;
z[2] = __builtin_addc(x[2], y[2], carryin, &carryout);
carryin = carryout;
z[3] = __builtin_addc(x[3], y[3], carryin, &carryout);The complete list of builtins are:
unsigned char __builtin_addcb (unsigned char x, unsigned char y, unsigned char carryin, unsigned char *carryout);
unsigned short __builtin_addcs (unsigned short x, unsigned short y, unsigned short carryin, unsigned short *carryout);
unsigned __builtin_addc (unsigned x, unsigned y, unsigned carryin, unsigned *carryout);
unsigned long __builtin_addcl (unsigned long x, unsigned long y, unsigned long carryin, unsigned long *carryout);
unsigned long long __builtin_addcll(unsigned long long x, unsigned long long y, unsigned long long carryin, unsigned long long *carryout);
unsigned char __builtin_subcb (unsigned char x, unsigned char y, unsigned char carryin, unsigned char *carryout);
unsigned short __builtin_subcs (unsigned short x, unsigned short y, unsigned short carryin, unsigned short *carryout);
unsigned __builtin_subc (unsigned x, unsigned y, unsigned carryin, unsigned *carryout);
unsigned long __builtin_subcl (unsigned long x, unsigned long y, unsigned long carryin, unsigned long *carryout);
unsigned long long __builtin_subcll(unsigned long long x, unsigned long long y, unsigned long long carryin, unsigned long long *carryout);Clang provides a set of builtins that implement checked arithmetic for security critical applications in a manner that is fast and easily expressable in C. As an example of their usage:
errorcode_t security_critical_application(...) {
unsigned x, y, result;
...
if (__builtin_mul_overflow(x, y, &result))
return kErrorCodeHackers;
...
use_multiply(result);
...
}Clang provides the following checked arithmetic builtins:
bool __builtin_add_overflow (type1 x, type2 y, type3 *sum);
bool __builtin_sub_overflow (type1 x, type2 y, type3 *diff);
bool __builtin_mul_overflow (type1 x, type2 y, type3 *prod);
bool __builtin_uadd_overflow (unsigned x, unsigned y, unsigned *sum);
bool __builtin_uaddl_overflow (unsigned long x, unsigned long y, unsigned long *sum);
bool __builtin_uaddll_overflow(unsigned long long x, unsigned long long y, unsigned long long *sum);
bool __builtin_usub_overflow (unsigned x, unsigned y, unsigned *diff);
bool __builtin_usubl_overflow (unsigned long x, unsigned long y, unsigned long *diff);
bool __builtin_usubll_overflow(unsigned long long x, unsigned long long y, unsigned long long *diff);
bool __builtin_umul_overflow (unsigned x, unsigned y, unsigned *prod);
bool __builtin_umull_overflow (unsigned long x, unsigned long y, unsigned long *prod);
bool __builtin_umulll_overflow(unsigned long long x, unsigned long long y, unsigned long long *prod);
bool __builtin_sadd_overflow (int x, int y, int *sum);
bool __builtin_saddl_overflow (long x, long y, long *sum);
bool __builtin_saddll_overflow(long long x, long long y, long long *sum);
bool __builtin_ssub_overflow (int x, int y, int *diff);
bool __builtin_ssubl_overflow (long x, long y, long *diff);
bool __builtin_ssubll_overflow(long long x, long long y, long long *diff);
bool __builtin_smul_overflow (int x, int y, int *prod);
bool __builtin_smull_overflow (long x, long y, long *prod);
bool __builtin_smulll_overflow(long long x, long long y, long long *prod);Each builtin performs the specified mathematical operation on the first two arguments and stores the result in the third argument. If possible, the result will be equal to mathematically-correct result and the builtin will return 0. Otherwise, the builtin will return 1 and the result will be equal to the unique value that is equivalent to the mathematically-correct result modulo two raised to the k power, where k is the number of bits in the result type. The behavior of these builtins is well-defined for all argument values.
The first three builtins work generically for operands of any integer type, including boolean types. The operands need not have the same type as each other, or as the result. The other builtins may implicitly promote or convert their operands before performing the operation.
