Google Git Google Git
Sign in
llvm/llvm-project/refs/heads/users/void/array-bounds-accessed/./clang/docs/LanguageExtensions.md
blob: 3db8d083c407569d1b938ca9ce4fb3a06320a7c5 [file] [view] [edit]
# Clang Language Extensions
```{toctree}
:hidden: true
ObjectiveCLiterals
BlockLanguageSpec
Block-ABI-Apple
AutomaticReferenceCounting
PointerAuthentication
MatrixTypes
CXXTypeAwareAllocators
```
## Introduction
This document describes the language extensions provided by Clang. In addition
to the language extensions listed here, Clang aims to support a broad range of
GCC extensions. Please see the [GCC manual](https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html) for more information on
these extensions.
(langext-feature_check)=
(langext-feature-check)=
## Feature Checking Macros
Language extensions can be very useful, but only if you know you can depend on
them. In order to allow fine-grained feature checks, we support three builtin
function-like macros. This allows you to directly test for a feature in your
code without having to resort to something like autoconf or fragile "compiler
version checks".
### `__has_builtin`
This function-like macro takes a single identifier argument that is the name of
a builtin function, a builtin pseudo-function (taking one or more type
arguments), or a builtin template.
It evaluates to 1 if the builtin is supported or 0 if not.
It can be used like this:
```c++
#ifndef __has_builtin // Optional of course.
#define __has_builtin(x) 0 // Compatibility with non-clang compilers.
#endif
...
#if __has_builtin(__builtin_trap)
__builtin_trap();
#else
abort();
#endif
...
```
:::{note}
Prior to Clang 10, `__has_builtin` could not be used to detect most builtin
pseudo-functions.
`__has_builtin` should not be used to detect support for a builtin macro;
use `#ifdef` instead.
When compiling with target offloading, `__has_builtin` only considers the
currently active target.
:::
### `__has_constexpr_builtin`
This function-like macro takes a single identifier argument that is the name of
a builtin function, a builtin pseudo-function (taking one or more type
arguments), or a builtin template.
It evaluates to 1 if the builtin is supported and can be constant evaluated or
0 if not. It can be used for writing conditionally constexpr code like this:
```c++
#ifndef __has_constexpr_builtin // Optional of course.
#define __has_constexpr_builtin(x) 0 // Compatibility with non-clang compilers.
#endif
...
#if __has_constexpr_builtin(__builtin_fmax)
constexpr
#endif
double money_fee(double amount) {
return __builtin_fmax(amount * 0.03, 10.0);
}
...
```
For example, `__has_constexpr_builtin` is used in libcxx's implementation of
the `<cmath>` header file to conditionally make a function constexpr whenever
the constant evaluation of the corresponding builtin (for example,
`std::fmax` calls `__builtin_fmax`) is supported in Clang.
(langext-__has_feature-__has_extension)=
(langext-has-feature-has-extension)=
### `__has_feature` and `__has_extension`
These function-like macros take a single identifier argument that is the name
of a feature. `__has_feature` evaluates to 1 if the feature is both
supported by Clang and standardized in the current language standard or 0 if
not (but see {ref}`below <langext-has-feature-back-compat>`), while
`__has_extension` evaluates to 1 if the feature is supported by Clang in the
current language (either as a language extension or a standard language
feature) or 0 if not. They can be used like this:
```c++
#ifndef __has_feature // Optional of course.
#define __has_feature(x) 0 // Compatibility with non-clang compilers.
#endif
#ifndef __has_extension
#define __has_extension __has_feature // Compatibility with pre-3.0 compilers.
#endif
...
#if __has_feature(cxx_rvalue_references)
// This code will only be compiled with the -std=c++11 and -std=gnu++11
// options, because rvalue references are only standardized in C++11.
#endif
#if __has_extension(cxx_rvalue_references)
// This code will be compiled with the -std=c++11, -std=gnu++11, -std=c++98
// and -std=gnu++98 options, because rvalue references are supported as a
// language extension in C++98.
#endif
```
(langext-has-feature-back-compat)=
For backward compatibility, `__has_feature` can also be used to test
for support for non-standardized features, i.e., features not prefixed `c_`,
`cxx_` or `objc_`.
Another use of `__has_feature` is to check for compiler features not related
to the language standard, such as {doc}`AddressSanitizer
<AddressSanitizer>`.
If the `-pedantic-errors` option is given, `__has_extension` is equivalent
to `__has_feature`.
The feature tag is described along with the language feature below.
The feature name or extension name can also be specified with a preceding and
following `__` (double underscore) to avoid interference from a macro with
the same name. For instance, `__cxx_rvalue_references__` can be used instead
of `cxx_rvalue_references`.
### `__has_cpp_attribute`
This function-like macro is available in C++20 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 be either 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](https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations)
for the list of values returned for standards-based attributes. If the attribute
is not supported by the current compilation target, this macro evaluates to 0.
It can be used like this:
```c++
#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`.
### `__has_c_attribute`
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:
```c
#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`.
### `__has_attribute`
This function-like macro takes a single identifier argument that is the name of
a GNU-style attribute. It evaluates to 1 if the attribute is supported by the
current compilation target, or 0 if not. It can be used like this:
```c++
#ifndef __has_attribute // Optional of course.
#define __has_attribute(x) 0 // Compatibility with non-clang compilers.
#endif
...
#if __has_attribute(always_inline)
#define ALWAYS_INLINE __attribute__((always_inline))
#else
#define ALWAYS_INLINE
#endif
...
```
The attribute name can also be specified with a preceding and following `__`
(double underscore) to avoid interference from a macro with the same name. For
instance, `__always_inline__` can be used instead of `always_inline`.
### `__has_declspec_attribute`
This function-like macro takes a single identifier argument that is the name of
an attribute implemented as a Microsoft-style `__declspec` attribute. It
evaluates to 1 if the attribute is supported by the current compilation target,
or 0 if not. It can be used like this:
```c++
#ifndef __has_declspec_attribute // Optional of course.
#define __has_declspec_attribute(x) 0 // Compatibility with non-clang compilers.
#endif
...
#if __has_declspec_attribute(dllexport)
#define DLLEXPORT __declspec(dllexport)
#else
#define DLLEXPORT
#endif
...
```
The attribute name can also be specified with a preceding and following `__`
(double underscore) to avoid interference from a macro with the same name. For
instance, `__dllexport__` can be used instead of `dllexport`.
### `__is_identifier`
This function-like macro takes a single identifier argument that might be either
a reserved word or a regular identifier. It evaluates to 1 if the argument is just
a regular identifier and not a reserved word, in the sense that it can then be
used as the name of a user-defined function or variable. Otherwise it evaluates
to 0. It can be used like this:
```c++
...
#ifdef __is_identifier // Compatibility with non-clang compilers.
#if __is_identifier(__wchar_t)
typedef wchar_t __wchar_t;
#endif
#endif
__wchar_t WideCharacter;
...
```
## Include File Checking Macros
Not all developments systems have the same include files. The
{ref}`__has_include <langext-has-include>` and {ref}`__has_include_next <langext-has-include-next>` macros allow
you to check for the existence of an include file before doing a possibly
failing `#include` directive. Include file checking macros must be used
as expressions in `#if` or `#elif` preprocessing directives.
(langext-__has_include)=
(langext-has-include)=
### `__has_include`
This function-like macro takes a single file name string argument that is the
name of an include file. It evaluates to 1 if the file can be found using the
include paths, or 0 otherwise:
```c++
// Note the two possible file name string formats.
#if __has_include("myinclude.h") && __has_include(<stdint.h>)
# include "myinclude.h"
#endif
```
To test for this feature, use `#if defined(__has_include)`:
```c++
// To avoid problems with non-clang compilers not having this macro.
#if defined(__has_include)
#if __has_include("myinclude.h")
# include "myinclude.h"
#endif
#endif
```
(langext-__has_include_next)=
(langext-has-include-next)=
### `__has_include_next`
This function-like macro takes a single file name string argument that is the
name of an include file. It is like `__has_include` except that it looks for
the second instance of the given file found in the include paths. It evaluates
to 1 if the second instance of the file can be found using the include paths,
or 0 otherwise:
```c++
// Note the two possible file name string formats.
#if __has_include_next("myinclude.h") && __has_include_next(<stdint.h>)
# include_next "myinclude.h"
#endif
// To avoid problems with non-clang compilers not having this macro.
#if defined(__has_include_next)
#if __has_include_next("myinclude.h")
# include_next "myinclude.h"
#endif
#endif
```
Note that `__has_include_next`, like the GNU extension `#include_next`
directive, is intended for use in headers only, and will issue a warning if
used in the top-level compilation file. A warning will also be issued if an
absolute path is used in the file argument.
### `__has_warning`
This function-like macro takes a string literal that represents a command line
option for a warning and returns `true` if that is a valid warning option.
```c++
#if __has_warning("-Wformat")
...
#endif
```
(languageextensions-builtin-macros)=
## Builtin Macros
`__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. This is a standard feature in C2y but
is an extension in earlier language modes and in C++. This macro can only be
expanded 2147483647 times at most.
`__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)`".
`__clang_literal_encoding__`
: Defined to a narrow string literal that represents the current encoding of
narrow string literals, e.g., `"hello"`. This macro typically expands to
"UTF-8" (but may change in the future if the
`-fexec-charset="Encoding-Name"` option is implemented.)
`__clang_wide_literal_encoding__`
: Defined to a narrow string literal that represents the current encoding of
wide string literals, e.g., `L"hello"`. This macro typically expands to
"UTF-16" or "UTF-32" (but may change in the future if the
`-fwide-exec-charset="Encoding-Name"` option is implemented.)
## Implementation-defined keywords
### \_\_datasizeof
`__datasizeof` behaves like `sizeof`, except that it returns the size of the
type ignoring tail padding.
### \_BitInt, \_ExtInt
Clang supports the C23 `_BitInt(N)` feature as an extension in older C modes
and in C++. This type was previously implemented in Clang with the same
semantics, but spelled `_ExtInt(N)`. This spelling has been deprecated in
favor of the standard type.
Note: the ABI for `_BitInt(N)` is still in the process of being stabilized,
so this type should not yet be used in interfaces that require ABI stability.
### C keywords supported in all language modes
Clang supports `_Alignas`, `_Alignof`, `_Atomic`, `_Complex`,
`_Generic`, `_Imaginary`, `_Noreturn`, `_Static_assert`,
`_Thread_local`, and `_Float16` in all language modes with the C semantics.
### \_\_alignof, \_\_alignof\_\_
`__alignof` and `__alignof__` return, in contrast to `_Alignof` and
`alignof`, the preferred alignment of a type. This may be larger than the
required alignment for improved performance.
### \_\_extension\_\_
`__extension__` suppresses extension diagnostics in the statement it is
prepended to.
### \_\_auto_type
`__auto_type` behaves the same as `auto` in C++11 but is available in all
language modes.
### \_\_imag, \_\_imag\_\_
`__imag` and `__imag__` can be used to get the imaginary part of a complex
value.
### \_\_real, \_\_real\_\_
`__real` and `__real__` can be used to get the real part of a complex value.
### \_\_asm, \_\_asm\_\_
`__asm` and `__asm__` are alternate spellings for `asm`, but available in
all language modes.
### \_\_complex, \_\_complex\_\_
`__complex` and `__complex__` are alternate spellings for `_Complex`.
### \_\_const, \_\_const\_\_, \_\_volatile, \_\_volatile\_\_, \_\_restrict, \_\_restrict\_\_
These are alternate spellings for their non-underscore counterparts, but are
available in all language modes.
### \_\_decltype
`__decltype` is an alternate spelling for `decltype`, but is also available
in C++ modes before C++11.
### \_\_inline, \_\_inline\_\_
`__inline` and `__inline__` are alternate spellings for `inline`, but are
available in all language modes.
### \_\_nullptr
`__nullptr` is an alternate spelling for `nullptr`. It is available in all C and C++ language modes.
### \_\_signed, \_\_signed\_\_
`__signed` and `__signed__` are alternate spellings for `signed`.
`__unsigned` and `__unsigned__` are **not** supported.
### \_\_typeof, \_\_typeof\_\_, \_\_typeof_unqual, \_\_typeof_unqual\_\_
`__typeof` and `__typeof__` are alternate spellings for `typeof`, but are
available in all language modes. These spellings result in the operand,
retaining all qualifiers.
`__typeof_unqual` and `__typeof_unqual__` are alternate spellings for the
C23 `typeof_unqual` type specifier, but are available in all language modes.
These spellings result in the type of the operand, stripping all qualifiers.
### \_\_char16_t, \_\_char32_t
`__char16_t` and `__char32_t` are alternate spellings for `char16_t` and
`char32_t` respectively, but are also available in C++ modes before C++11.
They are only supported in C++. `__char8_t` is not available.
% FIXME: This should list all the keyword extensions
(langext-vectors)=
## Vectors and Extended Vectors
Supports the GCC, OpenCL, AltiVec, NEON, SVE and RVV vector extensions.
OpenCL vector types are created using the `ext_vector_type` attribute. It
supports the `V.xyzw` syntax and other tidbits as seen in OpenCL. An example
is:
```c++
typedef float float4 __attribute__((ext_vector_type(4)));
typedef float float2 __attribute__((ext_vector_type(2)));
float4 foo(float2 a, float2 b) {
float4 c;
c.xz = a;
c.yw = b;
return c;
}
```
Query for this feature with `__has_attribute(ext_vector_type)`.
Giving `-maltivec` option to clang enables support for AltiVec vector syntax
and functions. For example:
```c++
vector float foo(vector int a) {
vector int b;
b = vec_add(a, a) + a;
return (vector float)b;
}
```
NEON vector types are created using `neon_vector_type` and
`neon_polyvector_type` attributes. For example:
```c++
typedef __attribute__((neon_vector_type(8))) int8_t int8x8_t;
typedef __attribute__((neon_polyvector_type(16))) poly8_t poly8x16_t;
int8x8_t foo(int8x8_t a) {
int8x8_t v;
v = a;
return v;
}
```
GCC vector types are created using the `vector_size(N)` attribute. The
argument `N` specifies the number of bytes that will be allocated for an
object of this type. The size has to be a multiple of the size of the vector
element type. For example:
```c++
// OK: This declares a vector type with four 'int' elements
typedef int int4 __attribute__((vector_size(4 * sizeof(int))));
// ERROR: '11' is not a multiple of sizeof(int)
typedef int int_impossible __attribute__((vector_size(11)));
int4 foo(int4 a) {
int4 v;
v = a;
return v;
}
```
### Boolean Vectors
Clang also supports the `ext_vector_type` attribute with boolean element types in
C and C++. For example:
```c++
// legal for Clang, error for GCC:
typedef bool bool4 __attribute__((ext_vector_type(4)));
// Objects of bool4 type hold 8 bits, sizeof(bool4) == 1
bool4 foo(bool4 a) {
bool4 v;
v = a;
return v;
}
```
Boolean vectors are a Clang extension of the ext vector type. Boolean vectors
are intended, though not guaranteed, to map to vector mask registers. The size
parameter of a boolean vector type is the number of bits in the vector. The
boolean vector is dense and each bit in the boolean vector is one vector
element. Query for this feature with `__has_feature(ext_vector_type_boolean)`.
The semantics of boolean vectors borrows from C bit-fields with the following
differences:
- Distinct boolean vectors are always distinct memory objects (there is no
packing).
- Only the operators `?:`, `!`, `~`, `|`, `&`, `^` and comparison are allowed on
boolean vectors.
- Casting a scalar bool value to a boolean vector type means broadcasting the
scalar value onto all lanes (same as general ext_vector_type).
The size and alignment are both the number of bits rounded up to the next power
of two, but the alignment is at most the maximum vector alignment of the
target.
A boolean vector can be used in a ternary `?:` operator to select vector
elements of a different type.
```c++
typedef int int4 __attribute__((ext_vector_type(4)));
typedef bool bool4 __attribute__((ext_vector_type(4)));
int4 blend(bool4 cond, int4 a, int4 b) { return cond ? a : b; }
```
### Vector Literals
Vector literals can be used to create vectors from a set of scalars, or
vectors. Either parentheses or braces form can be used. In the parentheses
form the number of literal values specified must be one, i.e., referring to a
scalar value, or must match the size of the vector type being created. If a
single scalar literal value is specified, the scalar literal value will be
replicated to all the components of the vector type. In the brackets form any
number of literals can be specified. For example:
```c++
typedef int v4si __attribute__((__vector_size__(16)));
typedef float float4 __attribute__((ext_vector_type(4)));
typedef float float2 __attribute__((ext_vector_type(2)));
v4si vsi = (v4si){1, 2, 3, 4};
float4 vf = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
vector int vi1 = (vector int)(1); // vi1 will be (1, 1, 1, 1).
vector int vi2 = (vector int){1}; // vi2 will be (1, 0, 0, 0).
vector int vi3 = (vector int)(1, 2); // error
vector int vi4 = (vector int){1, 2}; // vi4 will be (1, 2, 0, 0).
vector int vi5 = (vector int)(1, 2, 3, 4);
float4 vf = (float4)((float2)(1.0f, 2.0f), (float2)(3.0f, 4.0f));
```
### Vector Operations
The table below shows the support for each operation by vector extension. A
dash indicates that an operation is not accepted according to a corresponding
specification.
| Operator | OpenCL | AltiVec | GCC | NEON | SVE | RVV |
| --- | --- | --- | --- | --- | --- | --- |
| [] | yes | yes | yes | yes | yes | yes |
| unary operators +, -- | yes | yes | yes | yes | yes | yes |
| ++, -- -- | yes | yes | yes | no | no | no |
| +,--,*,/,% | yes | yes | yes | yes | yes | yes |
| bitwise operators &,\|,^,~ | yes | yes | yes | yes | yes | yes |
| >>,<< | yes | yes | yes | yes | yes | yes |
| !, &&, \|\| | yes | -- | yes | yes | yes | yes |
| ==, !=, >, <, >=, <= | yes | yes | yes | yes | yes | yes |
| = | yes | yes | yes | yes | yes | yes |
| ?: <a class="footnote-reference brackets" href="#id5" id="id1" role="doc-noteref">[1]</a> | yes | -- | yes | yes | yes | yes |
| sizeof | yes | yes | yes | yes | yes <a class="footnote-reference brackets" href="#vls" id="id2" role="doc-noteref">[2]</a> | yes <a class="footnote-reference brackets" href="#vls" id="id3" role="doc-noteref">[2]</a> |
| C-style cast | yes | yes | yes | no | no | yes |
| reinterpret_cast | yes | no | yes | no | no | yes |
| static_cast | yes | no | yes | no | no | yes |
| const_cast | no | no | no | no | no | no |
| address &v[i] | no | no | no <a class="footnote-reference brackets" href="#id6" id="id4" role="doc-noteref">[3]</a> | no | no | no |
Both SVE and RVV define sizeless vector types which cannot be used in globals,
structs, unions, or arrays. Both provide an attribute (`arm_sve_vector_bits`
and `riscv_rvv_vector_bits` respectively) to create fixed-length
vector-length-specific (VLS) variants that remove these restrictions. Using
these attributes requires the command line option `-msve-vector-bits=<N>` or
`-mrvv-vector-bits=<N>` respectively. For SVE, the operators above are
supported on both sizeless and VLS types. For RVV, the operators are only
supported on VLS types.
See also {ref}`langext-builtin-shufflevector`, {ref}`langext-builtin-convertvector`.
<aside class="footnote-list brackets">
<aside class="footnote brackets" id="id5" role="doc-footnote">
<span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id1">1</a><span class="fn-bracket">]</span></span>
<p>ternary operator(?:) has different behaviors depending on the condition
operand's vector type. If the condition is a GNU vector (i.e.,
<code>__vector_size__</code>), a NEON vector, an SVE vector or an RVV vector,
it's only available in C++ and uses normal bool conversions (that is, != 0).
If it's an extension (OpenCL) vector, it's only available in C and OpenCL C.
And it selects based on the signedness of the condition operands (OpenCL v1.1 s6.3.9).</p>
</aside>
<aside class="footnote brackets" id="vls" role="doc-footnote">
<span class="label"><span class="fn-bracket">[</span>2<span class="fn-bracket">]</span></span>
<span class="backrefs">(<a role="doc-backlink" href="#id2">1</a>,<a role="doc-backlink" href="#id3">2</a>)</span>
<p>sizeof can only be used on vector length specific SVE and RVV types.</p>
</aside>
<aside class="footnote brackets" id="id6" role="doc-footnote">
<span class="label"><span class="fn-bracket">[</span><a role="doc-backlink" href="#id4">3</a><span class="fn-bracket">]</span></span>
<p>Clang does not allow the address of an element to be taken while GCC
allows this. This is intentional for vectors with a boolean element type and
not implemented otherwise.</p>
</aside>
</aside>
### Vector Builtins
**Note: The implementation of vector builtins is work-in-progress and incomplete.**
In addition to the operators mentioned above, Clang provides a set of builtins
to perform additional operations on certain scalar and vector types.
Let `T` be one of the following types:
- an integer type (as in C23 6.2.5p22), but excluding enumerated types and `bool`
- the standard floating types `float` or `double`
- a half-precision floating point type, if one is supported on the target
- a vector type.
For scalar types, consider the operation applied to a vector with a single element.
*Vector Size*
To determine the number of elements in a vector, use `__builtin_vectorelements()`.
For fixed-sized vectors, e.g., defined via `__attribute__((vector_size(N)))` or ARM
NEON's vector types (e.g., `uint16x8_t`), this returns the constant number of
elements at compile-time. For scalable vectors, e.g., SVE or RISC-V V, the number of
elements is not known at compile-time and is determined at runtime. This builtin can
be used, e.g., to increment the loop-counter in vector-type agnostic loops.
*Elementwise Builtins*
Each builtin returns a vector equivalent to applying the specified operation
elementwise to the input.
Unless specified otherwise operation(±0) = ±0 and operation(±infinity) = ±infinity
The elementwise intrinsics `__builtin_elementwise_popcount`,
`__builtin_elementwise_bitreverse`, `__builtin_elementwise_add_sat`,
`__builtin_elementwise_sub_sat`, `__builtin_elementwise_max`,
`__builtin_elementwise_min`, `__builtin_elementwise_abs`,
`__builtin_elementwise_clzg`, `__builtin_elementwise_ctzg`, and
`__builtin_elementwise_fma` can be called in a `constexpr` context.
No implicit promotion of integer types takes place. The mixing of integer types
of different sizes and signs is forbidden in binary and ternary builtins.
| Name | Operation | Supported element types |
| ---------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | -------------------------------------------- |
| T \_\_builtin_elementwise_abs(T x) | return the absolute value of a number x; the absolute value of the most negative integer remains the most negative integer | signed integer and floating point types |
| T \_\_builtin_elementwise_fma(T x, T y, T z) | fused multiply add, (x * y) + z. | floating point types |
| T \_\_builtin_elementwise_ceil(T x) | return the smallest integral value greater than or equal to x | floating point types |
| T \_\_builtin_elementwise_sin(T x) | return the sine of x interpreted as an angle in radians | floating point types |
| T \_\_builtin_elementwise_cos(T x) | return the cosine of x interpreted as an angle in radians | floating point types |
| T \_\_builtin_elementwise_tan(T x) | return the tangent of x interpreted as an angle in radians | floating point types |
| T \_\_builtin_elementwise_asin(T x) | return the arcsine of x interpreted as an angle in radians | floating point types |
| T \_\_builtin_elementwise_acos(T x) | return the arccosine of x interpreted as an angle in radians | floating point types |
| T \_\_builtin_elementwise_atan(T x) | return the arctangent of x interpreted as an angle in radians | floating point types |
| T \_\_builtin_elementwise_atan2(T y, T x) | return the arctangent of y/x | floating point types |
| T \_\_builtin_elementwise_sinh(T x) | return the hyperbolic sine of angle x in radians | floating point types |
| T \_\_builtin_elementwise_cosh(T x) | return the hyperbolic cosine of angle x in radians | floating point types |
| T \_\_builtin_elementwise_tanh(T x) | return the hyperbolic tangent of angle x in radians | floating point types |
| T \_\_builtin_elementwise_floor(T x) | return the largest integral value less than or equal to x | floating point types |
| T \_\_builtin_elementwise_log(T x) | return the natural logarithm of x | floating point types |
| T \_\_builtin_elementwise_log2(T x) | return the base 2 logarithm of x | floating point types |
| T \_\_builtin_elementwise_log10(T x) | return the base 10 logarithm of x | floating point types |
| T \_\_builtin_elementwise_popcount(T x) | return the number of 1 bits in x | integer types |
| T \_\_builtin_elementwise_pow(T x, T y) | return x raised to the power of y | floating point types |
| T \_\_builtin_elementwise_bitreverse(T x) | return the integer represented after reversing the bits of x | integer types |
| T \_\_builtin_elementwise_exp(T x) | returns the base-e exponential, e^x, of the specified value | floating point types |
| T \_\_builtin_elementwise_exp2(T x) | returns the base-2 exponential, 2^x, of the specified value | floating point types |
| T \_\_builtin_elementwise_exp10(T x) | returns the base-10 exponential, 10^x, of the specified value | floating point types |
| T \_\_builtin_elementwise_ldexp(T x, IntT y) | returns the product of x and 2 raised to the power y. y must be an integer type matching the shape of x. | T: floating point types, IntT: integer types |
| T \_\_builtin_elementwise_sqrt(T x) | return the square root of a floating-point number | floating point types |
| T \_\_builtin_elementwise_roundeven(T x) | round x to the nearest integer value in floating point format, rounding halfway cases to even (that is, to the nearest value that is an even integer), regardless of the current rounding direction. | floating point types |
| T \_\_builtin_elementwise_round(T x) | round x to the nearest integer value in floating point format, rounding halfway cases away from zero, regardless of the current rounding direction. May raise floating-point exceptions. | floating point types |
| T \_\_builtin_elementwise_trunc(T x) | return the integral value nearest to but no larger in magnitude than x | floating point types |
| T \_\_builtin_elementwise_nearbyint(T x) | round x to the nearest integer value in floating point format, rounding according to the current rounding direction. May not raise the inexact floating-point exception. This is treated the same as `__builtin_elementwise_rint` unless {ref}`FENV_ACCESS is enabled <floating-point-environment>`. | floating point types |
| T \_\_builtin_elementwise_rint(T x) | round x to the nearest integer value in floating point format, rounding according to the current rounding direction. May raise floating-point exceptions. This is treated the same as `__builtin_elementwise_nearbyint` unless {ref}`FENV_ACCESS is enabled <floating-point-environment>`. | floating point types |
| T \_\_builtin_elementwise_canonicalize(T x) | return the platform specific canonical encoding of a floating-point number | floating point types |
| T \_\_builtin_elementwise_copysign(T x, T y) | return the magnitude of x with the sign of y. | floating point types |
| T \_\_builtin_elementwise_fmod(T x, T y) | return the floating-point remainder of (x/y) whose sign matches the sign of x. | floating point types |
| T \_\_builtin_elementwise_max(T x, T y) | return x or y, whichever is larger For floating point types, follows semantics of maxNum in IEEE 754-2008. See [LangRef](http://llvm.org/docs/LangRef.html#i-fminmax-family) for the comparison. | integer floating point types (deprecated) |
| T \_\_builtin_elementwise_min(T x, T y) | return x or y, whichever is smaller For floating point types, follows semantics of minNum in IEEE 754-2008. See [LangRef](http://llvm.org/docs/LangRef.html#i-fminmax-family) for the comparison. | integer floating point types (deprecated) |
| T \_\_builtin_elementwise_maxnum(T x, T y) | return x or y, whichever is larger. Follows IEEE 754-2008 semantics (maxNum) with +0.0>-0.0. See [LangRef](http://llvm.org/docs/LangRef.html#i-fminmax-family) for the comparison. | floating point types |
| T \_\_builtin_elementwise_minnum(T x, T y) | return x or y, whichever is smaller. Follows IEEE 754-2008 semantics (minNum) with +0.0>-0.0. See [LangRef](http://llvm.org/docs/LangRef.html#i-fminmax-family) for the comparison. | floating point types |
| T \_\_builtin_elementwise_add_sat(T x, T y) | return the sum of x and y, clamped to the range of representable values for the signed/unsigned integer type. | integer types |
| T \_\_builtin_elementwise_sub_sat(T x, T y) | return the difference of x and y, clamped to the range of representable values for the signed/unsigned integer type. | integer types |
| T \_\_builtin_elementwise_maximum(T x, T y) | return x or y, whichever is larger. Follows IEEE 754-2019 semantics, see [LangRef](http://llvm.org/docs/LangRef.html#i-fminmax-family) for the comparison. | floating point types |
| T \_\_builtin_elementwise_minimum(T x, T y) | return x or y, whichever is smaller. Follows IEEE 754-2019 semantics, see [LangRef](http://llvm.org/docs/LangRef.html#i-fminmax-family) for the comparison. | floating point types |
| T \_\_builtin_elementwise_maximumnum(T x, T y) | return x or y, whichever is larger. Follows IEEE 754-2019 semantics, see [LangRef](http://llvm.org/docs/LangRef.html#i-fminmax-family) for the comparison. | floating point types |
| T \_\_builtin_elementwise_minimumnum(T x, T y) | return x or y, whichever is smaller. Follows IEEE 754-2019 semantics, see [LangRef](http://llvm.org/docs/LangRef.html#i-fminmax-family) for the comparison. | floating point types |
| T \_\_builtin_elementwise_fshl(T x, T y, T z) | perform a funnel shift left. Concatenate x and y (x is the most significant bits of the wide value), the combined value is shifted left by z (modulo the bit width of the original arguments), and the most significant bits are extracted to produce a result that is the same size as the original arguments. | integer types |
| T \_\_builtin_elementwise_fshr(T x, T y, T z) | perform a funnel shift right. Concatenate x and y (x is the most significant bits of the wide value), the combined value is shifted right by z (modulo the bit width of the original arguments), and the least significant bits are extracted to produce a result that is the same size as the original arguments. | integer types |
| T \_\_builtin_elementwise_clzg(T x[, T y]) | return the number of leading 0 bits in the first argument. If the first argument is 0 and an optional second argument is provided, the second argument is returned. It is undefined behaviour if the first argument is 0 and no second argument is provided. | integer types |
| T \_\_builtin_elementwise_ctzg(T x[, T y]) | return the number of trailing 0 bits in the first argument. If the first argument is 0 and an optional second argument is provided, the second argument is returned. It is undefined behaviour if the first argument is 0 and no second argument is provided. | integer types |
| T \_\_builtin_elementwise_clmul(T x, T y) | perform a carry-less multiplication of x and y, returning the least significant bits of the wide result. | integer types |
| T \_\_builtin_elementwise_pext(T x, T m) | extract bits from x selected by the mask m, pack them contiguously into the least significant bits of the result, and zero the rest. | integer types |
| T \_\_builtin_elementwise_pdep(T x, T m) | deposit the least significant bits of x at the positions where m has a 1-bit, and zero the rest. | integer types |
*Reduction Builtins*
Each builtin returns a scalar equivalent to applying the specified
operation(x, y) as recursive even-odd pairwise reduction to all vector
elements. `operation(x, y)` is repeatedly applied to each non-overlapping
even-odd element pair with indices `i * 2` and `i * 2 + 1` with
`i in [0, Number of elements / 2)`. If the number of elements is not a
power of 2, the vector is widened with neutral elements for the reduction
at the end to the next power of 2.
These reductions support both fixed-sized and scalable vector types.
The integer reduction intrinsics, including `__builtin_reduce_max`,
`__builtin_reduce_min`, `__builtin_reduce_add`, `__builtin_reduce_mul`,
`__builtin_reduce_and`, `__builtin_reduce_or`, and `__builtin_reduce_xor`,
can be called in a `constexpr` context.
Example:
```c++
__builtin_reduce_add([e3, e2, e1, e0]) = __builtin_reduced_add([e3 + e2, e1 + e0])
= (e3 + e2) + (e1 + e0)
```
Let `VT` be a vector type and `ET` the element type of `VT`.
| Name | Operation | Supported element types |
| ----------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------- |
| ET \_\_builtin_reduce_max(VT a) | return the largest element of the vector. The floating point result will always be a number unless all elements of the vector are NaN. | integer and floating point types |
| ET \_\_builtin_reduce_min(VT a) | return the smallest element of the vector. The floating point result will always be a number unless all elements of the vector are NaN. | integer and floating point types |
| ET \_\_builtin_reduce_add(VT a) | + | integer types |
| ET \_\_builtin_reduce_mul(VT a) | \* | integer types |
| ET \_\_builtin_reduce_and(VT a) | & | integer types |
| ET \_\_builtin_reduce_or(VT a) | \| | integer types |
| ET \_\_builtin_reduce_xor(VT a) | ^ | integer types |
| ET \_\_builtin_reduce_maximum(VT a) | return the largest element of the vector. Follows IEEE 754-2019 semantics, see [LangRef](http://llvm.org/docs/LangRef.html#i-fminmax-family) for the comparison. | floating point types |
| ET \_\_builtin_reduce_minimum(VT a) | return the smallest element of the vector. Follows IEEE 754-2019 semantics, see [LangRef](http://llvm.org/docs/LangRef.html#i-fminmax-family) for the comparison. | floating point types |
| ET \_\_builtin_reduce_assoc_fadd(VT a[, ET s]) | associative floating-point add reduction. | floating point types |
| ET \_\_builtin_reduce_in_order_fadd(VT a, ET s) | in order floating-point add reduction, initializing the accumulator with `(ET)s`, then adding each lane of the `a` in-order, starting from lane 0. The additions cannot be reassociated. | floating point types |
*Masked Builtins*
Each builtin accesses memory according to a provided boolean mask. These are
provided as `__builtin_masked_load` and `__builtin_masked_store`. The first
argument is always a boolean mask vector. The `__builtin_masked_load` builtin
takes an optional third vector argument that will be used for the result of the
masked-off lanes. These builtins assume the memory is unaligned, use
`__builtin_assume_aligned` if alignment is desired.
The `__builtin_masked_expand_load` and `__builtin_masked_compress_store`
builtins have the same interface but store the result in consecutive indices.
Effectively this performs the `if (mask[i]) val[i] = ptr[j++]` and `if
(mask[i]) ptr[j++] = val[i]` pattern respectively.
The `__builtin_masked_gather` and `__builtin_masked_scatter` builtins handle
non-sequential memory access for vector types. These use a base pointer and a
vector of integer indices to gather memory into a vector type or scatter it to
separate indices.
Example:
```c++
using v8b = bool [[clang::ext_vector_type(8)]];
using v8i = int [[clang::ext_vector_type(8)]];
v8i load(v8b mask, int *ptr) { return __builtin_masked_load(mask, ptr); }
v8i load_expand(v8b mask, int *ptr) {
return __builtin_masked_expand_load(mask, ptr);
}
void store(v8b mask, v8i val, int *ptr) {
__builtin_masked_store(mask, val, ptr);
}
void store_compress(v8b mask, v8i val, int *ptr) {
__builtin_masked_compress_store(mask, val, ptr);
}
v8i gather(v8b mask, v8i idx, int *ptr) {
return __builtin_masked_gather(mask, idx, ptr);
}
void scatter(v8b mask, v8i val, v8i idx, int *ptr) {
__builtin_masked_scatter(mask, idx, val, ptr);
}
```
## Matrix Types
Clang provides an extension for matrix types, which is currently being
implemented. See {ref}`the draft specification <matrixtypes>` for more details.
For example, the code below uses the matrix types extension to multiply two 4x4
float matrices and add the result to a third 4x4 matrix.
```c++
typedef float m4x4_t __attribute__((matrix_type(4, 4)));
m4x4_t f(m4x4_t a, m4x4_t b, m4x4_t c) {
return a + b * c;
}
```
The matrix type extension also supports operations on a matrix and a scalar.
```c++
typedef float m4x4_t __attribute__((matrix_type(4, 4)));
m4x4_t f(m4x4_t a) {
return (a + 23) * 12;
}
```
The matrix type extension supports division on a matrix and a scalar but not on a matrix and a matrix.
```c++
typedef float m4x4_t __attribute__((matrix_type(4, 4)));
m4x4_t f(m4x4_t a) {
a = a / 3.0;
return a;
}
```
The matrix type extension supports compound assignments for addition, subtraction, and multiplication on matrices
and on a matrix and a scalar, provided their types are consistent.
```c++
typedef float m4x4_t __attribute__((matrix_type(4, 4)));
m4x4_t f(m4x4_t a, m4x4_t b) {
a += b;
a -= b;
a *= b;
a += 23;
a -= 12;
return a;
}
```
The matrix type extension supports explicit casts. Implicit type conversion between matrix types is not allowed.
```c++
typedef int ix5x5 __attribute__((matrix_type(5, 5)));
typedef float fx5x5 __attribute__((matrix_type(5, 5)));
fx5x5 f1(ix5x5 i, fx5x5 f) {
return (fx5x5) i;
}
template <typename X>
using matrix_4_4 = X __attribute__((matrix_type(4, 4)));
void f2() {
matrix_5_5<double> d;
matrix_5_5<int> i;
i = (matrix_5_5<int>)d;
i = static_cast<matrix_5_5<int>>(d);
}
```
The matrix type extension supports column and row major memory layouts, but not
all builtins are supported with row-major layout. The layout defaults to column
major and can be specified using `-fmatrix-memory-layout`. To enable column
major layout, use `-fmatrix-memory-layout=column-major`, and for row major
layout use `-fmatrix-memory-layout=row-major`
## Half-Precision Floating Point
Clang supports three half-precision (16-bit) floating point types:
`__fp16`, `_Float16` and `__bf16`. These types are supported
in all language modes, but their support differs between targets.
A target is said to have "native support" for a type if the target
processor offers instructions for directly performing basic arithmetic
on that type. In the absence of native support, a type can still be
supported if the compiler can emulate arithmetic on the type by promoting
to `float`; see below for more information on this emulation.
- `__fp16` is supported on all targets. The special semantics of this
type mean that no arithmetic is ever performed directly on `__fp16` values;
see below.
- `_Float16` is supported on the following targets:
- 32-bit ARM (natively on some architecture versions)
- 64-bit ARM (AArch64) (natively on ARMv8.2a and above)
- AMDGPU (natively)
- NVPTX (natively)
- SPIR (natively)
- X86 (if SSE2 is available; natively if AVX512-FP16 is also available)
- RISC-V (natively if Zfh or Zhinx is available)
- SystemZ (emulated)
- LoongArch (emulated)
- `__bf16` is supported on the following targets (currently never natively):
- 32-bit ARM
- 64-bit ARM (AArch64)
- RISC-V
- X86 (when SSE2 is available)
- LoongArch
(For X86, SSE2 is available on 64-bit and all recent 32-bit processors.)
`__fp16` and `_Float16` both use the binary16 format from IEEE
754-2008, which provides a 5-bit exponent and an 11-bit significand
(including the implicit leading 1). `__bf16` uses the [bfloat16](https://en.wikipedia.org/wiki/Bfloat16_floating-point_format) format,
which provides an 8-bit exponent and an 8-bit significand; this is the same
exponent range as `float`, just with greatly reduced precision.
`_Float16` and `__bf16` follow the usual rules for arithmetic
floating-point types. Most importantly, this means that arithmetic operations
on operands of these types are formally performed in the type and produce
values of the type. `__fp16` does not follow those rules: most operations
immediately promote operands of type `__fp16` to `float`, and so
arithmetic operations are defined to be performed in `float` and so result in
a value of type `float` (unless further promoted because of other operands).
See below for more information on the exact specifications of these types.
When compiling arithmetic on `_Float16` and `__bf16` for a target without
native support, Clang will perform the arithmetic in `float`, inserting
extensions and truncations as necessary. This can be done in a way that
exactly matches the operation-by-operation behavior of native support,
but that can require many extra truncations and extensions. By default,
when emulating `_Float16` and `__bf16` arithmetic using `float`, Clang
does not truncate intermediate operands back to their true type unless the
operand is the result of an explicit cast or assignment. This is generally
much faster but can generate different results from strict operation-by-operation
emulation. Usually the results are more precise. This is permitted by the
C and C++ standards under the rules for excess precision in intermediate operands;
see the discussion of evaluation formats in the C standard and [expr.pre] in
the C++ standard.
The use of excess precision can be independently controlled for these two
types with the `-ffloat16-excess-precision=` and
`-fbfloat16-excess-precision=` options. Valid values include:
- `none`: meaning to perform strict operation-by-operation emulation
- `standard`: meaning that excess precision is permitted under the rules
described in the standard, i.e., never across explicit casts or statements
- `fast`: meaning that excess precision is permitted whenever the
optimizer sees an opportunity to avoid truncations; currently this has no
effect beyond `standard`
The `_Float16` type is an interchange floating type specified in
ISO/IEC TS 18661-3:2015 ("Floating-point extensions for C"). It will
be supported on more targets as they define ABIs for it.
The `__bf16` type is a non-standard extension, but it generally follows
the rules for arithmetic interchange floating types from ISO/IEC TS
18661-3:2015. In previous versions of Clang, it was a storage-only type
that forbade arithmetic operations. It will be supported on more targets
as they define ABIs for it.
The `__fp16` type was originally an ARM extension and is specified
by the [ARM C Language Extensions](https://github.com/ARM-software/acle/releases).
Clang uses the `binary16` format from IEEE 754-2008 for `__fp16`,
not the ARM alternative format. Operators that expect arithmetic operands
immediately promote `__fp16` operands to `float`.
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.
## Pragmas
### #pragma export
Clang supports the export pragma used to indicate an
external symbol is to be exported from the shared library being built. The
syntax for the pragma is:
```c++
#pragma export (name)
```
where `name` is the name of the external function or variable to be
exported. The symbol needs to have external linkage. The pragma may appear
before or after the declaration of `name`, but must precede the
definition. The pragma must also appear at file scope. If `name` is not
defined, the pragma will have no effect. The pragma needs to be specified
in the same translation unit as `name` is defined.
The pragma has the same effect as adding `__attribute__((visibility("default")))`
to the declaration of `name`.
In C++, the function being exported must be declared as `extern "C"`. If the
function has overloads, the pragma only applies to the overload with `extern "C"`
linkage. For example:
```c++
#pragma export(func)
int func(double) { return 0; }
extern "C" int func(int) { return 4;}
```
In the code above the pragma will export `func(int)` but not `func(double)`.
If none of the overloads are declared with `extern "C"` a warning will be
generated saying the pragma didn't resolve to a declaration. For example:
```c++
#pragma export(func)
int func(double) { return 0; } // warning: failed to resolve '#pragma export' to a declaration
```
## Overflow behavior types
Clang provides an extension that allows developers to annotate integer types
with explicit overflow behavior. This enables fine-grained control over whether
arithmetic operations should wrap on overflow (with two's complement semantics)
or be checked for overflow (trapping or reporting via sanitizers).
This feature is experimental and must be enabled with the `-cc1` option
`-fexperimental-overflow-behavior-types`.
Query for this feature with `__has_extension(overflow_behavior_types)`.
### Syntax
Overflow behavior can be specified using either attribute syntax or keyword
syntax:
**Attribute syntax:**
```c
typedef int __attribute__((overflow_behavior(wrap))) wrapping_int;
typedef int __attribute__((overflow_behavior(trap))) checked_int;
```
**Keyword syntax:**
```c
typedef int __ob_wrap wrapping_int;
typedef int __ob_trap checked_int;
```
The annotation can also be applied directly to variable declarations:
```c
int __ob_wrap counter = 0;
unsigned __ob_trap safe_index = 0;
```
### Semantics
**wrap behavior:**
When an integer type is annotated with `wrap` (or `__ob_wrap`), arithmetic
operations on values of that type use two's complement wrapping semantics on
overflow. This behavior is well-defined regardless of signedness. Compilers
must not optimize based on the assumption that overflow does not occur.