Query for this feature with __has_builtin(__builtin_add_overflow), etc.
double __builtin_canonicalize(double);
float __builtin_canonicalizef(float);
long double__builtin_canonicalizel(long double);Returns the platform specific canonical encoding of a floating point number. This canonicalization is useful for implementing certain numeric primitives such as frexp. See LLVM canonicalize intrinsic for more information on the semantics.
Clang provides constant expression evaluation support for builtins forms of
the following functions from the C standard library <string.h> header:
memchrmemcmpstrchrstrcmpstrlenstrncmpwcschrwcscmpwcslenwcsncmpwmemchrwmemcmp
In each case, the builtin form has the name of the C library function prefixed
by __builtin_. Example:
void *p = __builtin_memchr("foobar", 'b', 5);In addition to the above, one further builtin is provided:
char *__builtin_char_memchr(const char *haystack, int needle, size_t size);__builtin_char_memchr(a, b, c) is identical to
(char*)__builtin_memchr(a, b, c) except that its use is permitted within
constant expressions in C++11 onwards (where a cast from void* to char*
is disallowed in general).
Support for constant expression evaluation for the above builtins be detected
with __has_feature(cxx_constexpr_string_builtins).
There are two atomic builtins with min/max in-memory comparison and swap. The syntax and semantics are similar to GCC-compatible __atomic_* builtins.
__atomic_fetch_min__atomic_fetch_max
The builtins work with signed and unsigned integers and require to specify memory ordering. The return value is the original value that was stored in memory before comparison.
Example:
unsigned int val = __atomic_fetch_min(unsigned int *pi, unsigned int ui, __ATOMIC_RELAXED);The third argument is one of the memory ordering specifiers __ATOMIC_RELAXED,
__ATOMIC_CONSUME, __ATOMIC_ACQUIRE, __ATOMIC_RELEASE,
__ATOMIC_ACQ_REL, or __ATOMIC_SEQ_CST following C++11 memory model semantics.
In terms or aquire-release ordering barriers these two operations are always considered as operations with load-store semantics, even when the original value is not actually modified after comparison.
Clang provides a set of builtins which are intended to be used to implement
C11's <stdatomic.h> header. These builtins provide the semantics of the
_explicit form of the corresponding C11 operation, and are named with a
__c11_ prefix. The supported operations, and the differences from
the corresponding C11 operations, are:
__c11_atomic_init__c11_atomic_thread_fence__c11_atomic_signal_fence__c11_atomic_is_lock_free(The argument is the size of the_Atomic(...)object, instead of its address)__c11_atomic_store__c11_atomic_load__c11_atomic_exchange__c11_atomic_compare_exchange_strong__c11_atomic_compare_exchange_weak__c11_atomic_fetch_add__c11_atomic_fetch_sub__c11_atomic_fetch_and__c11_atomic_fetch_or__c11_atomic_fetch_xor
The macros __ATOMIC_RELAXED, __ATOMIC_CONSUME, __ATOMIC_ACQUIRE,
__ATOMIC_RELEASE, __ATOMIC_ACQ_REL, and __ATOMIC_SEQ_CST are
provided, with values corresponding to the enumerators of C11's
memory_order enumeration.
(Note that Clang additionally provides GCC-compatible __atomic_*
builtins and OpenCL 2.0 __opencl_atomic_* builtins. The OpenCL 2.0
atomic builtins are an explicit form of the corresponding OpenCL 2.0
builtin function, and are named with a __opencl_ prefix. The macros
__OPENCL_MEMORY_SCOPE_WORK_ITEM, __OPENCL_MEMORY_SCOPE_WORK_GROUP,
__OPENCL_MEMORY_SCOPE_DEVICE, __OPENCL_MEMORY_SCOPE_ALL_SVM_DEVICES,
and __OPENCL_MEMORY_SCOPE_SUB_GROUP are provided, with values
corresponding to the enumerators of OpenCL's memory_scope enumeration.)
Clang provides overloaded builtins giving direct access to the three key ARM instructions for implementing atomic operations.
T __builtin_arm_ldrex(const volatile T *addr);
T __builtin_arm_ldaex(const volatile T *addr);
int __builtin_arm_strex(T val, volatile T *addr);
int __builtin_arm_stlex(T val, volatile T *addr);
void __builtin_arm_clrex(void);The types T currently supported are:
- Integer types with width at most 64 bits (or 128 bits on AArch64).
- Floating-point types
- Pointer types.
Note that the compiler does not guarantee it will not insert stores which clear
the exclusive monitor in between an ldrex type operation and its paired
strex. In practice this is only usually a risk when the extra store is on
the same cache line as the variable being modified and Clang will only insert
stack stores on its own, so it is best not to use these operations on variables
with automatic storage duration.
Also, loads and stores may be implicit in code written between the ldrex and
strex. Clang will not necessarily mitigate the effects of these either, so
care should be exercised.
For these reasons the higher level atomic primitives should be preferred where possible.
Clang provides overloaded builtins allowing generation of non-temporal memory accesses.
T __builtin_nontemporal_load(T *addr);
void __builtin_nontemporal_store(T value, T *addr);The types T currently supported are:
- Integer types.
- Floating-point types.
- Vector types.
Note that the compiler does not guarantee that non-temporal loads or stores will be used.
Warning
This is a work in progress. Compatibility across Clang/LLVM releases is not guaranteed.
Clang provides experimental builtins to support C++ Coroutines as defined by https://wg21.link/P0057. The following four are intended to be used by the standard library to implement std::experimental::coroutine_handle type.
Syntax:
void __builtin_coro_resume(void *addr);
void __builtin_coro_destroy(void *addr);
bool __builtin_coro_done(void *addr);
void *__builtin_coro_promise(void *addr, int alignment, bool from_promise)Example of use:
template <> struct coroutine_handle<void> {
void resume() const { __builtin_coro_resume(ptr); }
void destroy() const { __builtin_coro_destroy(ptr); }
bool done() const { return __builtin_coro_done(ptr); }
// ...
protected:
void *ptr;
};
template <typename Promise> struct coroutine_handle : coroutine_handle<> {
// ...
Promise &promise() const {
return *reinterpret_cast<Promise *>(
__builtin_coro_promise(ptr, alignof(Promise), /*from-promise=*/false));
}
static coroutine_handle from_promise(Promise &promise) {
coroutine_handle p;
p.ptr = __builtin_coro_promise(&promise, alignof(Promise),
/*from-promise=*/true);
return p;
}
};Other coroutine builtins are either for internal clang use or for use during development of the coroutine feature. See Coroutines in LLVM for more information on their semantics. Note that builtins matching the intrinsics that take token as the first parameter (llvm.coro.begin, llvm.coro.alloc, llvm.coro.free and llvm.coro.suspend) omit the token parameter and fill it to an appropriate value during the emission.
Syntax:
size_t __builtin_coro_size()
void *__builtin_coro_frame()
void *__builtin_coro_free(void *coro_frame)
void *__builtin_coro_id(int align, void *promise, void *fnaddr, void *parts)
bool __builtin_coro_alloc()
void *__builtin_coro_begin(void *memory)
void __builtin_coro_end(void *coro_frame, bool unwind)
char __builtin_coro_suspend(bool final)
bool __builtin_coro_param(void *original, void *copy)Note that there is no builtin matching the llvm.coro.save intrinsic. LLVM automatically will insert one if the first argument to llvm.coro.suspend is token none. If a user calls __builin_suspend, clang will insert token none as the first argument to the intrinsic.
Clang provides experimental builtins to support C++ standard library implementation
of std::experimental::source_location as specified in http://wg21.link/N4600.
With the exception of __builtin_COLUMN, these builtins are also implemented by
GCC.
Syntax:
const char *__builtin_FILE();
const char *__builtin_FUNCTION();
unsigned __builtin_LINE();
unsigned __builtin_COLUMN(); // Clang onlyExample of use:
void my_assert(bool pred, int line = __builtin_LINE(), // Captures line of caller
const char* file = __builtin_FILE(),
const char* function = __builtin_FUNCTION()) {
if (pred) return;
printf("%s:%d assertion failed in function %s\n", file, line, function);
std::abort();
}
struct MyAggregateType {
int x;
int line = __builtin_LINE(); // captures line where aggregate initialization occurs
};
static_assert(MyAggregateType{42}.line == __LINE__);
struct MyClassType {
int line = __builtin_LINE(); // captures line of the constructor used during initialization
constexpr MyClassType(int) { assert(line == __LINE__); }
};Description:
The builtins __builtin_LINE, __builtin_FUNCTION, and __builtin_FILE return
the values, at the "invocation point", for __LINE__, __FUNCTION__, and
__FILE__ respectively. These builtins are constant expressions.
When the builtins appear as part of a default function argument the invocation point is the location of the caller. When the builtins appear as part of a default member initializer, the invocation point is the location of the constructor or aggregate initialization used to create the object. Otherwise the invocation point is the same as the location of the builtin.