**trap behavior:**
When an integer type is annotated with `trap` (or `__ob_trap`), arithmetic
operations on values of that type are checked for overflow. If overflow occurs,
the program traps or reports via sanitizers (depending on compiler settings).
### Integer Promotions and Conversions
Overflow behavior types follow standard C integer promotion and conversion
rules. The overflow behavior annotation is preserved through implicit
promotions and conversions.
**Standard promotion rules apply:**
Integer literals without a suffix have type `int` (or a larger type if the
value requires it), following normal C rules. When such a literal is used in
an operation with an overflow behavior type, standard promotion rules determine
the result type, and the overflow behavior is propagated:
```c
typedef int __ob_wrap wrap_int;
wrap_int x = 100;
wrap_int y = x + 1; // 1 is promoted; result is __ob_wrap int
```
**Combining different overflow behaviors:**
When operands have different overflow behaviors, `trap` takes precedence
over `wrap`:
```c
typedef int __ob_wrap wrap_int;
typedef int __ob_trap trap_int;
wrap_int a = 1;
trap_int b = 2;
auto c = a + b; // Result is __ob_trap int (trap dominates)
```
**Conversion to standard types:**
When an overflow behavior type is converted to a standard integer type (without
an overflow behavior annotation), the overflow behavior is discarded. Compilers
may warn about this:
```c
typedef int __ob_wrap wrap_int;
wrap_int w = 42;
int i = w; // Warning: discards overflow behavior
```
### Interaction with Compiler Flags
Overflow behavior annotations take precedence over global compiler flags:
- A `wrap` type wraps on overflow even when `-ftrapv` is enabled.
- A `trap` type is checked for overflow even when `-fwrapv` is enabled.
This allows mixing different overflow behaviors within the same program,
enabling developers to selectively apply wrapping or checking to specific
types while using different defaults elsewhere.
### Incompatible Assignments
Direct assignment between types with different overflow behaviors (`wrap`
vs `trap`) is an error:
```c
int __ob_wrap w;
int __ob_trap t;
w = t; // Error: incompatible overflow behaviors
```
For more detailed documentation, see {doc}`OverflowBehaviorTypes`.
## Messages on `deprecated` and `unavailable` Attributes
An optional string message can be added to the `deprecated` and
`unavailable` attributes. For example:
```c++
void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));
```
If the deprecated or unavailable declaration is used, the message will be
incorporated into the appropriate diagnostic:
```none
harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!!
[-Wdeprecated-declarations]
explode();
^
```
Query for this feature with
`__has_extension(attribute_deprecated_with_message)` and
`__has_extension(attribute_unavailable_with_message)`.
## Attributes on Enumerators
Clang allows attributes to be written on individual enumerators. This allows
enumerators to be deprecated, made unavailable, etc. The attribute must appear
after the enumerator name and before any initializer, like so:
```c++
enum OperationMode {
OM_Invalid,
OM_Normal,
OM_Terrified __attribute__((deprecated)),
OM_AbortOnError __attribute__((deprecated)) = 4
};
```
Attributes on the `enum` declaration do not apply to individual enumerators.
Query for this feature with `__has_extension(enumerator_attributes)`.
## C++11 Attributes on using-declarations
Clang allows C++-style `[[]]` attributes to be written on using-declarations.
For instance:
```c++
[[clang::using_if_exists]] using foo::bar;
using foo::baz [[clang::using_if_exists]];
```
You can test for support for this extension with
`__has_extension(cxx_attributes_on_using_declarations)`.
## 'User-Specified' System Frameworks
Clang provides a mechanism by which frameworks can be built in such a way that
they will always be treated as being "system frameworks", even if they are not
present in a system framework directory. This can be useful to system
framework developers who want to be able to test building other applications
with development builds of their framework, including the manner in which the
compiler changes warning behavior for system headers.
Framework developers can opt-in to this mechanism by creating a
"`.system_framework`" file at the top-level of their framework. That is, the
framework should have contents like:
```none
.../TestFramework.framework
.../TestFramework.framework/.system_framework
.../TestFramework.framework/Headers
.../TestFramework.framework/Headers/TestFramework.h
...
```
Clang will treat the presence of this file as an indicator that the framework
should be treated as a system framework, regardless of how it was found in the
framework search path. For consistency, we recommend that such files never be
included in installed versions of the framework.
## Checks for Standard Language Features
The `__has_feature` macro can be used to query if certain standard language
features are enabled. The `__has_extension` macro can be used to query if
language features are available as an extension when compiling for a standard
which does not provide them. The features which can be tested are listed here.
Since Clang 3.4, the C++ SD-6 feature test macros are also supported.
These are macros with names of the form `__cpp_<feature_name>`, and are
intended to be a portable way to query the supported features of the compiler.
See [the C++ status page](https://clang.llvm.org/cxx_status.html#ts) for
information on the version of SD-6 supported by each Clang release, and the
macros provided by that revision of the recommendations.
### C++98
The features listed below are part of the C++98 standard. These features are
enabled by default when compiling C++ code.
#### C++ exceptions
Use `__has_feature(cxx_exceptions)` to determine if C++ exceptions have been
enabled. For example, compiling code with `-fno-exceptions` disables C++
exceptions.
#### C++ RTTI
Use `__has_feature(cxx_rtti)` to determine if C++ RTTI has been enabled. For
example, compiling code with `-fno-rtti` disables the use of RTTI.
### C++11
The features listed below are part of the C++11 standard. As a result, all
these features are enabled with the `-std=c++11` or `-std=gnu++11` option
when compiling C++ code.
#### C++11 SFINAE includes access control
Use `__has_feature(cxx_access_control_sfinae)` or
`__has_extension(cxx_access_control_sfinae)` to determine whether
access-control errors (e.g., calling a private constructor) are considered to
be template argument deduction errors (aka SFINAE errors), per [C++ DR1170](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1170).
#### C++11 alias templates
Use `__has_feature(cxx_alias_templates)` or
`__has_extension(cxx_alias_templates)` to determine if support for C++11's
alias declarations and alias templates is enabled.
#### C++11 alignment specifiers
Use `__has_feature(cxx_alignas)` or `__has_extension(cxx_alignas)` to
determine if support for alignment specifiers using `alignas` is enabled.
Use `__has_feature(cxx_alignof)` or `__has_extension(cxx_alignof)` to
determine if support for the `alignof` keyword is enabled.
#### C++11 attributes
Use `__has_feature(cxx_attributes)` or `__has_extension(cxx_attributes)` to
determine if support for attribute parsing with C++11's square bracket notation
is enabled.
#### C++11 generalized constant expressions
Use `__has_feature(cxx_constexpr)` to determine if support for generalized
constant expressions (e.g., `constexpr`) is enabled.
#### C++11 `decltype()`
Use `__has_feature(cxx_decltype)` or `__has_extension(cxx_decltype)` to
determine if support for the `decltype()` specifier is enabled. C++11's
`decltype` does not require type-completeness of a function call expression.
Use `__has_feature(cxx_decltype_incomplete_return_types)` or
`__has_extension(cxx_decltype_incomplete_return_types)` to determine if
support for this feature is enabled.
#### C++11 default template arguments in function templates
Use `__has_feature(cxx_default_function_template_args)` or
`__has_extension(cxx_default_function_template_args)` to determine if support
for default template arguments in function templates is enabled.
#### C++11 `default`ed functions
Use `__has_feature(cxx_defaulted_functions)` or
`__has_extension(cxx_defaulted_functions)` to determine if support for
defaulted function definitions (with `= default`) is enabled.
#### C++11 delegating constructors
Use `__has_feature(cxx_delegating_constructors)` to determine if support for
delegating constructors is enabled.
#### C++11 `deleted` functions
Use `__has_feature(cxx_deleted_functions)` or
`__has_extension(cxx_deleted_functions)` to determine if support for deleted
function definitions (with `= delete`) is enabled.
#### C++11 explicit conversion functions
Use `__has_feature(cxx_explicit_conversions)` to determine if support for
`explicit` conversion functions is enabled.
#### C++11 generalized initializers
Use `__has_feature(cxx_generalized_initializers)` to determine if support for
generalized initializers (using braced lists and `std::initializer_list`) is
enabled.
#### C++11 implicit move constructors/assignment operators
Use `__has_feature(cxx_implicit_moves)` to determine if Clang will implicitly
generate move constructors and move assignment operators where needed.
#### C++11 inheriting constructors
Use `__has_feature(cxx_inheriting_constructors)` to determine if support for
inheriting constructors is enabled.
#### C++11 inline namespaces
Use `__has_feature(cxx_inline_namespaces)` or
`__has_extension(cxx_inline_namespaces)` to determine if support for inline
namespaces is enabled.
#### C++11 lambdas
Use `__has_feature(cxx_lambdas)` or `__has_extension(cxx_lambdas)` to
determine if support for lambdas is enabled.
#### C++11 local and unnamed types as template arguments
Use `__has_feature(cxx_local_type_template_args)` or
`__has_extension(cxx_local_type_template_args)` to determine if support for
local and unnamed types as template arguments is enabled.
#### C++11 noexcept
Use `__has_feature(cxx_noexcept)` or `__has_extension(cxx_noexcept)` to
determine if support for noexcept exception specifications is enabled.
#### C++11 in-class non-static data member initialization
Use `__has_feature(cxx_nonstatic_member_init)` to determine whether in-class
initialization of non-static data members is enabled.
#### C++11 `nullptr`
Use `__has_feature(cxx_nullptr)` or `__has_extension(cxx_nullptr)` to
determine if support for `nullptr` is enabled.
#### C++11 `override control`
Use `__has_feature(cxx_override_control)` or
`__has_extension(cxx_override_control)` to determine if support for the
override control keywords is enabled.
#### C++11 reference-qualified functions
Use `__has_feature(cxx_reference_qualified_functions)` or
`__has_extension(cxx_reference_qualified_functions)` to determine if support
for reference-qualified functions (e.g., member functions with `&` or `&&`
applied to `*this`) is enabled.
#### C++11 range-based `for` loop
Use `__has_feature(cxx_range_for)` or `__has_extension(cxx_range_for)` to
determine if support for the range-based for loop is enabled.
#### C++11 raw string literals
Use `__has_feature(cxx_raw_string_literals)` to determine if support for raw
string literals (e.g., `R"x(foo\bar)x"`) is enabled.
#### C++11 rvalue references
Use `__has_feature(cxx_rvalue_references)` or
`__has_extension(cxx_rvalue_references)` to determine if support for rvalue
references is enabled.
#### C++11 `static_assert()`
Use `__has_feature(cxx_static_assert)` or
`__has_extension(cxx_static_assert)` to determine if support for compile-time
assertions using `static_assert` is enabled.
#### C++11 `thread_local`
Use `__has_feature(cxx_thread_local)` to determine if support for
`thread_local` variables is enabled.
#### C++11 type inference
Use `__has_feature(cxx_auto_type)` or `__has_extension(cxx_auto_type)` to
determine C++11 type inference is supported using the `auto` specifier. If
this is disabled, `auto` will instead be a storage class specifier, as in C
or C++98.
#### C++11 strongly typed enumerations
Use `__has_feature(cxx_strong_enums)` or
`__has_extension(cxx_strong_enums)` to determine if support for strongly
typed, scoped enumerations is enabled.
#### C++11 trailing return type
Use `__has_feature(cxx_trailing_return)` or
`__has_extension(cxx_trailing_return)` to determine if support for the
alternate function declaration syntax with trailing return type is enabled.
#### C++11 Unicode string literals
Use `__has_feature(cxx_unicode_literals)` to determine if support for Unicode
string literals is enabled.
#### C++11 unrestricted unions
Use `__has_feature(cxx_unrestricted_unions)` to determine if support for
unrestricted unions is enabled.
#### C++11 user-defined literals
Use `__has_feature(cxx_user_literals)` to determine if support for
user-defined literals is enabled.
#### C++11 variadic templates
Use `__has_feature(cxx_variadic_templates)` or
`__has_extension(cxx_variadic_templates)` to determine if support for
variadic templates is enabled.
### C++14
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.
#### C++14 binary literals
Use `__has_feature(cxx_binary_literals)` or
`__has_extension(cxx_binary_literals)` to determine whether
binary literals (for instance, `0b10010`) are recognized. Clang supports this
feature as an extension in all language modes.
#### C++14 contextual conversions
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.
#### C++14 decltype(auto)
Use `__has_feature(cxx_decltype_auto)` or
`__has_extension(cxx_decltype_auto)` to determine if support
for the `decltype(auto)` placeholder type is enabled.
#### C++14 default initializers for aggregates
Use `__has_feature(cxx_aggregate_nsdmi)` or
`__has_extension(cxx_aggregate_nsdmi)` to determine if support
for default initializers in aggregate members is enabled.
#### C++14 digit separators
Use `__cpp_digit_separators` to determine if support for digit separators
using single quotes (for instance, `10'000`) is enabled. At this time, there
is no corresponding `__has_feature` name.
#### C++14 generalized lambda capture
Use `__has_feature(cxx_init_captures)` or
`__has_extension(cxx_init_captures)` to determine if support for
lambda captures with explicit initializers is enabled
(for instance, `[n(0)] { return ++n; }`).
#### C++14 generic lambdas
Use `__has_feature(cxx_generic_lambdas)` or
`__has_extension(cxx_generic_lambdas)` to determine if support for generic
(polymorphic) lambdas is enabled
(for instance, `[] (auto x) { return x + 1; }`).
#### C++14 relaxed constexpr
Use `__has_feature(cxx_relaxed_constexpr)` or
`__has_extension(cxx_relaxed_constexpr)` to determine if variable
declarations, local variable modification, and control flow constructs
are permitted in `constexpr` functions.
#### C++14 return type deduction
Use `__has_feature(cxx_return_type_deduction)` or
`__has_extension(cxx_return_type_deduction)` to determine if support
for return type deduction for functions (using `auto` as a return type)
is enabled.
#### C++14 runtime-sized arrays
Use `__has_feature(cxx_runtime_array)` or
`__has_extension(cxx_runtime_array)` to determine if support
for arrays of runtime bound (a restricted form of variable-length arrays)
is enabled.
Clang's implementation of this feature is incomplete.
#### C++14 variable templates
Use `__has_feature(cxx_variable_templates)` or
`__has_extension(cxx_variable_templates)` to determine if support for
templated variable declarations is enabled.
#### C++ type aware allocators
Use `__has_extension(cxx_type_aware_allocators)` to determine the existence of
support for the future C++2d type aware allocator feature. For full details, see
{doc}`C++ Type Aware Allocators <CXXTypeAwareAllocators>` for additional details.
### C11
The features listed below are part of the C11 standard. As a result, all these
features are enabled with the `-std=c11` or `-std=gnu11` option when
compiling C code. Additionally, because these features are all
backward-compatible, they are available as extensions in all language modes.
#### C11 alignment specifiers
Use `__has_feature(c_alignas)` or `__has_extension(c_alignas)` to determine
if support for alignment specifiers using `_Alignas` is enabled.
Use `__has_feature(c_alignof)` or `__has_extension(c_alignof)` to determine
if support for the `_Alignof` keyword is enabled.
#### C11 atomic operations
Use `__has_feature(c_atomic)` or `__has_extension(c_atomic)` to determine
if support for atomic types using `_Atomic` is enabled. Clang also provides
{ref}`a set of builtins <langext-c11-atomic>` which can be used to implement
the `<stdatomic.h>` operations on `_Atomic` types. Use
`__has_include(<stdatomic.h>)` to determine if C11's `<stdatomic.h>` header
is available.
Clang will use the system's `<stdatomic.h>` header when one is available, and
will otherwise use its own. When using its own, implementations of the atomic
operations are provided as macros. In the cases where C11 also requires a real
function, this header provides only the declaration of that function (along
with a shadowing macro implementation), and you must link to a library which
provides a definition of the function if you use it instead of the macro.
#### C11 generic selections
Use `__has_feature(c_generic_selections)` or
`__has_extension(c_generic_selections)` to determine if support for generic
selections is enabled.
As an extension, the C11 generic selection expression is available in all
languages supported by Clang. The syntax is the same as that given in the C11
standard.
In C, type compatibility is decided according to the rules given in the
appropriate standard, but in C++, which lacks the type compatibility rules used
in C, types are considered compatible only if they are equivalent.
Clang also supports an extended form of `_Generic` with a controlling type
rather than a controlling expression. Unlike with a controlling expression, a
controlling type argument does not undergo any conversions and thus is suitable
for use when trying to match qualified types, incomplete types, or function
types. Variable-length array types lack the necessary compile-time information
to resolve which association they match with and thus are not allowed as a
controlling type argument.
Use `__has_extension(c_generic_selection_with_controlling_type)` to determine
if support for this extension is enabled.
#### C11 `_Static_assert()`
Use `__has_feature(c_static_assert)` or `__has_extension(c_static_assert)`
to determine if support for compile-time assertions using `_Static_assert` is
enabled.
#### C11 `_Thread_local`
Use `__has_feature(c_thread_local)` or `__has_extension(c_thread_local)`
to determine if support for `_Thread_local` variables is enabled.
### C2y
The features listed below are part of the C2y standard. As a result, all these
features are enabled with the `-std=c2y` or `-std=gnu2y` option when
compiling C code.
#### C2y `_Countof`
Use `__has_feature(c_countof)` (in C2y or later mode) or
`__has_extension(c_countof)` (in C23 or earlier mode) to determine if support
for the `_Countof` operator is enabled. This feature is not available in C++
mode.
### Modules
Use `__has_feature(modules)` to determine if Modules have been enabled.
For example, compiling code with `-fmodules` enables the use of Modules.
More information can be found [here](Modules.md).
## Language Extensions Back-ported to Previous Standards
| Feature | Feature Test Macro | Introduced In | Backported To |
| --------------------------------------------- | ---------------------------------- | ------------- | ------------- |
| variadic templates | \_\_cpp_variadic_templates | C++11 | C++03 |
| Alias templates | \_\_cpp_alias_templates | C++11 | C++03 |
| Non-static data member initializers | \_\_cpp_nsdmi | C++11 | C++03 |
| Range-based `for` loop | \_\_cpp_range_based_for | C++11 | C++03 |
| RValue references | \_\_cpp_rvalue_references | C++11 | C++03 |
| Attributes | \_\_cpp_attributes | C++11 | C++03 |
| Lambdas | \_\_cpp_lambdas | C++11 | C++03 |
| Generalized lambda captures | \_\_cpp_init_captures | C++14 | C++03 |
| Generic lambda expressions | \_\_cpp_generic_lambdas | C++14 | C++03 |
| variable templates | \_\_cpp_variable_templates | C++14 | C++03 |
| Binary literals | \_\_cpp_binary_literals | C++14 | C++03 |
| Relaxed constexpr | \_\_cpp_constexpr | C++14 | C++11 |
| Static assert with no message | \_\_cpp_static_assert >= 201411L | C++17 | C++11 |
| Pack expansion in generalized lambda-capture | \_\_cpp_init_captures | C++17 | C++03 |
| `if constexpr` | \_\_cpp_if_constexpr | C++17 | C++11 |
| fold expressions | \_\_cpp_fold_expressions | C++17 | C++03 |
| Lambda capture of \*this by value | \_\_cpp_capture_star_this | C++17 | C++03 |
| Attributes on enums | \_\_cpp_enumerator_attributes | C++17 | C++03 |
| Guaranteed copy elision | \_\_cpp_guaranteed_copy_elision | C++17 | C++03 |
| Hexadecimal floating literals | \_\_cpp_hex_float | C++17 | C++03 |
| `inline` variables | \_\_cpp_inline_variables | C++17 | C++03 |
| Attributes on namespaces | \_\_cpp_namespace_attributes | C++17 | C++11 |
| Structured bindings | \_\_cpp_structured_bindings | C++17 | C++03 |
| template template arguments | \_\_cpp_template_template_args | C++17 | C++03 |
| Familiar template syntax for generic lambdas | \_\_cpp_generic_lambdas | C++20 | C++03 |
| `static operator[]` | \_\_cpp_multidimensional_subscript | C++20 | C++03 |
| Designated initializers | \_\_cpp_designated_initializers | C++20 | C++03 |
| Conditional `explicit` | \_\_cpp_conditional_explicit | C++20 | C++03 |
| `using enum` | \_\_cpp_using_enum | C++20 | C++03 |
| `auto` parameters | | C++20 | C++03 |
| `if consteval` | \_\_cpp_if_consteval | C++23 | C++20 |
| `static operator()` | \_\_cpp_static_call_operator | C++23 | C++03 |
| Attributes on Lambda-Expressions | | C++23 | C++11 |
| Attributes on Structured Bindings | \_\_cpp_structured_bindings | C++26 | C++03 |
| Packs in Structured Bindings | \_\_cpp_structured_bindings | C++26 | C++03 |
| Structured binding declaration as a condition | \_\_cpp_structured_bindings | C++26 | C++98 |
| Static assert with user-generated message | \_\_cpp_static_assert >= 202306L | C++26 | C++11 |
| Pack Indexing | \_\_cpp_pack_indexing | C++26 | C++03 |
| `= delete ("should have a reason");` | \_\_cpp_deleted_function | C++26 | C++03 |
| Variadic Friends | \_\_cpp_variadic_friend | C++26 | C++03 |
| Trivial Relocatability | \_\_cpp_trivial_relocatability | C++26 | C++03 |
|``auto()`` cast | \_\_cpp_auto_cast | C++26 | C++03 |
| Pack Indexing for Template Names | \_\_cpp_pack_indexing >= 202606L | C++2d | C++03 |
| Designated initializers (N494) | | C99 | C89 |
| `_Complex` (N693) | | C99 | C89, C++ |
| `_Bool` (N815) | | C99 | C89 |
| Variable-length arrays (N683) | | C99 | C89, C++ |
| Flexible array members | | C99 | C89, C++ |
| static and type quals in arrays | | C99 | C89 |
| `long long` (N601) | | C99 | C89 |
| Hexadecimal floating constants (N308) | | C99 | C89 |
| Compound literals (N716) | | C99 | C89, C++ |
| `//` comments (N644) | | C99 | C89 |
| Mixed declarations and code (N740) | | C99 | C89 |
| init-statement in for (N740) | | C99 | C89 |
| Variadic macros (N707) | | C99 | C89 |
| Empty macro arguments (N570) | | C99 | C89 |
| Trailing comma in enum declaration | | C99 | C89 |
| Implicit `return 0` in `main` | | C99 | C89 |
| `__func__` (N611) | | C99 | C89 |
| `_Generic` (N1441) | | C11 | C89, C++ |
| `_Static_assert` (N1330) | | C11 | C89, C++ |
| `_Atomic` (N1485) | | C11 | C89, C++ |
| `_Thread_local` (N1364) | | C11 | C89, C++ |
| Array & element qualification (N2607) | | C23 | C89 |
| Attributes (N2335) | | C23 | C89 |
| `#embed` (N3017) | | C23 | C89, C++ |
| Enum with fixed underlying type (N3030) | | C23 | C89 |
| `#warning` (N2686) | | C23 | C89 |
| `_BitInt` (N3035) | | C23 | C89, C++ |
| Binary literals (N2549) | | C23 | C89 |
| Unnamed parameters in a function definition | | C23 | C89 |
| Free positioning of labels (N2508) | | C23 | C89 |
| `#elifdef` (N2645) | | C23 | C89 |
| `__has_include` (N2799) | | C23 | C89 |
| Octal literals prefixed with `0o` or `0O` | | C2y | C89, C++ |
| `_Countof` (N3369, N3469) | | C2y | C89 |
| `_Generic` with a type operand (N3260) | | C2y | C89, C++ |
| `++`/`--` on `_Complex` value (N3259) | | C2y | C89, C++ |
| `__COUNTER__` (N3457) | | C2y | C89, C++ |
| If declarations (N3356) | | C2y | C89 |
## Builtin type aliases
Clang provides a few builtin aliases to improve the throughput of certain metaprogramming facilities.