When the invocation point of __builtin_FUNCTION is not a function scope the
empty string is returned.
Clang's non-standard C++11 attributes live in the clang attribute
namespace.
Clang supports GCC's gnu attribute namespace. All GCC attributes which
are accepted with the __attribute__((foo)) syntax are also accepted as
[[gnu::foo]]. This only extends to attributes which are specified by GCC
(see the list of GCC function attributes, GCC variable
attributes, and
GCC type attributes). As with the GCC
implementation, these attributes must appertain to the declarator-id in a
declaration, which means they must go either at the start of the declaration or
immediately after the name being declared.
For example, this applies the GNU unused attribute to a and f, and
also applies the GNU noreturn attribute to f.
[[gnu::unused]] int a, f [[gnu::noreturn]] ();Clang supports some language features conditionally on some targets.
Clang implements the __dmb, __dsb and __isb intrinsics as defined
in the ARM C Language Extensions Release 2.0.
Note that these intrinsics are implemented as motion barriers that block
reordering of memory accesses and side effect instructions. Other instructions
like simple arithmetic may be reordered around the intrinsic. If you expect to
have no reordering at all, use inline assembly instead.
The X86 backend has these language extensions:
Annotating a pointer with address space #256 causes it to be code generated relative to the X86 GS segment register, address space #257 causes it to be relative to the X86 FS segment, and address space #258 causes it to be relative to the X86 SS segment. Note that this is a very very low-level feature that should only be used if you know what you're doing (for example in an OS kernel).
Here is an example:
#define GS_RELATIVE __attribute__((address_space(256)))
int foo(int GS_RELATIVE *P) {
return *P;
}Which compiles to (on X86-32):
_foo:
movl 4(%esp), %eax
movl %gs:(%eax), %eax
retPowerPC64/PowerPC64le supports the builtin function __builtin_setrnd to set
the floating point rounding mode. This function will use the least significant
two bits of integer argument to set the floating point rounding mode.
double __builtin_setrnd(int mode);The effective values for mode are:
- 0 - round to nearest
- 1 - round to zero
- 2 - round to +infinity
- 3 - round to -infinity
Note that the mode argument will modulo 4, so if the int argument is greater
than 3, it will only use the least significant two bits of the mode.
Namely, __builtin_setrnd(102)) is equal to __builtin_setrnd(2).
PowerPC64/PowerPC64le supports the builtin function __builtin_setrnd to set
the floating point rounding mode. This function will use the least significant
two bits of integer argument to set the floating point rounding mode.
double __builtin_setrnd(int mode);The effective values for mode are:
- 0 - round to nearest
- 1 - round to zero
- 2 - round to +infinity
- 3 - round to -infinity
Note that the mode argument will modulo 4, so if the integer argument is greater
than 3, it will only use the least significant two bits of the mode.
Namely, __builtin_setrnd(102)) is equal to __builtin_setrnd(2).
PowerPC64/PowerPC64le supports the builtin function __builtin_setrnd to set
the floating point rounding mode. This function will use the least significant
two bits of integer argument to set the floating point rounding mode.
double __builtin_setrnd(int mode);The effective values for mode are:
- 0 - round to nearest
- 1 - round to zero
- 2 - round to +infinity
- 3 - round to -infinity
Note that the mode argument will modulo 4, so if the integer argument is greater
than 3, it will only use the least significant two bits of the mode.
Namely, __builtin_setrnd(102)) is equal to __builtin_setrnd(2).
The PowerPC architecture specifies instructions implementing cache operations. Clang provides builtins that give direct programmer access to these cache instructions.
Currently the following builtins are implemented in clang:
__builtin_dcbf copies the contents of a modified block from the data cache
to main memory and flushes the copy from the data cache.
Syntax:
void __dcbf(const void* addr); /* Data Cache Block Flush */Example of Use:
int a = 1;
__builtin_dcbf (&a);Clang supports additional attributes that are useful for documenting program invariants and rules for static analysis tools, such as the Clang Static Analyzer. These attributes are documented in the analyzer's list of source-level annotations.
Use __has_feature(address_sanitizer) to check if the code is being built
with :doc:`AddressSanitizer`.
Use __has_feature(thread_sanitizer) to check if the code is being built
with :doc:`ThreadSanitizer`.
Use __has_feature(memory_sanitizer) to check if the code is being built
with :doc:`MemorySanitizer`.
Use __has_feature(safe_stack) to check if the code is being built
with :doc:`SafeStack`.
Clang provides a mechanism for selectively disabling optimizations in functions and methods.
To disable optimizations in a single function definition, the GNU-style or C++11
non-standard attribute optnone can be used.
// The following functions will not be optimized.
// GNU-style attribute
__attribute__((optnone)) int foo() {
// ... code
}
// C++11 attribute
[[clang::optnone]] int bar() {
// ... code
}To facilitate disabling optimization for a range of function definitions, a
range-based pragma is provided. Its syntax is #pragma clang optimize
followed by off or on.
All function definitions in the region between an off and the following
on will be decorated with the optnone attribute unless doing so would
conflict with explicit attributes already present on the function (e.g. the
ones that control inlining).
#pragma clang optimize off
// This function will be decorated with optnone.
int foo() {
// ... code
}
// optnone conflicts with always_inline, so bar() will not be decorated.
__attribute__((always_inline)) int bar() {
// ... code
}
#pragma clang optimize onIf no on is found to close an off region, the end of the region is the
end of the compilation unit.
Note that a stray #pragma clang optimize on does not selectively enable
additional optimizations when compiling at low optimization levels. This feature
can only be used to selectively disable optimizations.
The pragma has an effect on functions only at the point of their definition; for function templates, this means that the state of the pragma at the point of an instantiation is not necessarily relevant. Consider the following example:
template<typename T> T twice(T t) {
return 2 * t;
}
#pragma clang optimize off
template<typename T> T thrice(T t) {
return 3 * t;
}
int container(int a, int b) {
return twice(a) + thrice(b);
}
#pragma clang optimize onIn 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.
The #pragma clang loop directive is used to specify hints for optimizing the
subsequent for, while, do-while, or c++11 range-based for loop. The directive
provides options for vectorization, interleaving, unrolling and
distribution. Loop hints can be specified before any loop and will be ignored if
the optimization is not safe to apply.
A vectorized loop performs multiple iterations of the original loop in parallel using vector instructions. The instruction set of the target processor determines which vector instructions are available and their vector widths. This restricts the types of loops that can be vectorized. The vectorizer automatically determines if the loop is safe and profitable to vectorize. A vector instruction cost model is used to select the vector width.
Interleaving multiple loop iterations allows modern processors to further improve instruction-level parallelism (ILP) using advanced hardware features, such as multiple execution units and out-of-order execution. The vectorizer uses a cost model that depends on the register pressure and generated code size to select the interleaving count.
Vectorization is enabled by vectorize(enable) and interleaving is enabled
by interleave(enable). This is useful when compiling with -Os to
manually enable vectorization or interleaving.
#pragma clang loop vectorize(enable)
#pragma clang loop interleave(enable)
for(...) {
...
}The vector width is specified by vectorize_width(_value_) and the interleave
count is specified by interleave_count(_value_), where
_value_ is a positive integer. This is useful for specifying the optimal
width/count of the set of target architectures supported by your application.
#pragma clang loop vectorize_width(2)
#pragma clang loop interleave_count(2)
for(...) {
...
}Specifying a width/count of 1 disables the optimization, and is equivalent to
vectorize(disable) or interleave(disable).
Unrolling a loop reduces the loop control overhead and exposes more opportunities for ILP. Loops can be fully or partially unrolled. Full unrolling eliminates the loop and replaces it with an enumerated sequence of loop iterations. Full unrolling is only possible if the loop trip count is known at compile time. Partial unrolling replicates the loop body within the loop and reduces the trip count.