### \_\_builtin_common_type
```c++
template <template <class... Args> class BaseTemplate,
template <class TypeMember> class HasTypeMember,
class HasNoTypeMember,
class... Ts>
using __builtin_common_type = ...;
```
This alias is used for implementing `std::common_type`. If `std::common_type` should contain a `type` member,
it is an alias to `HasTypeMember<TheCommonType>`. Otherwise it is an alias to `HasNoTypeMember`. The
`BaseTemplate` is usually `std::common_type`. `Ts` are the arguments to `std::common_type`.
### \_\_type_pack_element
```c++
template <std::size_t Index, class... Ts>
using __type_pack_element = ...;
```
This alias returns the type at `Index` in the parameter pack `Ts`.
### \_\_make_integer_seq
```c++
template <template <class IntSeqT, IntSeqT... Ints> class IntSeq, class T, T N>
using __make_integer_seq = ...;
```
This alias returns `IntSeq` instantiated with `IntSeqT = T` and `Ints` being the pack `0, ..., N - 1`.
### \_\_builtin_dedup_pack
```c++
template <class... Ts>
using __builtin_dedup_pack = ...;
```
This alias takes a template parameter pack `Ts` and produces a new unexpanded pack containing the unique types
from `Ts`, with the order of the first occurrence of each type preserved.
It is useful in template metaprogramming to normalize type lists.
The resulting pack can be expanded in contexts like template argument lists or base specifiers.
**Example of Use**:
```c++
template <typename...> struct TypeList;
// The resulting type is TypeList<int, double, char>
template <typename ...ExtraTypes>
using MyTypeList = TypeList<__builtin_dedup_pack<int, double, int, char, double, ExtraTypes...>...>;
```
**Limitations**:
- This builtin can only be used inside a template.
- The resulting pack is currently only supported for expansion in template argument lists and base specifiers.
- This builtin cannot be assigned to a template template parameter.
### \_\_builtin_sort_pack
```c++
template <class... Ts>
using __builtin_sort_pack = ...;
```
This alias takes a template parameter pack `Ts` and produces a new unexpanded pack containing the same types
sorted by [`__builtin_type_order`](#builtin-type-order).
The resulting pack can be expanded in contexts like template argument lists or base specifiers.
**Example of Use**:
```c++
template <typename...> struct TypeList;
// Combined with `__builtin_dedup_pack` to canonicalize a type list.
template <typename ...ExtraTypes>
using MyTypeList = TypeList<
__builtin_sort_pack<__builtin_dedup_pack<int, double, ExtraTypes...>...>...>;
```
**Limitations**:
- This builtin can only be used inside a template.
- The resulting pack is currently only supported for expansion in template argument lists and base specifiers.
- This builtin cannot be assigned to a template template parameter.
## Type Trait Primitives
Type trait primitives are special builtin constant expressions that can be used
by the standard C++ library to facilitate or simplify the implementation of
user-facing type traits in the `<type_traits>` header.
They are not intended to be used directly by user code because they are
implementation-defined and subject to change -- as such they're tied closely to
the supported set of system headers, currently:
- LLVM's own libc++
- GNU libstdc++
- The Microsoft standard C++ library
Clang supports the [GNU C++ type traits](https://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html) and a subset of the
[Microsoft Visual C++ type traits](<https://msdn.microsoft.com/en-us/library/ms177194(v=VS.100).aspx>),
as well as nearly all of the
[Embarcadero C++ type traits](<http://docwiki.embarcadero.com/RADStudio/Rio/en/Type_Trait_Functions_(C%2B%2B11)_Index>).
The following type trait primitives are supported by Clang. Those traits marked
(C++) provide implementations for type traits specified by the C++ standard;
`__X(...)` has the same semantics and constraints as the corresponding
`std::X_t<...>` or `std::X_v<...>` type trait.
- `__array_rank(type)` (Embarcadero):
Returns the number of levels of array in the type `type`:
`0` if `type` is not an array type, and
`__array_rank(element) + 1` if `type` is an array of `element`.
- `__array_extent(type, dim)` (Embarcadero):
The `dim`'th array bound in the type `type`, or `0` if
`dim >= __array_rank(type)`.
- `__builtin_is_implicit_lifetime` (C++, GNU, Microsoft)
- `__builtin_is_virtual_base_of` (C++, GNU, Microsoft)
- `__can_pass_in_regs` (C++)
Returns whether a class can be passed in registers under the current
ABI. This type can only be applied to unqualified class types.
This is not a portable type trait.
- `__has_nothrow_assign` (GNU, Microsoft, Embarcadero):
Deprecated, use `__is_nothrow_assignable` instead.
- `__has_nothrow_move_assign` (GNU, Microsoft):
Deprecated, use `__is_nothrow_assignable` instead.
- `__has_nothrow_copy` (GNU, Microsoft):
Deprecated, use `__is_nothrow_constructible` instead.
- `__has_nothrow_constructor` (GNU, Microsoft):
Deprecated, use `__is_nothrow_constructible` instead.
- `__has_trivial_assign` (GNU, Microsoft, Embarcadero):
Deprecated, use `__is_trivially_assignable` instead.
- `__has_trivial_move_assign` (GNU, Microsoft):
Deprecated, use `__is_trivially_assignable` instead.
- `__has_trivial_copy` (GNU, Microsoft):
Deprecated, use `__is_trivially_copyable` instead.
- `__has_trivial_constructor` (GNU, Microsoft):
Deprecated, use `__is_trivially_constructible` instead.
- `__has_trivial_move_constructor` (GNU, Microsoft):
Deprecated, use `__is_trivially_constructible` instead.
- `__has_trivial_destructor` (GNU, Microsoft, Embarcadero):
Deprecated, use `__is_trivially_destructible` instead.
- `__has_unique_object_representations` (C++, GNU)
- `__has_virtual_destructor` (C++, GNU, Microsoft, Embarcadero)
- `__is_abstract` (C++, GNU, Microsoft, Embarcadero)
- `__is_aggregate` (C++, GNU, Microsoft)
- `__is_arithmetic` (C++, Embarcadero)
- `__is_array` (C++, Embarcadero)
- `__is_assignable` (C++, MSVC 2015)
- `__is_base_of` (C++, GNU, Microsoft, Embarcadero)
- `__is_bounded_array` (C++, GNU, Microsoft, Embarcadero)
- `__is_class` (C++, GNU, Microsoft, Embarcadero)
- `__is_complete_type(type)` (Embarcadero):
Return `true` if `type` is a complete type.
Warning: this trait is dangerous because it can return different values at
different points in the same program.
- `__is_compound` (C++, Embarcadero)
- `__is_const` (C++, Embarcadero)
- `__is_constructible` (C++, MSVC 2013)
- `__is_convertible` (C++, Embarcadero)
- `__is_nothrow_convertible` (C++, GNU)
- `__is_convertible_to` (Microsoft):
Synonym for `__is_convertible`.
- `__is_destructible` (C++, MSVC 2013)
- `__is_empty` (C++, GNU, Microsoft, Embarcadero)
- `__is_enum` (C++, GNU, Microsoft, Embarcadero)
- `__is_final` (C++, GNU, Microsoft)
- `__is_floating_point` (C++, Embarcadero)
- `__is_function` (C++, Embarcadero)
- `__is_fundamental` (C++, Embarcadero)
- `__is_integral` (C++, Embarcadero)
- `__is_interface_class` (Microsoft):
Returns `false`, even for types defined with `__interface`.
- `__is_layout_compatible` (C++, GNU, Microsoft)
- `__is_literal` (Clang):
Synonym for `__is_literal_type`.
- `__is_literal_type` (C++, GNU, Microsoft):
Note, the corresponding standard trait was deprecated in C++17
and removed in C++20.
- `__is_lvalue_reference` (C++, Embarcadero)
- `__is_member_object_pointer` (C++, Embarcadero)
- `__is_member_function_pointer` (C++, Embarcadero)
- `__is_member_pointer` (C++, Embarcadero)
- `__is_nothrow_assignable` (C++, MSVC 2013)
- `__is_nothrow_constructible` (C++, MSVC 2013)
- `__is_nothrow_destructible` (C++, MSVC 2013)
- `__is_object` (C++, Embarcadero)
- `__is_pod` (C++, GNU, Microsoft, Embarcadero):
Note, the corresponding standard trait was deprecated in C++20.
- `__is_pointer` (C++, Embarcadero)
- `__is_pointer_interconvertible_base_of` (C++, GNU, Microsoft)
- `__is_polymorphic` (C++, GNU, Microsoft, Embarcadero)
- `__is_reference` (C++, Embarcadero)
- `__is_rvalue_reference` (C++, Embarcadero)
- `__is_same` (C++, Embarcadero)
- `__is_same_as` (GCC): Synonym for `__is_same`.
- `__is_scalar` (C++, Embarcadero)
- `__is_scoped_enum` (C++, GNU, Microsoft, Embarcadero)
- `__is_sealed` (Microsoft):
Synonym for `__is_final`.
- `__is_signed` (C++, Embarcadero):
Returns false for enumeration types, and returns true for floating-point
types. Note, before Clang 10, returned true for enumeration types if the
underlying type was signed, and returned false for floating-point types.
- `__is_standard_layout` (C++, GNU, Microsoft, Embarcadero)
- `__is_trivial` (C++, GNU, Microsoft, Embarcadero)
- `__is_trivially_assignable` (C++, GNU, Microsoft)
- `__is_trivially_constructible` (C++, GNU, Microsoft)
- `__is_trivially_copyable` (C++, GNU, Microsoft)
- `__is_trivially_destructible` (C++, MSVC 2013)
- `__is_trivially_relocatable` (Clang) (Deprecated,
use `__builtin_is_cpp_trivially_relocatable` instead).
Returns true if moving an object
of the given type, and then destroying the source object, is known to be
functionally equivalent to copying the underlying bytes and then dropping the
source object on the floor. This is true of trivial types,
C++26 relocatable types, and types which
were made trivially relocatable via the `clang::trivial_abi` attribute.
This trait is deprecated and should be replaced by
`__builtin_is_cpp_trivially_relocatable`. Note, however, that it is generally
unsafe to relocate a C++-relocatable type with `memcpy` or `memmove`;
use `__builtin_trivially_relocate`.
- `__builtin_is_cpp_trivially_relocatable` (C++): Returns true if an object
is trivially relocatable, as defined by the C++26 standard [meta.unary.prop].
Note that when relocating the caller code should ensure that if the object is polymorphic,
the dynamic type is of the most derived type. Padding bytes should not be copied.
- `__is_trivially_equality_comparable` (Clang): Returns true if comparing two
objects of the provided type is known to be equivalent to comparing their
object representations. Note that types containing padding bytes are never
trivially equality comparable.
- `__is_unbounded_array` (C++, GNU, Microsoft, Embarcadero)
- `__is_union` (C++, GNU, Microsoft, Embarcadero)
- `__is_unsigned` (C++, Embarcadero):
Returns false for enumeration types. Note, before Clang 13, returned true for
enumeration types if the underlying type was unsigned.
- `__is_void` (C++, Embarcadero)
- `__is_volatile` (C++, Embarcadero)
- `__reference_binds_to_temporary(T, U)` (Clang): Determines whether a
reference of type `T` bound to an expression of type `U` would bind to a
materialized temporary object. If `T` is not a reference type, the result
is false. Note this trait will also return false when the initialization of
`T` from `U` is ill-formed.
Deprecated, use `__reference_constructs_from_temporary`.
- `__reference_constructs_from_temporary(T, U)` (C++)
Returns true if a reference `T` can be direct-initialized from a temporary of type
a non-cv-qualified `U`.
- `__reference_converts_from_temporary(T, U)` (C++)
: Returns true if a reference `T` can be copy-initialized from a temporary of type
a non-cv-qualified `U`.
- `__underlying_type` (C++, GNU, Microsoft)
- `__builtin_lt_synthesizes_from_spaceship`, `__builtin_gt_synthesizes_from_spaceship`,
`__builtin_le_synthesizes_from_spaceship`, `__builtin_ge_synthesizes_from_spaceship` (Clang):
These builtins can be used to determine whether the corresponding operator is synthesized from a spaceship operator.
- (builtin-type-order)=
`__builtin_type_order` (C++): Returns `std::strong_ordering::less` if `T` precedes `U` in an
implementation-defined total ordering of all types, `std::strong_ordering::greater` if `U` precedes `T`,
and `std::strong_ordering::equal` if they are the same type.
The order is stable across translation units for a given platform/ABI.
In addition, the following expression traits are supported:
- `__is_lvalue_expr(e)` (Embarcadero):
Returns true if `e` is an lvalue expression.
Deprecated, use `__is_lvalue_reference(decltype((e)))` instead.
- `__is_rvalue_expr(e)` (Embarcadero):
Returns true if `e` is a prvalue expression.
Deprecated, use `!__is_reference(decltype((e)))` instead.
There are multiple ways to detect support for a type trait `__X` in the
compiler, depending on the oldest version of Clang you wish to support.
- From Clang 10 onwards, `__has_builtin(__X)` can be used.
- From Clang 6 onwards, `!__is_identifier(__X)` can be used.
- From Clang 3 onwards, `__has_feature(X)` can be used, but only supports
the following traits:
- `__has_nothrow_assign`
- `__has_nothrow_copy`
- `__has_nothrow_constructor`
- `__has_trivial_assign`
- `__has_trivial_copy`
- `__has_trivial_constructor`
- `__has_trivial_destructor`
- `__has_virtual_destructor`
- `__is_abstract`
- `__is_base_of`
- `__is_class`
- `__is_constructible`
- `__is_convertible_to`
- `__is_empty`
- `__is_enum`
- `__is_final`
- `__is_literal`
- `__is_standard_layout`
- `__is_pod`
- `__is_polymorphic`
- `__is_sealed`
- `__is_trivial`
- `__is_trivially_assignable`
- `__is_trivially_constructible`
- `__is_trivially_copyable`
- `__is_union`
- `__underlying_type`
A simplistic usage example as might be seen in standard C++ headers follows:
```c++
#if __has_builtin(__is_convertible_to)
template<typename From, typename To>
struct is_convertible_to {
static const bool value = __is_convertible_to(From, To);
};
#else
// Emulate type trait for compatibility with other compilers.
#endif
```
(builtin_structured_binding_size-doc)=
(builtin-structured-binding-size-doc)=
### \_\_builtin_structured_binding_size (C++)
The `__builtin_structured_binding_size(T)` type trait returns
the *structured binding size* ([dcl.struct.bind]) of type `T`
This is equivalent to the size of the pack `p` in `auto&& [...p] = declval<T&>();`.
If the argument cannot be decomposed, `__builtin_structured_binding_size(T)`
is not a valid expression (`__builtin_structured_binding_size` is SFINAE-friendly).
builtin arrays, builtin SIMD vectors,
builtin complex types, *tuple-like* types, and decomposable class types
are decomposable types.
A type is considered a valid *tuple-like* if `std::tuple_size_v<T>` is a valid expression,
even if there is no valid `std::tuple_element` specialization or suitable
`get` function for that type.
```c++
template<std::size_t Idx, typename T>
requires (Idx < __builtin_structured_binding_size(T))
decltype(auto) constexpr get_binding(T&& obj) {
auto && [...p] = std::forward<T>(obj);
return p...[Idx];
}
struct S { int a = 0, b = 42; };
static_assert(__builtin_structured_binding_size(S) == 2);
static_assert(get_binding<1>(S{}) == 42);
```
## Blocks
The syntax and high-level language feature description is in
{doc}`BlockLanguageSpec<BlockLanguageSpec>`. Implementation and ABI details for
the clang implementation are in {doc}`Block-ABI-Apple<Block-ABI-Apple>`.
Query for this feature with `__has_extension(blocks)`.
## ASM Goto with Output Constraints
Outputs may be used along any branches from the `asm goto` whether the
branches are taken or not.
Query for this feature with `__has_extension(gnu_asm_goto_with_outputs)`.
Prior to clang-16, the output may only be used safely when the indirect
branches are not taken. Query for this difference with
`__has_extension(gnu_asm_goto_with_outputs_full)`.
When using tied-outputs (i.e., outputs that are inputs and outputs, not just
outputs) with the `+r` constraint, there is a hidden input that's created
before the label, so numeric references to operands must account for that.
```c++
int foo(int x) {
// %0 and %1 both refer to x
// %l2 refers to err
asm goto("# %0 %1 %l2" : "+r"(x) : : : err);
return x;
err:
return -1;
}
```
This was changed to match GCC in clang-13; for better portability, symbolic
references can be used instead of numeric references.
```c++
int foo(int x) {
asm goto("# %[x] %l[err]" : [x]"+r"(x) : : : err);
return x;
err:
return -1;
}
```
## ASM Goto versus Branch Target Enforcement
Some target architectures implement branch target enforcement, by requiring
indirect (register-controlled) branch instructions to jump only to locations
marked by a special instruction (such as AArch64 `bti`).
The assembler code inside an `asm goto` statement is expected not to use a
branch instruction of that kind to transfer control to any of its destination
labels. Therefore, using a label in an `asm goto` statement does not cause
clang to put a `bti` or equivalent instruction at the label.
## Constexpr strings in GNU ASM statements
In C++11 mode (and greater), Clang supports specifying the template,
constraints, and clobber strings with a parenthesized constant expression
producing an object with the following member functions
```c++
constexpr const char* data() const;
constexpr size_t size() const;
```
such as `std::string`, `std::string_view`, `std::vector<char>`.
This mechanism follows the same rules as `static_assert` messages in
C++26, see `[dcl.pre]/p12`.
Query for this feature with `__has_extension(gnu_asm_constexpr_strings)`.
```c++
int foo() {
asm((std::string_view("nop")) ::: (std::string_view("memory")));
}
```
## Objective-C Features
### Related result types
According to Cocoa conventions, Objective-C methods with certain names
("`init`", "`alloc`", etc.) always return objects that are an instance of
the receiving class's type. Such methods are said to have a "related result
type", meaning that a message send to one of these methods will have the same
static type as an instance of the receiver class. For example, given the
following classes:
```objc
@interface NSObject
+ (id)alloc;
- (id)init;
@end
@interface NSArray : NSObject
@end
```
and this common initialization pattern
```objc
NSArray *array = [[NSArray alloc] init];
```
the type of the expression `[NSArray alloc]` is `NSArray*` because
`alloc` implicitly has a related result type. Similarly, the type of the
expression `[[NSArray alloc] init]` is `NSArray*`, since `init` has a
related result type and its receiver is known to have the type `NSArray *`.
If neither `alloc` nor `init` had a related result type, the expressions
would have had type `id`, as declared in the method signature.
A method with a related result type can be declared by using the type
`instancetype` as its result type. `instancetype` is a contextual keyword
that is only permitted in the result type of an Objective-C method, e.g.
```objc
@interface A
+ (instancetype)constructAnA;
@end
```
The related result type can also be inferred for some methods. To determine
whether a method has an inferred related result type, the first word in the
camel-case selector (e.g., "`init`" in "`initWithObjects`") is considered,
and the method will have a related result type if its return type is compatible
with the type of its class and if:
- the first word is "`alloc`" or "`new`", and the method is a class method,
or
- the first word is "`autorelease`", "`init`", "`retain`", or "`self`",
and the method is an instance method.
If a method with a related result type is overridden by a subclass method, the
subclass method must also return a type that is compatible with the subclass
type. For example:
```objc
@interface NSString : NSObject
- (NSUnrelated *)init; // incorrect usage: NSUnrelated is not NSString or a superclass of NSString
@end
```
Related result types only affect the type of a message send or property access
via the given method. In all other respects, a method with a related result
type is treated the same way as method that returns `id`.