If unroll(enable) is specified the unroller will attempt to fully unroll the
loop if the trip count is known at compile time. If the fully unrolled code size
is greater than an internal limit the loop will be partially unrolled up to this
limit. If the trip count is not known at compile time the loop will be partially
unrolled with a heuristically chosen unroll factor.
#pragma clang loop unroll(enable)
for(...) {
...
}If unroll(full) is specified the unroller will attempt to fully unroll the
loop if the trip count is known at compile time identically to
unroll(enable). However, with unroll(full) the loop will not be unrolled
if the loop count is not known at compile time.
#pragma clang loop unroll(full)
for(...) {
...
}The unroll count can be specified explicitly with unroll_count(_value_) where
_value_ is a positive integer. If this value is greater than the trip count the
loop will be fully unrolled. Otherwise the loop is partially unrolled subject
to the same code size limit as with unroll(enable).
#pragma clang loop unroll_count(8)
for(...) {
...
}Unrolling of a loop can be prevented by specifying unroll(disable).
Loop Distribution allows splitting a loop into multiple loops. This is beneficial for example when the entire loop cannot be vectorized but some of the resulting loops can.
If distribute(enable)) is specified and the loop has memory dependencies
that inhibit vectorization, the compiler will attempt to isolate the offending
operations into a new loop. This optimization is not enabled by default, only
loops marked with the pragma are considered.
#pragma clang loop distribute(enable)
for (i = 0; i < N; ++i) {
S1: A[i + 1] = A[i] + B[i];
S2: C[i] = D[i] * E[i];
}This loop will be split into two loops between statements S1 and S2. The second loop containing S2 will be vectorized.
Loop Distribution is currently not enabled by default in the optimizer because it can hurt performance in some cases. For example, instruction-level parallelism could be reduced by sequentializing the execution of the statements S1 and S2 above.
If Loop Distribution is turned on globally with
-mllvm -enable-loop-distribution, specifying distribute(disable) can
be used the disable it on a per-loop basis.
For convenience multiple loop hints can be specified on a single line.
#pragma clang loop vectorize_width(4) interleave_count(8)
for(...) {
...
}If an optimization cannot be applied any hints that apply to it will be ignored.
For example, the hint vectorize_width(4) is ignored if the loop is not
proven safe to vectorize. To identify and diagnose optimization issues use
-Rpass, -Rpass-missed, and -Rpass-analysis command line options. See the
user guide for details.
The #pragma clang fp pragma allows floating-point options to be specified
for a section of the source code. This pragma can only appear at file scope or
at the start of a compound statement (excluding comments). When using within a
compound statement, the pragma is active within the scope of the compound
statement.
Currently, only FP contraction can be controlled with the pragma. #pragma
clang fp contract specifies whether the compiler should contract a multiply
and an addition (or subtraction) into a fused FMA operation when supported by
the target.
The pragma can take three values: on, fast and off. The on
option is identical to using #pragma STDC FP_CONTRACT(ON) and it allows
fusion as specified the language standard. The fast option allows fusiong
in cases when the language standard does not make this possible (e.g. across
statements in C)
for(...) {
#pragma clang fp contract(fast)
a = b[i] * c[i];
d[i] += a;
}The pragma can also be used with off which turns FP contraction off for a
section of the code. This can be useful when fast contraction is otherwise
enabled for the translation unit with the -ffp-contract=fast flag.
The #pragma clang attribute directive can be used to apply an attribute to
multiple declarations. The #pragma clang attribute push variation of the
directive pushes a new "scope" of #pragma clang attribute that attributes
can be added to. The #pragma clang attribute (...) variation adds an
attribute to that scope, and the #pragma clang attribute pop variation pops
the scope. You can also use #pragma clang attribute push (...), which is a
shorthand for when you want to add one attribute to a new scope. Multiple push
directives can be nested inside each other.