Use `__has_feature(objc_instancetype)` to determine whether the
`instancetype` contextual keyword is available.
### Automatic reference counting
Clang provides support for {doc}`automated reference counting
<AutomaticReferenceCounting>` in Objective-C, which eliminates the need
for manual `retain`/`release`/`autorelease` message sends. There are 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.
(objc-weak)=
### Weak references
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.
(objc-fixed-enum)=
### Enumerations with a fixed underlying type
Clang provides support for C++11 enumerations with a fixed underlying type
within Objective-C and C [prior to C23](https://open-std.org/JTC1/SC22/WG14/www/docs/n3030.htm). For example, one can write an enumeration type as:
```c++
typedef enum : unsigned char { Red, Green, Blue } Color;
```
This specifies that the underlying type, which is used to store the enumeration
value, is `unsigned char`.
Use `__has_feature(objc_fixed_enum)` to determine whether support for fixed
underlying types is available in Objective-C.
Use `__has_extension(c_fixed_enum)` to determine whether support for fixed
underlying types is available in C prior to C23. This will also report `true` in C23
and later modes as the functionality is available even if it's not an extension in
those modes.
Use `__has_feature(c_fixed_enum)` to determine whether support for fixed
underlying types is available in C23 and later.
### Enumerations with no enumerators
Clang provides support for Microsoft extensions to support enumerations with no enumerators.
```c++
typedef enum empty { } A;
```
### Microsoft Anonymous Structs and Unions
Clang provides support for a Microsoft extension that allows use of named struct or union types to
declare anonymous members inside another struct or union, making their fields directly accessible
from the enclosing type.
For example, consider the following code:
```c
struct Inner {
int x;
int y;
};
struct Outer {
struct Inner; /* Microsoft extension: named anonymous struct member */
};
void f(struct Outer *o) {
o->x = 1; /* accesses x member of anonymous member of type Inner directly */
o->y = 1; /* accesses x member of anonymous member of type Inner directly */
}
```
Without this extension, such declarations generate a warning that the declaration does not
declare anything, the associated member names are not available for access, and the layout
of types containing such declarations are affected accordingly.
This extension can be controlled independently of other Microsoft extensions:
`-fms-anonymous-structs`
: Enable named anonymous struct/union support
`-fno-ms-anonymous-structs`
: Disable anonymous struct/union support
This extension is also **implicitly enabled** when either of the following options is used:
- `-fms-extensions`
- `-fms-compatibility`
When multiple controlling options are specified, the last option on the command line takes
precedence.
### Interoperability with C++11 lambdas
Clang provides interoperability between C++11 lambdas and blocks-based APIs, by
permitting a lambda to be implicitly converted to a block pointer with the
corresponding signature. For example, consider an API such as `NSArray`'s
array-sorting method:
```objc
- (NSArray *)sortedArrayUsingComparator:(NSComparator)cmptr;
```
`NSComparator` is simply a typedef for the block pointer `NSComparisonResult
(^)(id, id)`, and parameters of this type are generally provided with block
literals as arguments. However, one can also use a C++11 lambda so long as it
provides the same signature (in this case, accepting two parameters of type
`id` and returning an `NSComparisonResult`):
```objc
NSArray *array = @[@"string 1", @"string 21", @"string 12", @"String 11",
@"String 02"];
const NSStringCompareOptions comparisonOptions
= NSCaseInsensitiveSearch | NSNumericSearch |
NSWidthInsensitiveSearch | NSForcedOrderingSearch;
NSLocale *currentLocale = [NSLocale currentLocale];
NSArray *sorted
= [array sortedArrayUsingComparator:[=](id s1, id s2) -> NSComparisonResult {
NSRange string1Range = NSMakeRange(0, [s1 length]);
return [s1 compare:s2 options:comparisonOptions
range:string1Range locale:currentLocale];
}];
NSLog(@"sorted: %@", sorted);
```
This code relies on an implicit conversion from the type of the lambda
expression (an unnamed, local class type called the *closure type*) to the
corresponding block pointer type. The conversion itself is expressed by a
conversion operator in that closure type that produces a block pointer with the
same signature as the lambda itself, e.g.,
```objc
operator NSComparisonResult (^)(id, id)() const;
```
This conversion function returns a new block that simply forwards the two
parameters to the lambda object (which it captures by copy), then returns the
result. The returned block is first copied (with `Block_copy`) and then
autoreleased. As an optimization, if a lambda expression is immediately
converted to a block pointer (as in the first example, above), then the block
is not copied and autoreleased: rather, it is given the same lifetime as a
block literal written at that point in the program, which avoids the overhead
of copying a block to the heap in the common case.
The conversion from a lambda to a block pointer is only available in
Objective-C++, and not in C++ with blocks, due to its use of Objective-C memory
management (autorelease).
### Object Literals and Subscripting
Clang provides support for {doc}`Object Literals and Subscripting
<ObjectiveCLiterals>` in Objective-C, which simplifies common Objective-C
programming patterns, makes programs more concise, and improves the safety of
container creation. There are several feature macros associated with object
literals and subscripting: `__has_feature(objc_array_literals)` tests the
availability of array literals; `__has_feature(objc_dictionary_literals)`
tests the availability of dictionary literals; `objc_constant_literals`
tests the availability of having number, array, and dictionary literals
emitted at compile time; `__has_feature(objc_subscripting)` tests the availability of object
subscripting.
### Objective-C Autosynthesis of Properties
Clang provides support for autosynthesis of declared properties. Using this
feature, clang provides default synthesis of those properties not declared
@dynamic and not having user-provided backing getter and setter methods.
`__has_feature(objc_default_synthesize_properties)` checks for availability
of this feature in version of clang being used.
(langext-objc-retain-release)=
### Objective-C retaining behavior attributes
In Objective-C, functions and methods are generally assumed to follow the
[Cocoa Memory Management](https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html)
conventions for ownership of object arguments and
return values. However, there are exceptions, and so Clang provides attributes
to allow these exceptions to be documented. These are used by ARC and the
[static analyzer](https://clang-analyzer.llvm.org) Some exceptions may be
better described using the `objc_method_family` attribute instead.
**Usage**: The `ns_returns_retained`, `ns_returns_not_retained`,
`ns_returns_autoreleased`, `cf_returns_retained`, and
`cf_returns_not_retained` attributes can be placed on methods and functions
that return Objective-C or CoreFoundation objects. They are commonly placed at
the end of a function prototype or method declaration:
```objc
id foo() __attribute__((ns_returns_retained));
- (NSString *)bar:(int)x __attribute__((ns_returns_retained));
```
The `*_returns_retained` attributes specify that the returned object has a +1
retain count. The `*_returns_not_retained` attributes specify that the return
object has a +0 retain count, even if the normal convention for its selector
would be +1. `ns_returns_autoreleased` specifies that the returned object is
+0, but is guaranteed to live at least as long as the next flush of an
autorelease pool.
**Usage**: The `ns_consumed` and `cf_consumed` attributes can be placed on
a 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.
```objc
void foo(__attribute__((ns_consumed)) NSString *string);
- (void) bar __attribute__((ns_consumes_self));
- (void) baz:(id) __attribute__((ns_consumed)) x;
```
Further examples of these attributes are available in the static analyzer's
{ref}`list of annotations for analysis <cocoa_mem>`.
Query for these features with `__has_attribute(ns_consumed)`,
`__has_attribute(ns_returns_retained)`, etc.
(langext-objective-c-available)=
### Objective-C @available
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 `-mmacos-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](AttributeReference.md#availability) 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:
```objc
void my_fun(NSSomeClass* var) {
// If fancyNewMethod was added in e.g., macOS 10.12, but the code is
// built with -mmacos-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())`:
```objc
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()`:
```objc
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](AttributeReference.md#availability) to it,
which will also suppress the warning and require that calls to my_fun() are
checked:
```objc
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.
(langext-overloading)=
### Objective-C++ ABI: protocol-qualifier mangling of parameters
Starting with LLVM 3.4, Clang produces a new mangling for parameters whose
type is a qualified-`id` (e.g., `id<Foo>`). This mangling allows such
parameters to be differentiated from those with the regular unqualified `id`
type.
This was a non-backward compatible mangling change to the ABI. This change
allows proper overloading, and also prevents mangling conflicts with template
parameters of protocol-qualified type.
Query the presence of this new mangling with
`__has_feature(objc_protocol_qualifier_mangling)`.
## Initializer lists for complex numbers in C
clang supports an extension which allows the following in C:
```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:
```c++
complex float x[] = { { 1.0f, 1.0f } }; // [0] = (1, 1)
complex float x[] = { 1.0f, 1.0f }; // [0] = (1, 0), [1] = (1, 0)
```
This extension also works in C++ mode, as far as that goes, but does not apply
to the C++ `std::complex`. (In C++11, list initialization allows the same
syntax to be used with `std::complex` with the same meaning.)
For GCC compatibility, `__builtin_complex(re, im)` can also be used to
construct a complex number from the given real and imaginary components.
## OpenCL Features
Clang supports internal OpenCL extensions documented below.
### `__cl_clang_bitfields`
With this extension it is possible to enable bitfields in structs
or unions using the OpenCL extension pragma mechanism detailed in
[the OpenCL Extension Specification, section 1.2](https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_Ext.html#extensions-overview).
Use of bitfields in OpenCL kernels can result in reduced portability as struct
layout is not guaranteed to be consistent when compiled by different compilers.
If structs with bitfields are used as kernel function parameters, it can result
in incorrect functionality when the layout is different between the host and
device code.
**Example of Use**:
```c++
#pragma OPENCL EXTENSION __cl_clang_bitfields : enable
struct with_bitfield {
unsigned int i : 5; // compiled - no diagnostic generated
};
#pragma OPENCL EXTENSION __cl_clang_bitfields : disable
struct without_bitfield {
unsigned int i : 5; // error - bitfields are not supported
};
```
### `__cl_clang_function_pointers`
With this extension it is possible to enable various language features that
are relying on function pointers using regular OpenCL extension pragma
mechanism detailed in [the OpenCL Extension Specification,
section 1.2](https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_Ext.html#extensions-overview).
In C++ for OpenCL this also enables:
- Use of member function pointers;
- Unrestricted use of references to functions;
- Virtual member functions.
Such functionality is not conformant and does not guarantee to compile
correctly in any circumstances. It can be used if:
- the kernel source does not contain call expressions to (member-) function
pointers, or virtual functions. For example, this extension can be used in
metaprogramming algorithms to be able to specify/detect types generically.
- the generated kernel binary does not contain indirect calls because they
are eliminated using compiler optimizations e.g., devirtualization.
- the selected target supports the function pointer like functionality e.g.
most CPU targets.
**Example of Use**:
```c++
#pragma OPENCL EXTENSION __cl_clang_function_pointers : enable
void foo()
{
void (*fp)(); // compiled - no diagnostic generated
}
#pragma OPENCL EXTENSION __cl_clang_function_pointers : disable
void bar()
{
void (*fp)(); // error - pointers to function are not allowed
}
```
### `__cl_clang_variadic_functions`
With this extension it is possible to enable variadic arguments in functions
using regular OpenCL extension pragma mechanism detailed in [the OpenCL
Extension Specification, section 1.2](https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_Ext.html#extensions-overview).
This is not conformant behavior and it can only be used portably when the
functions with variadic prototypes do not get generated in binary e.g., the
variadic prototype is used to specify a function type with any number of
arguments in metaprogramming algorithms in C++ for OpenCL.
This extension can also be used when the kernel code is intended for targets
supporting the variadic arguments e.g., majority of CPU targets.
**Example of Use**:
```c++
#pragma OPENCL EXTENSION __cl_clang_variadic_functions : enable
void foo(int a, ...); // compiled - no diagnostic generated
#pragma OPENCL EXTENSION __cl_clang_variadic_functions : disable
void bar(int a, ...); // error - variadic prototype is not allowed
```
### `__cl_clang_non_portable_kernel_param_types`
With this extension it is possible to enable the use of some restricted types
in kernel parameters specified in [C++ for OpenCL v1.0 s2.4](https://www.khronos.org/opencl/assets/CXX_for_OpenCL.html#kernel_function).
The restrictions can be relaxed using regular OpenCL extension pragma mechanism
detailed in [the OpenCL Extension Specification, section 1.2](https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_Ext.html#extensions-overview).
This is not a conformant behavior and it can only be used when the
kernel arguments are not accessed on the host side or the data layout/size
between the host and device is known to be compatible.
**Example of Use**:
```c++
// Plain Old Data type.
struct Pod {
int a;
int b;
};
// Not POD type because of the constructor.
// Standard layout type because there is only one access control.
struct OnlySL {
int a;
int b;
OnlySL() : a(0), b(0) {}
};
// Not standard layout type because of two different access controls.
struct NotSL {
int a;
private:
int b;
};
#pragma OPENCL EXTENSION __cl_clang_non_portable_kernel_param_types : enable
kernel void kernel_main(
Pod a,
OnlySL b,
global NotSL *c,
global OnlySL *d
);
#pragma OPENCL EXTENSION __cl_clang_non_portable_kernel_param_types : disable
```
### `__cl_clang_function_scope_local_variables`
This extension allows declaring variables in the local address space within
function scope, including non-kernel functions or nested scopes within a kernel,
using regular OpenCL extension pragma mechanism detailed in [the OpenCL
Extension Specification, section 1.2](https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_Ext.html#extensions-overview).
This relaxes the [Declaration Scopes and Variable Types](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_C.html#_usage_for_declaration_scopes_and_variable_types)
rule that limits local-address-space variable declarations to the outermost
compound statement inside the body of the kernel function.
To expose static local allocations at kernel scope, targets can either force-
inline non-kernel functions that declare local memory or pass a kernel-allocated
local buffer to those functions via an implicit argument.
```c++
#pragma OPENCL EXTENSION __cl_clang_function_scope_local_variables : enable
kernel void kernel1(...)
{
{
local float a; // compiled - no diagnostic generated
}
}
void foo()
{
local float c; // compiled - no diagnostic generated
}
#pragma OPENCL EXTENSION __cl_clang_function_scope_local_variables : disable
kernel void kernel2(...)
{
{
local float a; // error - variables in the local address space can only be declared in the outermost scope of a kernel function
}
}
void bar()
{
local float c; // error - non-kernel function variable cannot be declared in local address space
}
```
### Remove address space builtin function
`__remove_address_space` allows to derive types in C++ for OpenCL
that have address space qualifiers removed. This utility only affects
address space qualifiers, therefore, other type qualifiers such as
`const` or `volatile` remain unchanged.
**Example of Use**:
```c++
template<typename T>
void foo(T *par){
T var1; // error - local function variable with global address space
__private T var2; // error - conflicting address space qualifiers
__private __remove_address_space<T>::type var3; // var3 is __private int
}
void bar(){
__global int* ptr;
foo(ptr);
}
```
### Legacy 1.x atomics with generic address space
Clang allows the use of atomic functions from the OpenCL 1.x standards
with the generic address space pointer in C++ for OpenCL mode.
This is a non-portable feature and might not be supported by all
targets.
**Example of Use**:
```c++
void foo(__generic volatile unsigned int* a) {
atomic_add(a, 1);
}
```
## WebAssembly Features
Clang supports the WebAssembly features documented below. For further
information related to the semantics of the builtins, please refer to the [WebAssembly Specification](https://webassembly.github.io/spec/core/).
In this section, when we refer to reference types, we are referring to
WebAssembly reference types, not C++ reference types unless stated
otherwise.
### `__builtin_wasm_table_set`
This builtin function stores a value in a WebAssembly table.
It takes three arguments.
The first argument is the table to store a value into, the second
argument is the index to which to store the value into, and the
third argument is a value of reference type to store in the table.
It returns nothing.
```c++
static __externref_t table[0];
extern __externref_t JSObj;
void store(int index) {
__builtin_wasm_table_set(table, index, JSObj);
}
```
### `__builtin_wasm_table_get`
This builtin function is the counterpart to `__builtin_wasm_table_set`
and loads a value from a WebAssembly table of reference typed values.
It takes 2 arguments.
The first argument is a table of reference typed values and the
second argument is an index from which to load the value. It returns
the loaded reference typed value.
```c++
static __externref_t table[0];
__externref_t load(int index) {
__externref_t Obj = __builtin_wasm_table_get(table, index);
return Obj;
}
```
### `__builtin_wasm_table_size`
This builtin function returns the size of the WebAssembly table.
Takes the table as an argument and returns an unsigned integer (`size_t`)
with the current table size.
```c++
typedef void (*__funcref funcref_t)();
static funcref_t table[0];
size_t getSize() {
return __builtin_wasm_table_size(table);
}
```
### `__builtin_wasm_table_grow`
This builtin function grows the WebAssembly table by a certain amount.
Currently, as all WebAssembly tables created in C/C++ are zero-sized,
this always needs to be called to grow the table.
It takes three arguments. The first argument is the WebAssembly table
to grow. The second argument is the reference typed value to store in
the new table entries (the initialization value), and the third argument
is the amount to grow the table by. It returns the previous table size
or -1. It will return -1 if not enough space could be allocated.
```c++
typedef void (*__funcref funcref_t)();
static funcref_t table[0];
// grow returns the new table size or -1 on error.
int grow(funcref_t fn, int delta) {
int prevSize = __builtin_wasm_table_grow(table, fn, delta);
if (prevSize == -1)
return -1;
return prevSize + delta;
}
```
### `__builtin_wasm_table_fill`
This builtin function sets all the entries of a WebAssembly table to a given
reference typed value. It takes four arguments. The first argument is
the WebAssembly table, the second argument is the index that starts the
range, the third argument is the value to set in the new entries, and
the fourth and the last argument is the size of the range. It returns
nothing.
```c++
static __externref_t table[0];
// resets a table by setting all of its entries to a given value.
void reset(__externref_t Obj) {
int Size = __builtin_wasm_table_size(table);
__builtin_wasm_table_fill(table, 0, Obj, Size);
}
```
### `__builtin_wasm_table_copy`
This builtin function copies elements from a source WebAssembly table
to a possibly overlapping destination region. It takes five arguments.
The first argument is the destination WebAssembly table, and the second
argument is the source WebAssembly table. The third argument is the
destination index from where the copy starts, the fourth argument is the
source index from where the copy starts, and the fifth and last argument
is the number of elements to copy. It returns nothing.
```c++
static __externref_t tableSrc[0];
static __externref_t tableDst[0];
// Copy nelem elements from [src, src + nelem - 1] in tableSrc to
// [dst, dst + nelem - 1] in tableDst
void copy(int dst, int src, int nelem) {
__builtin_wasm_table_copy(tableDst, tableSrc, dst, src, nelem);
}
```
### `__builtin_wasm_memory_copy`
This builtin function copies bytes from a source memory to a possibly
overlapping destination region using the WebAssembly `memory.copy` instruction.
It takes five arguments:
1. Destination memory index (must be a constant integer)
2. Source memory index (must be a constant integer)
3. Destination pointer (`void *`)
4. Source pointer (`const void *`)
5. Number of bytes to copy (`size_t`)
It returns nothing. Note that unlike C `memcpy` or `memmove`, `memory.copy`
traps if either pointer is out of bounds even when the number of bytes is zero.
```c++
void copy(void *dst, const void *src, size_t n) {
__builtin_wasm_memory_copy(0, 0, dst, src, n);
}
```
### `__builtin_wasm_memory_fill`
This builtin function sets bytes in memory using the WebAssembly `memory.fill`
instruction. It takes four arguments:
1. Memory index (must be a constant integer)
2. Destination pointer (`void *`)
3. Byte value to set (passed as `int`, lowest 8 bits used)
4. Number of bytes to set (`size_t`)
It returns nothing. Note that unlike C `memset`, `memory.fill` traps if the
pointer is out of bounds even when the number of bytes is zero.
```c++
void fill(void *dst, int val, size_t n) {
__builtin_wasm_memory_fill(0, dst, val, n);
}
```
## Builtin Functions
Clang supports a number of builtin library functions with the same syntax as
GCC, including things like `__builtin_nan`, `__builtin_constant_p`,
`__builtin_choose_expr`, `__builtin_types_compatible_p`,
`__builtin_assume_aligned`, `__sync_fetch_and_add`, etc. In addition to
the GCC builtins, Clang supports a number of builtins that GCC does not, which
are listed here.
Please note that Clang does not and will not support all of the GCC builtins
for vector operations. Instead of using builtins, you should use the functions
defined in target-specific header files like `<xmmintrin.h>`, which define
portable wrappers for these. Many of the Clang versions of these functions are
implemented directly in terms of {ref}`extended vector support
<langext-vectors>` instead of builtins, in order to reduce the number of
builtins that we need to implement.
### `__builtin_alloca`
`__builtin_alloca` is used to dynamically allocate memory on the stack. Memory
is automatically freed upon function termination.
**Syntax**:
```c++
__builtin_alloca(size_t n)
```
**Example of Use**:
```c++
void init(float* data, size_t nbelems);
void process(float* data, size_t nbelems);
int foo(size_t n) {
auto mem = (float*)__builtin_alloca(n * sizeof(float));
init(mem, n);
process(mem, n);
/* mem is automatically freed at this point */
}
```
**Description**:
`__builtin_alloca` is meant to be used to allocate a dynamic amount of memory
on the stack. This amount is subject to stack allocation limits.
Query for this feature with `__has_builtin(__builtin_alloca)`.
### `__builtin_alloca_with_align`
`__builtin_alloca_with_align` is used to dynamically allocate memory on the
stack while controlling its alignment. Memory is automatically freed upon
function termination.
**Syntax**:
```c++
__builtin_alloca_with_align(size_t n, size_t align)
```
**Example of Use**:
```c++
void init(float* data, size_t nbelems);
void process(float* data, size_t nbelems);
int foo(size_t n) {
auto mem = (float*)__builtin_alloca_with_align(
n * sizeof(float),
CHAR_BIT * alignof(float));
init(mem, n);
process(mem, n);
/* mem is automatically freed at this point */
}
```
**Description**:
`__builtin_alloca_with_align` is meant to be used to allocate a dynamic amount of memory
on the stack. It is similar to `__builtin_alloca` but accepts a second
argument whose value is the alignment constraint, as a power of 2 in *bits*.
Query for this feature with `__has_builtin(__builtin_alloca_with_align)`.
(langext-__builtin_assume)=
(langext-builtin-assume)=
### `__builtin_assume`
`__builtin_assume` is used to provide the optimizer with a boolean
invariant that is defined to be true.
**Syntax**:
```c++
__builtin_assume(bool)
```
**Example of Use**:
```c++
int foo(int x) {
__builtin_assume(x != 0);
// The optimizer may short-circuit this check using the invariant.
if (x == 0)
return do_something();
return do_something_else();
}
```
**Description**:
The boolean argument to this function is defined to be true. The optimizer may
analyze the form of the expression provided as the argument and deduce from
that information used to optimize the program. If the condition is violated
during execution, the behavior is undefined. The argument itself is never
evaluated, so any side effects of the expression will be discarded.
Query for this feature with `__has_builtin(__builtin_assume)`.
(langext-__builtin_assume_separate_storage)=
(langext-builtin-assume-separate-storage)=
### `__builtin_assume_separate_storage`
`__builtin_assume_separate_storage` is used to provide the optimizer with the
knowledge that its two arguments point to separately allocated objects.
**Syntax**:
```c++
__builtin_assume_separate_storage(const volatile void *, const volatile void *)
```
**Example of Use**:
```c++
int foo(int *x, int *y) {
__builtin_assume_separate_storage(x, y);
*x = 0;
*y = 1;
// The optimizer may optimize this to return 0 without reloading from *x.
return *x;
}
```
**Description**:
The arguments to this function are assumed to point into separately allocated
storage (either different variable definitions or different dynamic storage
allocations). The optimizer may use this fact to aid in alias analysis. If the
arguments point into the same storage, the behavior is undefined. Note that the
definition of "storage" here refers to the outermost enclosing allocation of any
particular object (so for example, it's never correct to call this function
passing the addresses of fields in the same struct, elements of the same array,
etc.).
Query for this feature with `__has_builtin(__builtin_assume_separate_storage)`.
### `__builtin_assume_dereferenceable`
`__builtin_assume_dereferenceable` is used to provide the optimizer with the
knowledge that the pointer argument P is dereferenceable up to at least the
specified number of bytes.
**Syntax**:
```c++
__builtin_assume_dereferenceable(const void *, size_t)
```
**Example of Use**:
```c++
int foo(int *x, int y) {
__builtin_assume_dereferenceable(x, sizeof(int));
int z = 0;
if (y == 1) {
// The optimizer may execute the load of x unconditionally due to
// __builtin_assume_dereferenceable guaranteeing sizeof(int) bytes can
// be loaded speculatively without trapping.
z = *x;
}
return z;
}
```
**Description**:
The arguments to this function provide a start pointer `P` and a size `S`.
`S` must be at least 1 and a constant. The optimizer may assume that `S`
bytes are dereferenceable starting at `P`. Note that this does not necessarily
imply that `P` is non-null as `nullptr` can be dereferenced in some cases.
The assumption also does not imply that `P` is not dereferenceable past `S`
bytes.
Query for this feature with `__has_builtin(__builtin_assume_dereferenceable)`.
### `__builtin_offsetof`
`__builtin_offsetof` is used to implement the `offsetof` macro, which
calculates the offset (in bytes) to a given member of the given type.
**Syntax**:
```c++
__builtin_offsetof(type-name, member-designator)
```
**Example of Use**:
```c++
struct S {
char c;
int i;
struct T {
float f[2];
} t;
};
const int offset_to_i = __builtin_offsetof(struct S, i);
const int ext1 = __builtin_offsetof(struct U { int i; }, i); // C extension
const int offset_to_subobject = __builtin_offsetof(struct S, t.f[1]);
```
**Description**:
This builtin is usable in an integer constant expression which returns a value
of type `size_t`. The value returned is the offset in bytes to the subobject
designated by the member-designator from the beginning of an object of type
`type-name`. Clang extends the required standard functionality in the
following way:
- In C language modes, the first argument may be the definition of a new type.
Any type declared this way is scoped to the nearest scope containing the call
to the builtin.
Query for this feature with `__has_builtin(__builtin_offsetof)`.
### `__builtin_get_vtable_pointer`
`__builtin_get_vtable_pointer` loads and authenticates the primary vtable
pointer from an instance of a polymorphic C++ class. This builtin is needed
for directly loading the vtable pointer when on platforms using
{doc}`PointerAuthentication`.
**Syntax**:
```c++
__builtin_get_vtable_pointer(PolymorphicClass*)
```
**Example of Use**:
```c++
struct PolymorphicClass {
virtual ~PolymorphicClass();
};
PolymorphicClass anInstance;
const void* vtablePointer = __builtin_get_vtable_pointer(&anInstance);
```
**Description**:
The `__builtin_get_vtable_pointer` builtin loads the primary vtable
pointer from a polymorphic C++ type. If the target platform authenticates
vtable pointers, this builtin will perform the authentication and produce
the underlying raw pointer. The object being queried must be polymorphic,
and so must also be a complete type.
Query for this feature with `__has_builtin(__builtin_get_vtable_pointer)`.
### `__builtin_call_with_static_chain`
`__builtin_call_with_static_chain` is used to perform a static call while
updating the static chain register.
**Syntax**:
```c++
T __builtin_call_with_static_chain(T expr, void* ptr)
```
**Example of Use**:
```c++
auto v = __builtin_call_with_static_chain(foo(3), foo);
```
**Description**:
This builtin returns `expr` after checking that `expr` is a non-member
static call expression. The call to that expression is made while using `ptr`
as a function pointer stored in a dedicated register to implement *static chain*
calling convention, as used by some language to implement closures or nested
functions.
Query for this feature with `__has_builtin(__builtin_call_with_static_chain)`.
### `__builtin_readcyclecounter`
`__builtin_readcyclecounter` is used to access the cycle counter register (or
a similar low-latency, high-accuracy clock) on those targets that support it.
**Syntax**:
```c++
__builtin_readcyclecounter()
```
**Example of Use**:
```c++
unsigned long long t0 = __builtin_readcyclecounter();
do_something();
unsigned long long t1 = __builtin_readcyclecounter();
unsigned long long cycles_to_do_something = t1 - t0; // assuming no overflow
```
**Description**:
The `__builtin_readcyclecounter()` builtin returns the cycle counter value,
which may be either global or process/thread-specific depending on the target.
As the backing counters often overflow quickly (on the order of seconds) this
should only be used for timing small intervals. When not supported by the
target, the return value is always zero. This builtin takes no arguments and
produces an unsigned long long result.
Query for this feature with `__has_builtin(__builtin_readcyclecounter)`. Note
that even if present, its use may depend on run-time privilege or other OS
controlled state.
### `__builtin_readsteadycounter`
`__builtin_readsteadycounter` is used to access the fixed frequency counter
register (or a similar steady-rate clock) on those targets that support it.
The function is similar to `__builtin_readcyclecounter` above except that the
frequency is fixed, making it suitable for measuring elapsed time.
**Syntax**:
```c++
__builtin_readsteadycounter()
```
**Example of Use**:
```c++
unsigned long long t0 = __builtin_readsteadycounter();
do_something();
unsigned long long t1 = __builtin_readsteadycounter();
unsigned long long secs_to_do_something = (t1 - t0) / tick_rate;
```
**Description**:
The `__builtin_readsteadycounter()` builtin returns the frequency counter value.
When not supported by the target, the return value is always zero. This builtin
takes no arguments and produces an unsigned long long result. The builtin does
not guarantee any particular frequency, only that it is stable. Knowledge of the
counter's true frequency will need to be provided by the user.
Query for this feature with `__has_builtin(__builtin_readsteadycounter)`.
### `__builtin_cpu_supports`
**Syntax**:
```c++
int __builtin_cpu_supports(const char *features);
```
**Example of Use:**:
```c++
if (__builtin_cpu_supports("sve"))
sve_code();
```
**Description**:
The `__builtin_cpu_supports` function detects if the run-time CPU supports
features specified in string argument. It returns a positive integer if all
features are supported and 0 otherwise. Feature names are target specific. On
AArch64, features are combined using `+` like this
`__builtin_cpu_supports("flagm+sha3+lse+rcpc2+fcma+memtag+bti+sme2")`.
If a feature name is not supported, Clang will issue a warning and replace
builtin by the constant 0.
Query for this feature with `__has_builtin(__builtin_cpu_supports)`.
### `__builtin_dump_struct`
**Syntax**:
```c++
__builtin_dump_struct(&some_struct, some_printf_func, args...);
```
**Examples**:
```c++
struct S {
int x, y;
float f;
struct T {
int i;
} t;
};
void func(struct S *s) {
__builtin_dump_struct(s, printf);
}
```
Example output:
```none
struct S {
int x = 100
int y = 42
float f = 3.141593
struct T t = {
int i = 1997
}
}
```
```c++
#include <string>
struct T { int a, b; };
constexpr void constexpr_sprintf(std::string &out, const char *format,
auto ...args) {
// ...
}
constexpr std::string dump_struct(auto &x) {
std::string s;
__builtin_dump_struct(&x, constexpr_sprintf, s);
return s;
}
static_assert(dump_struct(T{1, 2}) == R"(struct T {
int a = 1
int b = 2
}
)");
```
**Description**:
The `__builtin_dump_struct` function is used to print the fields of a simple
structure and their values for debugging purposes. The first argument of the
builtin should be a pointer to a complete record type to dump. The second argument `f`
should be some callable expression, and can be a function object or an overload
set. The builtin calls `f`, passing any further arguments `args...`
followed by a `printf`-compatible format string and the corresponding
arguments. `f` may be called more than once, and `f` and `args` will be
evaluated once per call. In C++, `f` may be a template or overload set and
resolve to different functions for each call.
In the format string, a suitable format specifier will be used for builtin
types that Clang knows how to format. This includes standard builtin types, as
well as aggregate structures, `void*` (printed with `%p`), and `const
char*` (printed with `%s`). A `*%p` specifier will be used for a field
that Clang doesn't know how to format, and the corresponding argument will be a
pointer to the field. This allows a C++ templated formatting function to detect
this case and implement custom formatting. A `*` will otherwise not precede a
format specifier.
This builtin does not return a value.
This builtin can be used in constant expressions.
Query for this feature with `__has_builtin(__builtin_dump_struct)`
(langext-__builtin_shufflevector)=
(langext-builtin-shufflevector)=
### `__builtin_shufflevector`
`__builtin_shufflevector` is used to express generic vector
permutation/shuffle/swizzle operations. This builtin is also very important
for the implementation of various target-specific header files like
`<xmmintrin.h>`. This builtin can be used within constant expressions.
**Syntax**:
```c++
__builtin_shufflevector(vec1, vec2, index1, index2, ...)
```
**Examples**:
```c++
// identity operation - return 4-element vector v1.
__builtin_shufflevector(v1, v1, 0, 1, 2, 3)
// "Splat" element 0 of V1 into a 4-element result.
__builtin_shufflevector(V1, V1, 0, 0, 0, 0)
// Reverse 4-element vector V1.
__builtin_shufflevector(V1, V1, 3, 2, 1, 0)
// Concatenate every other element of 4-element vectors V1 and V2.
__builtin_shufflevector(V1, V2, 0, 2, 4, 6)
// Concatenate every other element of 8-element vectors V1 and V2.
__builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)
// Shuffle v1 with some elements being undefined. Not allowed in constexpr.
__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.
Values of -1 are not supported in constant expressions.
The result of `__builtin_shufflevector` is a vector with the same element
type as `vec1`/`vec2` but that has an element count equal to the number of
indices specified.
Query for this feature with `__has_builtin(__builtin_shufflevector)`.
(langext-__builtin_convertvector)=
(langext-builtin-convertvector)=
### `__builtin_convertvector`
`__builtin_convertvector` is used to express generic vector
type-conversion operations. The input vector and the output vector
type must have the same number of elements. This builtin can be used within
constant expressions.
**Syntax**:
```c++
__builtin_convertvector(src_vec, dst_vec_type)
```
**Examples**:
```c++
typedef double vector4double __attribute__((__vector_size__(32)));
typedef float vector4float __attribute__((__vector_size__(16)));
typedef short vector4short __attribute__((__vector_size__(8)));
vector4float vf; vector4short vs;
// convert from a vector of 4 floats to a vector of 4 doubles.
__builtin_convertvector(vf, vector4double)
// equivalent to:
(vector4double) { (double) vf[0], (double) vf[1], (double) vf[2], (double) vf[3] }
// convert from a vector of 4 shorts to a vector of 4 floats.
__builtin_convertvector(vs, vector4float)
// equivalent to:
(vector4float) { (float) vs[0], (float) vs[1], (float) vs[2], (float) vs[3] }
```
**Description**:
The first argument to `__builtin_convertvector` is a vector, and the second
argument is a vector type with the same number of elements as the first
argument.
The result of `__builtin_convertvector` is a vector with the same element
type as the second argument, with a value defined in terms of the action of a
C-style cast applied to each element of the first argument.
Query for this feature with `__has_builtin(__builtin_convertvector)`.
### `__builtin_bitreverse`
- `__builtin_bitreverse8`
- `__builtin_bitreverse16`
- `__builtin_bitreverse32`
- `__builtin_bitreverse64`
**Syntax**:
```c++
__builtin_bitreverse32(x)
```
**Examples**:
```c++
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`. These builtins can be used within constant expressions.
### `__builtin_rotateleft`
- `__builtin_rotateleft8`
- `__builtin_rotateleft16`
- `__builtin_rotateleft32`
- `__builtin_rotateleft64`
**Syntax**:
```c++
__builtin_rotateleft32(x, y)
```
**Examples**:
```c++
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. These builtins can be used within constant
expressions.
### `__builtin_rotateright`
- `__builtin_rotateright8`
- `__builtin_rotateright16`
- `__builtin_rotateright32`
- `__builtin_rotateright64`
**Syntax**:
```c++
__builtin_rotateright32(x, y)
```
**Examples**:
```c++
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. These builtins can be used within constant
expressions.
### `__builtin_stdc_rotate_left` and `__builtin_stdc_rotate_right`
**Syntax**:
```c
T __builtin_stdc_rotate_left(T value, count)
T __builtin_stdc_rotate_right(T value, count)
```
where `T` is any unsigned integer type and `count` is any integer type.
**Description**:
These builtins rotate the bits in `value` by `count` positions. The
`__builtin_stdc_rotate_left` builtin rotates bits to the left, while
`__builtin_stdc_rotate_right` rotates bits to the right. The first
argument (`value`) must be an unsigned integer type, including `_BitInt` types.
The second argument (`count`) can be any integer type. The rotation count is
normalized modulo the bit-width of the value being rotated, with negative
counts converted to equivalent positive rotations (e.g., rotating left
by `-1` is equivalent to rotating left by `BitWidth-1`). These builtins can
be used within constant expressions.
**Example of use**:
```c
unsigned char rotated_left = __builtin_stdc_rotate_left((unsigned char)0xB1, 3);
unsigned int rotated_right = __builtin_stdc_rotate_right(0x12345678U, 8);
unsigned char neg_rotate = __builtin_stdc_rotate_left((unsigned char)0xB1, -1);
unsigned _BitInt(20) value = 0xABCDE;
unsigned _BitInt(20) rotated = __builtin_stdc_rotate_left(value, 5);
```
### `__builtin_stdc_*` bit utilities
**Syntax**:
```c
unsigned int __builtin_stdc_leading_zeros(T value);
unsigned int __builtin_stdc_leading_ones(T value);
unsigned int __builtin_stdc_trailing_zeros(T value);
unsigned int __builtin_stdc_trailing_ones(T value);
unsigned int __builtin_stdc_first_leading_zero(T value);
unsigned int __builtin_stdc_first_leading_one(T value);
unsigned int __builtin_stdc_first_trailing_zero(T value);
unsigned int __builtin_stdc_first_trailing_one(T value);
unsigned int __builtin_stdc_count_zeros(T value);
unsigned int __builtin_stdc_count_ones(T value);
bool __builtin_stdc_has_single_bit(T value);
unsigned int __builtin_stdc_bit_width(T value);
T __builtin_stdc_bit_floor(T value);
T __builtin_stdc_bit_ceil(T value);
```
where `T` is any unsigned integer type except `bool` and enumeration types,
including `_BitInt` types.
**Description**:
These builtins implement the C23 `<stdbit.h>` operations. Following the C23
standard, `unsigned int` is used as the `generic_return_type` for count and
position queries (`leading_zeros`, `leading_ones`, `trailing_zeros`,
`trailing_ones`, `first_leading_zero`, `first_leading_one`,
`first_trailing_zero`, `first_trailing_one`, `count_zeros`,
`count_ones`, `bit_width`); `has_single_bit` returns `bool`; and
`bit_floor`/`bit_ceil` return the same type as the operand. Zero and
all-ones cases follow the C23 definitions. All are usable in constant
expressions.
`bool` and enumeration types are rejected as arguments because C23 does not
permit them for these functions.
As a Clang extension, `_BitInt` types of arbitrary widths are supported. C23
only requires support for bit-precise integers whose width matches a standard
or extended integer type.
**Examples**:
```c
unsigned _BitInt(9) x = 0x11;
unsigned int lz = __builtin_stdc_leading_zeros(x);
unsigned int tz = __builtin_stdc_trailing_zeros(x);
unsigned int fto = __builtin_stdc_first_trailing_one(x);
unsigned int cz = __builtin_stdc_count_zeros(x);
bool has_one = __builtin_stdc_has_single_bit(x);
unsigned _BitInt(9) ceilv = __builtin_stdc_bit_ceil((unsigned _BitInt(9))5);
unsigned _BitInt(9) floorv = __builtin_stdc_bit_floor((unsigned _BitInt(9))5);
```
### `__builtin_unreachable`
`__builtin_unreachable` is used to indicate that a specific point in the
program cannot be reached, even if the compiler might otherwise think it can.
This is useful to improve optimization and eliminates certain warnings. For
example, without the `__builtin_unreachable` in the example below, the
compiler assumes that the inline asm can fall through and prints a "function
declared '`noreturn`' should not return" warning.
**Syntax**:
```c++
__builtin_unreachable()
```
**Example of use**:
```c++
void myabort(void) __attribute__((noreturn));
void myabort(void) {
asm("int3");
__builtin_unreachable();
}
```
**Description**:
The `__builtin_unreachable()` builtin has completely undefined behavior.
Since it has undefined behavior, it is a statement that it is never reached and
the optimizer can take advantage of this to produce better code. This builtin
takes no arguments and produces a void result.
Query for this feature with `__has_builtin(__builtin_unreachable)`.
### `__builtin_unpredictable`
`__builtin_unpredictable` is used to indicate that a branch condition is
unpredictable by hardware mechanisms such as branch prediction logic.
**Syntax**:
```c++
__builtin_unpredictable(long long)
```
**Example of use**:
```c++
if (__builtin_unpredictable(x > 0)) {
foo();
}
```
**Description**:
The `__builtin_unpredictable()` builtin is expected to be used with control
flow conditions such as in `if` and `switch` statements.
Query for this feature with `__has_builtin(__builtin_unpredictable)`.
### `__builtin_expect`
`__builtin_expect` is used to indicate that the value of an expression is
anticipated to be the same as a statically known result.
**Syntax**:
```c++
long __builtin_expect(long expr, long val)
```
**Example of use**:
```c++
if (__builtin_expect(x, 0)) {
bar();
}
```
**Description**:
The `__builtin_expect()` builtin is typically used with control flow
conditions such as in `if` and `switch` statements to help branch
prediction. It means that its first argument `expr` is expected to take the
value of its second argument `val`. It always returns `expr`.
Query for this feature with `__has_builtin(__builtin_expect)`.
### `__builtin_expect_with_probability`
`__builtin_expect_with_probability` is similar to `__builtin_expect` but it
takes a probability as third argument.
**Syntax**:
```c++
long __builtin_expect_with_probability(long expr, long val, double p)
```
**Example of use**:
```c++
if (__builtin_expect_with_probability(x, 0, .3)) {
bar();
}
```
**Description**:
The `__builtin_expect_with_probability()` builtin is typically used with
control flow conditions such as in `if` and `switch` statements to help
branch prediction. It means that its first argument `expr` is expected to take
the value of its second argument `val` with probability `p`. `p` must be
within `[0.0 ; 1.0]` bounds. This builtin always returns the value of `expr`.
Query for this feature with `__has_builtin(__builtin_expect_with_probability)`.
### `__builtin_prefetch`
`__builtin_prefetch` is used to communicate with the cache handler to bring
data into the cache before it gets used.
**Syntax**:
```c++
void __builtin_prefetch(const void *addr, int rw=0, int locality=3)
```
**Example of use**:
```c++
__builtin_prefetch(a + i);
```
**Description**:
The `__builtin_prefetch(addr, rw, locality)` builtin is expected to be used to
avoid cache misses when the developer has a good understanding of which data
are going to be used next. `addr` is the address that needs to be brought into
the cache. `rw` indicates the expected access mode: `0` for *read* and `1`
for *write*. In case of *read write* access, `1` is to be used. `locality`
indicates the expected persistence of data in cache, from `0` which means that
data can be discarded from cache after its next use to `3` which means that
data is going to be reused a lot once in cache. `1` and `2` provide
intermediate behavior between these two extremes.
Query for this feature with `__has_builtin(__builtin_prefetch)`.
### `__sync_swap`
`__sync_swap` is used to atomically swap integers or pointers in memory.