The attributes that are used in the #pragma clang attribute directives
can be written using the GNU-style syntax:
#pragma clang attribute push (__attribute__((annotate("custom"))), apply_to = function)
void function(); // The function now has the annotate("custom") attribute
#pragma clang attribute popThe attributes can also be written using the C++11 style syntax:
#pragma clang attribute push ([[noreturn]], apply_to = function)
void function(); // The function now has the [[noreturn]] attribute
#pragma clang attribute popThe __declspec style syntax is also supported:
#pragma clang attribute push (__declspec(dllexport), apply_to = function)
void function(); // The function now has the __declspec(dllexport) attribute
#pragma clang attribute popA single push directive accepts only one attribute regardless of the syntax used.
Because multiple push directives can be nested, if you're writing a macro that
expands to _Pragma("clang attribute") it's good hygiene (though not
required) to add a namespace to your push/pop directives. A pop directive with a
namespace will pop the innermost push that has that same namespace. This will
ensure that another macro's pop won't inadvertently pop your attribute. Note
that an pop without a namespace will pop the innermost push without a
namespace. push``es with a namespace can only be popped by ``pop with the
same namespace. For instance:
#define ASSUME_NORETURN_BEGIN _Pragma("clang attribute AssumeNoreturn.push ([[noreturn]], apply_to = function)")
#define ASSUME_NORETURN_END _Pragma("clang attribute AssumeNoreturn.pop")
#define ASSUME_UNAVAILABLE_BEGIN _Pragma("clang attribute Unavailable.push (__attribute__((unavailable)), apply_to=function)")
#define ASSUME_UNAVAILABLE_END _Pragma("clang attribute Unavailable.pop")
ASSUME_NORETURN_BEGIN
ASSUME_UNAVAILABLE_BEGIN
void function(); // function has [[noreturn]] and __attribute__((unavailable))
ASSUME_NORETURN_END
void other_function(); // function has __attribute__((unavailable))
ASSUME_UNAVAILABLE_ENDWithout the namespaces on the macros, other_function will be annotated with
[[noreturn]] instead of __attribute__((unavailable)). This may seem like
a contrived example, but its very possible for this kind of situation to appear
in real code if the pragmas are spread out across a large file. You can test if
your version of clang supports namespaces on #pragma clang attribute with
__has_extension(pragma_clang_attribute_namespaces).
The set of declarations that receive a single attribute from the attribute stack
depends on the subject match rules that were specified in the pragma. Subject
match rules are specified after the attribute. The compiler expects an
identifier that corresponds to the subject set specifier. The apply_to
specifier is currently the only supported subject set specifier. It allows you
to specify match rules that form a subset of the attribute's allowed subject
set, i.e. the compiler doesn't require all of the attribute's subjects. For
example, an attribute like [[nodiscard]] whose subject set includes
enum, record and hasType(functionType), requires the presence of at
least one of these rules after apply_to:
#pragma clang attribute push([[nodiscard]], apply_to = enum)
enum Enum1 { A1, B1 }; // The enum will receive [[nodiscard]]
struct Record1 { }; // The struct will *not* receive [[nodiscard]]
#pragma clang attribute pop
#pragma clang attribute push([[nodiscard]], apply_to = any(record, enum))
enum Enum2 { A2, B2 }; // The enum will receive [[nodiscard]]
struct Record2 { }; // The struct *will* receive [[nodiscard]]
#pragma clang attribute pop
// This is an error, since [[nodiscard]] can't be applied to namespaces:
#pragma clang attribute push([[nodiscard]], apply_to = any(record, namespace))
#pragma clang attribute popMultiple match rules can be specified using the any match rule, as shown
in the example above. The any rule applies attributes to all declarations
that are matched by at least one of the rules in the any. It doesn't nest
and can't be used inside the other match rules. Redundant match rules or rules
that conflict with one another should not be used inside of any.