**Syntax**:
```c++
type __sync_swap(type *ptr, type value, ...)
```
**Example of Use**:
```c++
int old_value = __sync_swap(&value, new_value);
```
**Description**:
The `__sync_swap()` builtin extends the existing `__sync_*()` family of
atomic intrinsics to allow code to atomically swap the current value with the
new value. More importantly, it helps developers write more efficient and
correct code by avoiding expensive loops around
`__sync_bool_compare_and_swap()` or relying on the platform specific
implementation details of `__sync_lock_test_and_set()`. The
`__sync_swap()` builtin is a full barrier.
### `__builtin_addressof`
`__builtin_addressof` performs the functionality of the built-in `&`
operator, ignoring any `operator&` overload. This is useful in constant
expressions in C++11, where there is no other way to take the address of an
object that overloads `operator&`. Clang automatically adds
`[[clang::lifetimebound]]` to the parameter of `__builtin_addressof`.
**Example of use**:
```c++
template<typename T> constexpr T *addressof(T &value) {
return __builtin_addressof(value);
}
```
### `__builtin_function_start`
`__builtin_function_start` returns the address of a function body.
**Syntax**:
```c++
void *__builtin_function_start(function)
```
**Example of use**:
```c++
void a() {}
void *p = __builtin_function_start(a);
class A {
public:
void a(int n);
void a();
};
void A::a(int n) {}
void A::a() {}
void *pa1 = __builtin_function_start((void(A::*)(int)) &A::a);
void *pa2 = __builtin_function_start((void(A::*)()) &A::a);
```
**Description**:
The `__builtin_function_start` builtin accepts an argument that can be
constant-evaluated to a function, and returns the address of the function
body. This builtin is not supported on all targets.
The returned pointer may differ from the normally taken function address
and is not safe to call. For example, with `-fsanitize=cfi`, taking a
function address produces a callable pointer to a CFI jump table, while
`__builtin_function_start` returns an address that fails
{doc}`cfi-icall<ControlFlowIntegrity>` checks.
### `__builtin_operator_new` and `__builtin_operator_delete`
A call to `__builtin_operator_new(args)` is exactly the same as a call to
`::operator new(args)`, 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), and that the call is required to resolve to a
[replaceable global allocation function](https://en.cppreference.com/w/cpp/memory/new/operator_new).
Likewise, `__builtin_operator_delete` is exactly the same as a call to
`::operator delete(args)`, except that it permits optimizations
and that the call is required to resolve to a
[replaceable global deallocation function](https://en.cppreference.com/w/cpp/memory/new/operator_delete).
These builtins are intended for use in the implementation of `std::allocator`
and other similar allocation libraries, and are only available in C++.
Query for this feature with `__has_builtin(__builtin_operator_new)` or
`__has_builtin(__builtin_operator_delete)`:
> - If the value is at least `201802L`, the builtins behave as described above.
> - If the value is non-zero, the builtins may not support calling arbitrary
> replaceable global (de)allocation functions, but do support calling at least
> `::operator new(size_t)` and `::operator delete(void*)`.
### `__builtin_trivially_relocate`
**Syntax**:
```c
T* __builtin_trivially_relocate(T* dest, T* src, size_t count)
```
Trivially relocates `count` objects of relocatable, complete type `T`
from `src` to `dest` and returns `dest`.
This builtin is used to implement `std::trivially_relocate`.
### `__builtin_invoke`
**Syntax**:
```c++
template <class Callee, class... Args>
decltype(auto) __builtin_invoke(Callee&& callee, Args&&... args);
```
`__builtin_invoke` is equivalent to `std::invoke`.
### `__builtin_preserve_access_index`
`__builtin_preserve_access_index` specifies a code section where
array subscript access and structure/union member access are relocatable
under bpf compile-once run-everywhere framework. Debuginfo (typically
with `-g`) is needed, otherwise, the compiler will exit with an error.
The return type for the intrinsic is the same as the type of the
argument.
**Syntax**:
```c
type __builtin_preserve_access_index(type arg)
```
**Example of Use**:
```c
struct t {
int i;
int j;
union {
int a;
int b;
} c[4];
};
struct t *v = ...;
int *pb =__builtin_preserve_access_index(&v->c[3].b);
__builtin_preserve_access_index(v->j);
```
### `__builtin_debugtrap`
`__builtin_debugtrap` causes the program to stop its execution in such a way that a debugger can catch it.
**Syntax**:
```c++
__builtin_debugtrap()
```
**Description**
`__builtin_debugtrap` is lowered to the [`llvm.debugtrap`](https://llvm.org/docs/LangRef.html#llvm-debugtrap-intrinsic) builtin. It should have the same effect as setting a breakpoint on the line where the builtin is called.
Query for this feature with `__has_builtin(__builtin_debugtrap)`.
### `__builtin_trap`
`__builtin_trap` causes the program to stop its execution abnormally.
**Syntax**:
```c++
__builtin_trap()
```
**Description**
`__builtin_trap` is lowered to the [`llvm.trap`](https://llvm.org/docs/LangRef.html#llvm-trap-intrinsic) builtin.
Query for this feature with `__has_builtin(__builtin_trap)`.
### `__builtin_arm_trap`
`__builtin_arm_trap` is an AArch64 extension to `__builtin_trap` which also accepts a compile-time constant value, encoded directly into the trap instruction for later inspection.
**Syntax**:
```c++
__builtin_arm_trap(const unsigned short payload)
```
**Description**
`__builtin_arm_trap` is lowered to the `llvm.aarch64.break` builtin, and then to `brk #payload`.
### `__builtin_verbose_trap`
`__builtin_verbose_trap` causes the program to stop its execution abnormally
and shows a human-readable description of the reason for the termination when a
debugger is attached or in a symbolicated crash log.
**Syntax**:
```c++
__builtin_verbose_trap(const char *category, const char *reason)
```
**Description**
`__builtin_verbose_trap` is lowered to the [`llvm.trap`](https://llvm.org/docs/LangRef.html#llvm-trap-intrinsic) builtin.
Additionally, clang emits debugging information that represents an artificial
inline frame whose name encodes the category and reason strings passed to the builtin,
prefixed by a "magic" prefix.
For example, consider the following code:
```c++
void foo(int* p) {
if (p == nullptr)
__builtin_verbose_trap("check null", "Argument must not be null!");
}
```
The debugging information would look as if it were produced for the following code:
```c++
__attribute__((always_inline))
inline void "__clang_trap_msg$check null$Argument must not be null!"() {
__builtin_trap();
}
void foo(int* p) {
if (p == nullptr)
"__clang_trap_msg$check null$Argument must not be null!"();
}
```
However, the generated code would not actually contain a call to the artificial
function — it only exists in the debugging information.
Query for this feature with `__has_builtin(__builtin_verbose_trap)`. Note that
users need to enable debug information to enable this feature. A call to this
builtin is equivalent to a call to `__builtin_trap` if debug information isn't
enabled.
The optimizer can merge calls to trap with different messages, which degrades
the debugging experience.
### `__builtin_allow_runtime_check`
`__builtin_allow_runtime_check` returns true if the check at the current
program location should be executed. It is expected to be used to implement
`assert` like checks which can be safely removed by the optimizer.
**Syntax**:
```c++
bool __builtin_allow_runtime_check(const char* kind)
```
**Example of use**:
```c++
if (__builtin_allow_runtime_check("mycheck") && !ExpensiveCheck()) {
abort();
}
```
**Description**
`__builtin_allow_runtime_check` is lowered to the [llvm.allow.runtime.check](https://llvm.org/docs/LangRef.html#llvm-allow-runtime-check-intrinsic)
intrinsic.
The `__builtin_allow_runtime_check()` can be used within control structures
like `if` to guard expensive runtime checks. The return value is determined
by the following compiler options and may differ per call site:
- `-mllvm -lower-allow-check-percentile-cutoff-hot=N`: Disable checks in hot
code marked by the profile summary with a hotness cutoff in the range
`[0, 999999]` (a larger N disables more checks).
- `-mllvm -lower-allow-check-random-rate=P`: Keep a check with probability P,
a floating point number in the range `[0.0, 1.0]`.
- If both options are specified, a check is disabled if either condition is satisfied.
- If neither is specified, all checks are allowed.
Parameter `kind`, currently unused, is a string literal specifying the check
kind. Future compiler versions may use this to allow for more granular control,
such as applying different hotness cutoffs to different check kinds.
Query for this feature with `__has_builtin(__builtin_allow_runtime_check)`.
### `__builtin_nondeterministic_value`
`__builtin_nondeterministic_value` returns a valid nondeterministic value of the same type as the provided argument.
**Syntax**:
```c++
type __builtin_nondeterministic_value(type x)
```
**Examples**:
```c++
int x = __builtin_nondeterministic_value(x);
float y = __builtin_nondeterministic_value(y);
__m256i a = __builtin_nondeterministic_value(a);
```
**Description**
Each call to `__builtin_nondeterministic_value` returns a valid value of the type given by the argument.
The types currently supported are: integer types, floating-point types, vector types.
Query for this feature with `__has_builtin(__builtin_nondeterministic_value)`.
### `__builtin_sycl_unique_stable_name`
`__builtin_sycl_unique_stable_name()` is a builtin that takes a type and
produces a string literal containing a unique name for the type that is stable
across split compilations, mainly to support SYCL/Data Parallel C++ language.
In cases where the split compilation needs to share a unique token for a type
across the boundary (such as in an offloading situation), this name can be used
for lookup purposes, such as in the SYCL Integration Header.
The value of this builtin is computed entirely at compile time, so it can be
used in constant expressions. This value encodes lambda functions based on a
stable numbering order in which they appear in their local declaration contexts.
Once this builtin is evaluated in a constexpr context, it is erroneous to use
it in an instantiation which changes its value.
In order to produce the unique name, the current implementation of the builtin
uses Itanium mangling even if the host compilation uses a different name
mangling scheme at runtime. The mangler marks all the lambdas required to name
the SYCL kernel and emits a stable local ordering of the respective lambdas.
The resulting pattern is demanglable. When non-lambda types are passed to the
builtin, the mangler emits their usual pattern without any special treatment.
**Syntax**:
```c
// Computes a unique stable name for the given type.
constexpr const char * __builtin_sycl_unique_stable_name( type-id );
```
### `__builtin_popcountg`
`__builtin_popcountg` returns the number of 1 bits in the argument. The
argument can be of any unsigned integer type or fixed boolean vector.
**Syntax**:
```c++
int __builtin_popcountg(type x)
```
**Examples**:
```c++
unsigned int x = 1;
int x_pop = __builtin_popcountg(x);
unsigned long y = 3;
int y_pop = __builtin_popcountg(y);
unsigned _BitInt(128) z = 7;
int z_pop = __builtin_popcountg(z);
```
**Description**:
`__builtin_popcountg` is meant to be a type-generic alternative to the
`__builtin_popcount{,l,ll}` builtins, with support for other integer types,
such as `unsigned __int128` and C23 `unsigned _BitInt(N)`.
### `__builtin_clzg` and `__builtin_ctzg`
`__builtin_clzg` (respectively `__builtin_ctzg`) returns the number of
leading (respectively trailing) 0 bits in the first argument. The first argument
can be of any unsigned integer type or fixed boolean vector.
For boolean vectors, these builtins interpret the vector like a bit-field where
the ith element of the vector is bit i of the bit-field, counting from the
least significant end. `__builtin_clzg` returns the number of zero elements at
the end of the vector, while `__builtin_ctzg` returns the number of zero
elements at the start of the vector.
If the first argument is 0 and an optional second argument of `int` type is
provided, then the second argument is returned. If the first argument is 0, but
only one argument is provided, then the behavior is undefined.
**Syntax**:
```c++
int __builtin_clzg(type x[, int fallback])
int __builtin_ctzg(type x[, int fallback])
```
**Examples**:
```c++
unsigned int x = 1;
int x_lz = __builtin_clzg(x);
int x_tz = __builtin_ctzg(x);
unsigned long y = 2;
int y_lz = __builtin_clzg(y);
int y_tz = __builtin_ctzg(y);
unsigned _BitInt(128) z = 4;
int z_lz = __builtin_clzg(z);
int z_tz = __builtin_ctzg(z);
```
**Description**:
`__builtin_clzg` (respectively `__builtin_ctzg`) is meant to be a
type-generic alternative to the `__builtin_clz{,l,ll}` (respectively
`__builtin_ctz{,l,ll}`) builtins, with support for other integer types, such
as `unsigned __int128` and C23 `unsigned _BitInt(N)`.
### `__builtin_counted_by_ref`
`__builtin_counted_by_ref` returns a pointer to the count field from the
`counted_by` attribute.
The argument must be a flexible array member or a pointer with the `counted_by`
attribute. If the argument doesn't have the `counted_by` attribute, the builtin
returns `(void *)0`.
**Syntax**:
```c
T *__builtin_counted_by_ref(void *array)
```
**Examples**:
```c
#define alloc(P, FAM, COUNT) ({ \
size_t __ignored_assignment; \
typeof(P) __p = NULL; \
__p = malloc(MAX(sizeof(*__p), \
sizeof(*__p) + sizeof(*__p->FAM) * COUNT)); \
\
*_Generic( \
__builtin_counted_by_ref(__p->FAM), \
void *: &__ignored_assignment, \
default: __builtin_counted_by_ref(__p->FAM)) = COUNT; \
\
__p; \
})
```
**Description**:
The `__builtin_counted_by_ref` builtin allows the programmer to prevent a
common error associated with the `counted_by` attribute. When using the
`counted_by` attribute, the `count` field **must** be set before the
flexible array member or pointer can be accessed. Otherwise, the sanitizers may
view such accesses as false positives. For instance, it's not uncommon for
programmers to initialize the flexible array before setting the `count` field:
```c
struct s {
int dummy;
short count;
long array[] __attribute__((counted_by(count)));
};
struct s *ptr = malloc(sizeof(struct s) + sizeof(long) * COUNT);
for (int i = 0; i < COUNT; ++i)
ptr->array[i] = i;
ptr->count = COUNT;
```
Enforcing the rule that `ptr->count = COUNT;` must occur after every
allocation of a struct with a `counted_by` member is prone to failure in large
code bases. This builtin mitigates this for allocators (like in Linux) that are
implemented in a way where the counter assignment can happen automatically.
**Note:** The value returned by `__builtin_counted_by_ref` cannot be assigned
to a variable, have its address taken, or passed into or returned from a
function, because doing so violates bounds safety conventions.
(builtin_stack_address-doc)=
(builtin-stack-address-doc)=
### `__builtin_stack_address`
`__builtin_stack_address` returns the address that separates the current
function's (i.e. the one calling the builtin) stack space and the region of the
stack that may be modified by called functions. The semantics match those of
GCC's builtin of the same name.
**Syntax**:
```c++
void *__builtin_stack_address()
```
**Example**:
```c++
void *sp = __builtin_stack_address();
```
**Description**:
The address returned by `__builtin_stack_address` identifies the starting
address of the stack region that may be used by called functions.
On some architectures (e.g. x86), it's sufficient to return the value in the
stack pointer register directly. On others (e.g. SPARCv9), adjustments are
required to the value of the stack pointer register.
`__builtin_stack_address` performs the necessary adjustments and returns the
correct boundary address.
### Multiprecision Arithmetic Builtins
Clang provides a set of builtins which expose multiprecision arithmetic in a
manner amenable to C. They all have the following form:
```c
unsigned x = ..., y = ..., carryin = ..., carryout;
unsigned sum = __builtin_addc(x, y, carryin, &carryout);
```
Thus one can form a multiprecision addition chain in the following manner:
```c
unsigned *x, *y, *z, carryin=0, carryout;
z[0] = __builtin_addc(x[0], y[0], carryin, &carryout);
carryin = carryout;
z[1] = __builtin_addc(x[1], y[1], carryin, &carryout);
carryin = carryout;
z[2] = __builtin_addc(x[2], y[2], carryin, &carryout);
carryin = carryout;
z[3] = __builtin_addc(x[3], y[3], carryin, &carryout);
```
The complete list of builtins are:
```c
unsigned char __builtin_addcb (unsigned char x, unsigned char y, unsigned char carryin, unsigned char *carryout);
unsigned short __builtin_addcs (unsigned short x, unsigned short y, unsigned short carryin, unsigned short *carryout);
unsigned __builtin_addc (unsigned x, unsigned y, unsigned carryin, unsigned *carryout);
unsigned long __builtin_addcl (unsigned long x, unsigned long y, unsigned long carryin, unsigned long *carryout);
unsigned long long __builtin_addcll(unsigned long long x, unsigned long long y, unsigned long long carryin, unsigned long long *carryout);
unsigned char __builtin_subcb (unsigned char x, unsigned char y, unsigned char carryin, unsigned char *carryout);
unsigned short __builtin_subcs (unsigned short x, unsigned short y, unsigned short carryin, unsigned short *carryout);
unsigned __builtin_subc (unsigned x, unsigned y, unsigned carryin, unsigned *carryout);
unsigned long __builtin_subcl (unsigned long x, unsigned long y, unsigned long carryin, unsigned long *carryout);
unsigned long long __builtin_subcll(unsigned long long x, unsigned long long y, unsigned long long carryin, unsigned long long *carryout);
```
### Checked Arithmetic Builtins
Clang provides a set of builtins that implement checked arithmetic for security
critical applications in a manner that is fast and easily expressible in C. As
an example of their usage:
```c
errorcode_t security_critical_application(...) {
unsigned x, y, result;
...
if (__builtin_mul_overflow(x, y, &result))
return kErrorCodeHackers;
...
use_multiply(result);
...
}
```
Clang provides the following checked arithmetic builtins:
```c
bool __builtin_add_overflow (type1 x, type2 y, type3 *sum);
bool __builtin_sub_overflow (type1 x, type2 y, type3 *diff);
bool __builtin_mul_overflow (type1 x, type2 y, type3 *prod);
bool __builtin_uadd_overflow (unsigned x, unsigned y, unsigned *sum);
bool __builtin_uaddl_overflow (unsigned long x, unsigned long y, unsigned long *sum);
bool __builtin_uaddll_overflow(unsigned long long x, unsigned long long y, unsigned long long *sum);
bool __builtin_usub_overflow (unsigned x, unsigned y, unsigned *diff);
bool __builtin_usubl_overflow (unsigned long x, unsigned long y, unsigned long *diff);
bool __builtin_usubll_overflow(unsigned long long x, unsigned long long y, unsigned long long *diff);
bool __builtin_umul_overflow (unsigned x, unsigned y, unsigned *prod);
bool __builtin_umull_overflow (unsigned long x, unsigned long y, unsigned long *prod);
bool __builtin_umulll_overflow(unsigned long long x, unsigned long long y, unsigned long long *prod);
bool __builtin_sadd_overflow (int x, int y, int *sum);
bool __builtin_saddl_overflow (long x, long y, long *sum);
bool __builtin_saddll_overflow(long long x, long long y, long long *sum);
bool __builtin_ssub_overflow (int x, int y, int *diff);
bool __builtin_ssubl_overflow (long x, long y, long *diff);
bool __builtin_ssubll_overflow(long long x, long long y, long long *diff);
bool __builtin_smul_overflow (int x, int y, int *prod);
bool __builtin_smull_overflow (long x, long y, long *prod);
bool __builtin_smulll_overflow(long long x, long long y, long long *prod);
```
Each builtin performs the specified mathematical operation on the
first two arguments and stores the result in the third argument. If
possible, the result will be equal to mathematically-correct result
and the builtin will return 0. Otherwise, the builtin will return
1 and the result will be equal to the unique value that is equivalent
to the mathematically-correct result modulo two raised to the *k*
power, where *k* is the number of bits in the result type. The
behavior of these builtins is well-defined for all argument values.
The first three builtins work generically for operands of any integer type,
including boolean types. The operands need not have the same type as each
other, or as the result. The other builtins may implicitly promote or
convert their operands before performing the operation.
Query for this feature with `__has_builtin(__builtin_add_overflow)`, etc.
### Floating point builtins
### `__builtin_isfpclass`
`__builtin_isfpclass` is used to test if the specified floating-point values
fall into one of the specified floating-point classes.
**Syntax**:
```c++
int __builtin_isfpclass(fp_type expr, int mask)
int_vector __builtin_isfpclass(fp_vector expr, int mask)
```
**Example of use**:
```c++
if (__builtin_isfpclass(x, 448)) {
// `x` is positive finite value
...
}
```
**Description**:
The `__builtin_isfpclass()` builtin is a generalization of functions `isnan`,
`isinf`, `isfinite` and some others defined by the C standard. It tests if
the floating-point value, specified by the first argument, falls into any of data
classes, specified by the second argument. The latter is an integer constant
bitmask expression, in which each data class is represented by a bit
using the encoding:
| Mask value | Data class | Macro |
| ---------- | ------------------ | ------------------------ |
| 0x0001 | Signaling NaN | \_\_FPCLASS_SNAN |
| 0x0002 | Quiet NaN | \_\_FPCLASS_QNAN |
| 0x0004 | Negative infinity | \_\_FPCLASS_NEGINF |
| 0x0008 | Negative normal | \_\_FPCLASS_NEGNORMAL |
| 0x0010 | Negative subnormal | \_\_FPCLASS_NEGSUBNORMAL |
| 0x0020 | Negative zero | \_\_FPCLASS_NEGZERO |
| 0x0040 | Positive zero | \_\_FPCLASS_POSZERO |
| 0x0080 | Positive subnormal | \_\_FPCLASS_POSSUBNORMAL |
| 0x0100 | Positive normal | \_\_FPCLASS_POSNORMAL |
| 0x0200 | Positive infinity | \_\_FPCLASS_POSINF |
For convenience preprocessor defines macros for these values. The function
returns 1 if `expr` falls into one of the specified data classes, 0 otherwise.
In the example above the mask value 448 (0x1C0) contains the bits selecting
positive zero, positive subnormal and positive normal classes.