Clang supports the following match rules:
function: Can be used to apply attributes to functions. This includes C++ member functions, static functions, operators, and constructors/destructors.function(is_member): Can be used to apply attributes to C++ member functions. This includes members like static functions, operators, and constructors/destructors.hasType(functionType): Can be used to apply attributes to functions, C++ member functions, and variables/fields whose type is a function pointer. It does not apply attributes to Objective-C methods or blocks.type_alias: Can be used to apply attributes totypedefdeclarations and C++11 type aliases.record: Can be used to apply attributes tostruct,class, anduniondeclarations.record(unless(is_union)): Can be used to apply attributes only tostructandclassdeclarations.enum: Can be be used to apply attributes to enumeration declarations.enum_constant: Can be used to apply attributes to enumerators.variable: Can be used to apply attributes to variables, including local variables, parameters, global variables, and static member variables. It does not apply attributes to instance member variables or Objective-C ivars.variable(is_thread_local): Can be used to apply attributes to thread-local variables only.variable(is_global): Can be used to apply attributes to global variables only.variable(is_parameter): Can be used to apply attributes to parameters only.variable(unless(is_parameter)): Can be used to apply attributes to all the variables that are not parameters.field: Can be used to apply attributes to non-static member variables in a record. This includes Objective-C ivars.namespace: Can be used to apply attributes tonamespacedeclarations.objc_interface: Can be used to apply attributes to@interfacedeclarations.objc_protocol: Can be used to apply attributes to@protocoldeclarations.objc_category: Can be used to apply attributes to category declarations, including class extensions.objc_method: Can be used to apply attributes to Objective-C methods, including instance and class methods. Implicit methods like implicit property getters and setters do not receive the attribute.objc_method(is_instance): Can be used to apply attributes to Objective-C instance methods.objc_property: Can be used to apply attributes to@propertydeclarations.block: Can be used to apply attributes to block declarations. This does not include variables/fields of block pointer type.
The use of unless in match rules is currently restricted to a strict set of
sub-rules that are used by the supported attributes. That means that even though
variable(unless(is_parameter)) is a valid match rule,
variable(unless(is_thread_local)) is not.
Not all attributes can be used with the #pragma clang attribute directive.
Notably, statement attributes like [[fallthrough]] or type attributes
like address_space aren't supported by this directive. You can determine
whether or not an attribute is supported by the pragma by referring to the
:doc:`individual documentation for that attribute <AttributeReference>`.
The attributes are applied to all matching declarations individually, even when the attribute is semantically incorrect. The attributes that aren't applied to any declaration are not verified semantically.
The #pragma clang section directive provides a means to assign section-names
to global variables, functions and static variables.
The section names can be specified as:
#pragma clang section bss="myBSS" data="myData" rodata="myRodata" text="myText"The section names can be reverted back to default name by supplying an empty string to the section kind, for example:
#pragma clang section bss="" data="" text="" rodata=""The #pragma clang section directive obeys the following rules:
- The pragma applies to all global variable, statics and function declarations from the pragma to the end of the translation unit.
- The pragma clang section is enabled automatically, without need of any flags.
- This feature is only defined to work sensibly for ELF targets.
- If section name is specified through _attribute_((section("myname"))), then the attribute name gains precedence.
- Global variables that are initialized to zero will be placed in the named bss section, if one is present.
- The
#pragma clang sectiondirective does not does try to infer section-kind from the name. For example, naming a section ".bss.mySec" does NOT mean it will be a bss section name. - The decision about which section-kind applies to each global is taken in the back-end.
Once the section-kind is known, appropriate section name, as specified by the user using
#pragma clang sectiondirective, is applied to that global.
The #pragma comment(lib, ...) directive is supported on all ELF targets.
The second parameter is the library name (without the traditional Unix prefix of
lib). This allows you to provide an implicit link of dependent libraries.
Clang supports the builtin __builtin_dynamic_object_size, the semantics are
the same as GCC's __builtin_object_size (which Clang also supports), but
__builtin_dynamic_object_size can evaluate the object's size at runtime.
__builtin_dynamic_object_size is meant to be used as a drop-in replacement
for __builtin_object_size in libraries that support it.
For instance, here is a program that __builtin_dynamic_object_size will make
safer:
void copy_into_buffer(size_t size) {
char* buffer = malloc(size);
strlcpy(buffer, "some string", strlen("some string"));
// Previous line preprocesses to:
// __builtin___strlcpy_chk(buffer, "some string", strlen("some string"), __builtin_object_size(buffer, 0))
}Since the size of buffer can't be known at compile time, Clang will fold
__builtin_object_size(buffer, 0) into -1. However, if this was written
as __builtin_dynamic_object_size(buffer, 0), Clang will fold it into
size, providing some extra runtime safety.