`__builtin_isfpclass(x, 448)` would return true only if `x` if of any of
these data classes. Using suitable mask value, the function can implement any of
the standard classification functions, for example, `__builtin_isfpclass(x, 3)`
is identical to `isnan`, `__builtin_isfpclass(x, 504)` - to `isfinite`
and so on.
If the first argument is a vector, the function is equivalent to the set of
scalar calls of `__builtin_isfpclass` applied to the input elementwise.
The result of `__builtin_isfpclass` is a boolean value, if the first argument
is a scalar, or an integer vector with the same element count as the first
argument. The element type in this vector has the same bit length as the
element of the first argument type.
This function never raises floating-point exceptions and does not canonicalize
its input. The floating-point argument is not promoted, its data class is
determined based on its representation in its actual semantic type.
### `__builtin_canonicalize`
```c
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](https://llvm.org/docs/LangRef.html#llvm-canonicalize-intrinsic) for
more information on the semantics.
### `__builtin_flt_rounds` and `__builtin_set_flt_rounds`
```c
int __builtin_flt_rounds();
void __builtin_set_flt_rounds(int);
```
Returns and sets current floating point rounding mode. The encoding of returned
values and input parameters is same as the result of FLT_ROUNDS, specified by C
standard:
\- `0` - toward zero
\- `1` - to nearest, ties to even
\- `2` - toward positive infinity
\- `3` - toward negative infinity
\- `4` - to nearest, ties away from zero
The effect of passing some other value to `__builtin_flt_rounds` is
implementation-defined. `__builtin_set_flt_rounds` is currently only supported
to work on x86, x86_64, powerpc, powerpc64, Arm and AArch64 targets. These builtins
read and modify the floating-point environment, which is not always allowed and may
have unexpected behavior. Please see the section on [Accessing the floating point environment](UsersManual.md#accessing-the-floating-point-environment) for more information.
### String builtins
Clang provides constant expression evaluation support for builtins forms of
the following functions from the C standard library headers
`<string.h>` and `<wchar.h>`:
- `memchr`
- `memcmp` (and its deprecated BSD / POSIX alias `bcmp`)
- `strchr`
- `strcmp`
- `strlen`
- `strncmp`
- `wcschr`
- `wcscmp`
- `wcslen`
- `wcsncmp`
- `wmemchr`
- `wmemcmp`
In each case, the builtin form has the name of the C library function prefixed
by `__builtin_`. Example:
```c
void *p = __builtin_memchr("foobar", 'b', 5);
```
In addition to the above, one further builtin is provided:
```c
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).
Constant evaluation support for the `__builtin_mem*` functions is provided
only for arrays of `char`, `signed char`, `unsigned char`, or `char8_t`,
despite these functions accepting an argument of type `const void*`.
Support for constant expression evaluation for the above builtins can be detected
with `__has_feature(cxx_constexpr_string_builtins)`.
### Variadic function builtins
Clang provides several builtins for working with variadic functions from the C
standard library `<stdarg.h>` header:
- `__builtin_va_list`
A predefined typedef for the target-specific `va_list` type. It is undefined
behavior to use a byte-wise copy of this type produced by calling `memcpy`,
`memmove`, or similar. Valid explicit copies are only produced by calling
`va_copy` or `__builtin_va_copy`.
- `void __builtin_va_start(__builtin_va_list list, <parameter-name>)`
A builtin function for the target-specific `va_start` function-like macro.
The `parameter-name` argument is the name of the parameter preceding the
ellipsis (`...`) in the function signature. Alternatively, in C23 mode or
later, it may be the integer literal `0` if there is no parameter preceding
the ellipsis. This function initializes the given `__builtin_va_list` object.
It is undefined behavior to call this function on an already initialized
`__builtin_va_list` object.
- `void __builtin_c23_va_start(__builtin_va_list list, ...)`
A builtin function for the target-specific `va_start` function-like macro,
available only in C23 and later. The builtin accepts zero or one argument for
the ellipsis (`...`). If such an argument is provided, it should be the name
of the parameter preceding the ellipsis, which is used for compatibility with
C versions before C23. It is an error to provide two or more variadic arguments.
This function initializes the given `__builtin_va_list` object. It is
undefined behavior to call this function on an already initialized
`__builtin_va_list` object.
- `void __builtin_va_end(__builtin_va_list list)`
A builtin function for the target-specific `va_end` function-like macro. This
function finalizes the given `__builtin_va_list` object such that it is no
longer usable unless re-initialized with a call to `__builtin_va_start` or
`__builtin_va_copy`. It is undefined behavior to call this function with a
`list` that has not been initialized by either `__builtin_va_start` or
`__builtin_va_copy`.
- `<type-name> __builtin_va_arg(__builtin_va_list list, <type-name>)`
A builtin function for the target-specific `va_arg` function-like macro. This
function returns the value of the next variadic argument to the call. It is
undefined behavior to call this builtin when there is no next variadic argument
to retrieve or if the next variadic argument does not have a type compatible
with the given `type-name`. The return type of the function is the
`type-name` given as the second argument. It is undefined behavior to call
this function with a `list` that has not been initialized by either
`__builtin_va_start` or `__builtin_va_copy`.
- `void __builtin_va_copy(__builtin_va_list dest, __builtin_va_list src)`
A builtin function for the target-specific `va_copy` function-like macro.
This function initializes `dest` as a copy of `src`. It is undefined
behavior to call this function with an already initialized `dest` argument.
### Memory builtins
Clang provides constant expression evaluation support for builtin forms of the
following functions from the C standard library headers
`<string.h>` and `<wchar.h>`:
- `memcpy`
- `memmove`
- `wmemcpy`
- `wmemmove`
In each case, the builtin form has the name of the C library function prefixed
by `__builtin_`.
Constant evaluation support is only provided when the source and destination
are pointers to arrays with the same trivially copyable element type, and the
given size is an exact multiple of the element size that is no greater than
the number of elements accessible through the source and destination operands.
#### Guaranteed inlined copy
```c
void __builtin_memcpy_inline(void *dst, const void *src, size_t size);
```
`__builtin_memcpy_inline` has been designed as a building block for efficient
`memcpy` implementations. It is identical to `__builtin_memcpy` but also
guarantees not to call any external functions. See LLVM IR [llvm.memcpy.inline](https://llvm.org/docs/LangRef.html#llvm-memcpy-inline-intrinsic) intrinsic
for more information.
This is useful to implement a custom version of `memcpy`, implement a
`libc` memcpy or work around the absence of a `libc`.
Note that the `size` argument must be a compile time constant.
Note that this intrinsic cannot yet be called in a `constexpr` context.
#### Guaranteed inlined memset
```c
void __builtin_memset_inline(void *dst, int value, size_t size);
```
`__builtin_memset_inline` has been designed as a building block for efficient
`memset` implementations. It is identical to `__builtin_memset` but also
guarantees not to call any external functions. See LLVM IR [llvm.memset.inline](https://llvm.org/docs/LangRef.html#llvm-memset-inline-intrinsic) intrinsic
for more information.
This is useful to implement a custom version of `memset`, implement a
`libc` memset or work around the absence of a `libc`.
Note that the `size` argument must be a compile time constant.
Note that this intrinsic cannot yet be called in a `constexpr` context.
### `__is_bitwise_cloneable`
A type trait is used to check whether a type can be safely copied by memcpy.
**Syntax**:
```c++
bool __is_bitwise_cloneable(Type)
```
**Description**:
Objects of bitwise cloneable types can be bitwise copied by memcpy/memmove. The
Clang compiler warrants that this behavior is well defined, and won't be
broken by compiler optimizations and sanitizers.
For implicit-lifetime types, the lifetime of the new object is implicitly
started after the copy. For other types (e.g., classes with virtual methods),
the lifetime isn't started, and using the object results in undefined behavior
according to the C++ Standard.
This builtin can be used in constant expressions.
### Atomic Min/Max builtins with memory ordering
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.
Clang provides two additional atomic builtins with incrementing behavior. These
builtins perform an unsigned increment or decrement modulo a wrap-around value.
- `__atomic_fetch_uinc`
- `__atomic_fetch_udec`
See the LLVM IR [atomicrmw](https://llvm.org/docs/LangRef.html#atomicrmw-instruction)
instruction for the complete semantics of uinc_wrap and udec_wrap.
Atomic memory scopes are designed to assist optimizations for systems with
several levels of memory hierarchy like GPUs. The following memory scopes are
currently supported:
Example:
```c
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 of acquire-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.
(langext-__c11_atomic)=
(langext-c11-atomic)=
### \_\_c11_atomic builtins
Clang provides a set of builtins which are intended to be used to implement
C11's `<stdatomic.h>` header. These builtins provide the semantics of the
`_explicit` form of the corresponding C11 operation, and are named with a
`__c11_` prefix. The supported operations, and the differences from
the corresponding C11 operations, are:
- `__c11_atomic_init`
- `__c11_atomic_thread_fence`
- `__c11_atomic_signal_fence`
- `__c11_atomic_is_lock_free` (The argument is the size of the
`_Atomic(...)` object, instead of its address)
- `__c11_atomic_store`
- `__c11_atomic_load`
- `__c11_atomic_exchange`
- `__c11_atomic_compare_exchange_strong`
- `__c11_atomic_compare_exchange_weak`
- `__c11_atomic_fetch_add`
- `__c11_atomic_fetch_sub`
- `__c11_atomic_fetch_and`
- `__c11_atomic_fetch_or`
- `__c11_atomic_fetch_xor`
- `__c11_atomic_fetch_nand` (Nand is not presented in `<stdatomic.h>`)
- `__c11_atomic_fetch_max`
- `__c11_atomic_fetch_min`
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.)
(langext-__scoped_atomic)=
(langext-scoped-atomic)=
### \_\_scoped_atomic builtins
Clang provides a set of atomics taking a memory scope argument. These atomics
are identical to the standard GNU / GCC atomic builtins but taking an extra
memory scope argument. These are designed to be a generic alternative to the
`__opencl_atomic_*` builtin functions for targets that support atomic memory
scopes.
- `__MEMORY_SCOPE_SYSTEM`
- `__MEMORY_SCOPE_DEVICE`
- `__MEMORY_SCOPE_WRKGRP`
- `__MEMORY_SCOPE_CLUSTR`
- `__MEMORY_SCOPE_WVFRNT`
- `__MEMORY_SCOPE_SINGLE`
This controls whether or not the atomic operation is ordered with respect to the
whole system, the current device, an OpenCL workgroup, wavefront, or just a
single thread. If these are used on a target that does not support atomic
scopes, then they will behave exactly as the standard GNU atomic builtins.
### Low-level ARM exclusive memory builtins
Clang provides overloaded builtins giving direct access to the three key ARM
instructions for implementing atomic operations.
```c
T __builtin_arm_ldrex(const volatile T *addr);
T __builtin_arm_ldaex(const volatile T *addr);
int __builtin_arm_strex(T val, volatile T *addr);
int __builtin_arm_stlex(T val, volatile T *addr);
void __builtin_arm_clrex(void);
```
The types `T` currently supported are:
- Integer types with width at most 64 bits (or 128 bits on AArch64).
- Floating-point types
- Pointer types.
Note that the compiler does not guarantee it will not insert stores which clear
the exclusive monitor in between an `ldrex` type operation and its paired
`strex`. In practice this is only usually a risk when the extra store is on
the same cache line as the variable being modified and Clang will only insert
stack stores on its own, so it is best not to use these operations on variables
with automatic storage duration.
Also, loads and stores may be implicit in code written between the `ldrex` and
`strex`. Clang will not necessarily mitigate the effects of these either, so
care should be exercised.
For these reasons the higher level atomic primitives should be preferred where
possible.
### Non-temporal load/store builtins
Clang provides overloaded builtins allowing generation of non-temporal memory
accesses.
```c
T __builtin_nontemporal_load(T *addr);
void __builtin_nontemporal_store(T value, T *addr);
```
The types `T` currently supported are:
- Integer types.
- Floating-point types.
- Vector types.
Note that the compiler does not guarantee that non-temporal loads or stores
will be used.
### C++ Coroutines support builtins
:::{warning}
This is a work in progress. Compatibility across Clang/LLVM releases is not
guaranteed.
:::
Clang provides experimental builtins to support C++ Coroutines as defined by
<https://wg21.link/P0057>. The following four are intended to be used by the
standard library to implement the `std::coroutine_handle` type.
**Syntax**:
```c
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**:
```c++
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](https://llvm.org/docs/Coroutines.html#intrinsics) 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**:
```c
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)
```
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 `__builtin_suspend`, clang will insert `token none`
as the first argument to the intrinsic.
### Source location builtins
Clang provides builtins to support C++ standard library implementation
of `std::source_location` as specified in C++20. With the exception
of `__builtin_COLUMN`, `__builtin_FILE_NAME` and `__builtin_FUNCSIG`,
these builtins are also implemented by GCC.
**Syntax**:
```c
const char *__builtin_FILE();
const char *__builtin_FILE_NAME(); // Clang only
const char *__builtin_FUNCTION();
const char *__builtin_FUNCSIG(); // Microsoft
unsigned __builtin_LINE();
unsigned __builtin_COLUMN(); // Clang only
const std::source_location::__impl *__builtin_source_location();
```
**Example of use**:
```c++
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`, `__builtin_FUNCSIG`,
`__builtin_FILE` and `__builtin_FILE_NAME` return the values, at the
"invocation point", for `__LINE__`, `__FUNCTION__`, `__FUNCSIG__`,
`__FILE__` and `__FILE_NAME__` respectively. `__builtin_COLUMN` similarly
returns the column, though there is no corresponding macro. 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.
The builtin `__builtin_COLUMN` returns the offset from the start of the line,
beginning from column 1. [This may differ from other implementations.](https://eel.is/c++draft/support.srcloc#tab:support.srcloc.current-row-3-column-2-sentence-2)
The builtin `__builtin_source_location` returns a pointer to constant static
data of type `std::source_location::__impl`. This type must have already been
defined, and must contain exactly four fields: `const char *_M_file_name`,
`const char *_M_function_name`, `<any-integral-type> _M_line`, and
`<any-integral-type> _M_column`. The fields will be populated in the same
manner as the above four builtins, except that `_M_function_name` is populated
with `__PRETTY_FUNCTION__` rather than `__FUNCTION__`.
### Alignment builtins
Clang provides builtins to support checking and adjusting alignment of
pointers and integers.
These builtins can be used to avoid relying on implementation-defined behavior
of arithmetic on integers derived from pointers.
Additionally, these builtins retain type information and, unlike bitwise
arithmetic, they can perform semantic checking on the alignment value.
**Syntax**:
```c
Type __builtin_align_up(Type value, size_t alignment);
Type __builtin_align_down(Type value, size_t alignment);
bool __builtin_is_aligned(Type value, size_t alignment);
```
**Example of use**:
```c++
char* global_alloc_buffer;
void* my_aligned_allocator(size_t alloc_size, size_t alignment) {
char* result = __builtin_align_up(global_alloc_buffer, alignment);
// result now contains the value of global_alloc_buffer rounded up to the
// next multiple of alignment.
global_alloc_buffer = result + alloc_size;
return result;
}
void* get_start_of_page(void* ptr) {
return __builtin_align_down(ptr, PAGE_SIZE);
}
void example(char* buffer) {
if (__builtin_is_aligned(buffer, 64)) {
do_fast_aligned_copy(buffer);
} else {
do_unaligned_copy(buffer);
}
}
// In addition to pointers, the builtins can also be used on integer types
// and are evaluatable inside constant expressions.
static_assert(__builtin_align_up(123, 64) == 128, "");
static_assert(__builtin_align_down(123u, 64) == 64u, "");
static_assert(!__builtin_is_aligned(123, 64), "");
```
**Description**:
The builtins `__builtin_align_up`, `__builtin_align_down`, return their
first argument aligned up/down to the next multiple of the second argument.
If the value is already sufficiently aligned, it is returned unchanged.
The builtin `__builtin_is_aligned` returns whether the first argument is
aligned to a multiple of the second argument.
All of these builtins expect the alignment to be expressed as a number of bytes.
These builtins can be used for all integer types as well as (non-function)
pointer types. For pointer types, these builtins operate in terms of the integer
address of the pointer and return a new pointer of the same type (including
qualifiers such as `const`) with an adjusted address.
When aligning pointers up or down, the resulting value must be within the same
underlying allocation or one past the end (see C17 6.5.6p8, C++ [expr.add]).
This means that arbitrary integer values stored in pointer-type variables must
not be passed to these builtins. For those use cases, the builtins can still be
used, but the operation must be performed on the pointer cast to `uintptr_t`.
Null pointers are considered to be aligned to any requested alignment.
Therefore, `__builtin_is_aligned` evaluates to true for null pointer
arguments, and `__builtin_align_up` and `__builtin_align_down` preserve
the null pointer value.
If Clang can determine that the alignment is not a power of two at compile time,
it will result in a compilation failure. If the alignment argument is not a
power of two at run time, the behavior of these builtins is undefined.
## Non-standard C++11 Attributes
Clang's non-standard C++11 attributes live in the `clang` attribute
namespace.
Clang supports GCC's `gnu` attribute namespace. All GCC attributes which
are accepted with the `__attribute__((foo))` syntax are also accepted as
`[[gnu::foo]]`. This only extends to attributes which are specified by GCC
(see the list of [GCC function attributes](https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html), [GCC variable
attributes](https://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html), and
[GCC type attributes](https://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html)). As with the GCC
implementation, these attributes must appertain to the *declarator-id* in a
declaration, which means they must go either at the start of the declaration or
immediately after the name being declared.
For example, this applies the GNU `unused` attribute to `a` and `f`, and
also applies the GNU `noreturn` attribute to `f`.
Examples:
```c++
[[gnu::unused]] int a, f [[gnu::noreturn]] ();
```
## Target-Specific Extensions
Clang supports some language features conditionally on some targets.
### AMDGPU Language Extensions
#### \_\_builtin_amdgcn_fence
`__builtin_amdgcn_fence` emits a fence.
- `unsigned` atomic ordering, e.g., `__ATOMIC_ACQUIRE`
- `const char *` synchronization scope, e.g., `workgroup`
- Zero or more `const char *` address spaces names.
The address spaces arguments must be one of the following string literals:
- `"local"`
- `"global"`
If one or more address space name are provided, the code generator will attempt
to emit potentially faster instructions that order access to at least those
address spaces.
Emitting such instructions may not always be possible and the compiler is free
to fence more aggressively.
If no address spaces names are provided, all address spaces are fenced.
```c++
// Fence all address spaces.
__builtin_amdgcn_fence(__ATOMIC_SEQ_CST, "workgroup");
__builtin_amdgcn_fence(__ATOMIC_ACQUIRE, "agent");
// Fence only requested address spaces.
__builtin_amdgcn_fence(__ATOMIC_SEQ_CST, "workgroup", "local")
__builtin_amdgcn_fence(__ATOMIC_SEQ_CST, "workgroup", "local", "global")
```
#### \_\_builtin_amdgcn_processor_is and \_\_builtin_amdgcn_is_invocable
`__builtin_amdgcn_processor_is` and `__builtin_amdgcn_is_invocable` provide
a functional mechanism for programatically querying:
- the identity of the current target processor;
- the capability of the current target processor to invoke a particular builtin.
**Syntax**:
```c
__amdgpu_feature_predicate_t __builtin_amdgcn_processor_is(const char*);
__amdgpu_feature_predicate_t __builtin_amdgcn_is_invocable(builtin_name);
```
**Example of use**:
```c++
if (__builtin_amdgcn_processor_is("gfx1201") ||
__builtin_amdgcn_is_invocable(__builtin_amdgcn_s_sleep_var))
__builtin_amdgcn_s_sleep_var(x);
if (!__builtin_amdgcn_processor_is("gfx906"))
__builtin_amdgcn_s_wait_event_export_ready();
else if (__builtin_amdgcn_processor_is("gfx1010") ||
__builtin_amdgcn_processor_is("gfx1101"))
__builtin_amdgcn_s_ttracedata_imm(1);
while (__builtin_amdgcn_processor_is("gfx1101")) *p += x;
do {
break;
} while (__builtin_amdgcn_processor_is("gfx1010"));
for (; __builtin_amdgcn_processor_is("gfx1201"); ++*p) break;
if (__builtin_amdgcn_is_invocable(__builtin_amdgcn_s_wait_event_export_ready))
__builtin_amdgcn_s_wait_event_export_ready();
else if (__builtin_amdgcn_is_invocable(__builtin_amdgcn_s_ttracedata_imm))
__builtin_amdgcn_s_ttracedata_imm(1);
do {
break;
} while (
__builtin_amdgcn_is_invocable(__builtin_amdgcn_global_load_tr_b64_i32));
for (; __builtin_amdgcn_is_invocable(__builtin_amdgcn_permlane64); ++*p)
break;
```
**Description**:
The builtins return a value of type `__amdgpu_feature_predicate_t`, which is a
target specific type that behaves as if its C++ definition was the following:
```c++
struct __amdgpu_feature_predicate_t {
__amdgpu_feature_predicate_t() = delete;
__amdgpu_feature_predicate_t(const __amdgpu_feature_predicate_t&) = delete;
__amdgpu_feature_predicate_t(__amdgpu_feature_predicate_t&&) = delete;
explicit
operator bool() const noexcept;
};
```
The builtins can be used in C as well, wherein the
`__amdgpu_feature_predicate_t` type behaves as an opaque, forward declared
type with conditional automated conversion to `_Bool` when used as the
predicate argument to a control structure:
```c
struct __amdgpu_feature_predicate_t ret(); // Error
void arg(struct __amdgpu_feature_predicate_t); // Error
void local() {
struct __amdgpu_feature_predicate_t x; // Error
struct __amdgpu_feature_predicate_t y =
__builtin_amdgcn_processor_is("gfx900"); // Error
}
void valid_use() {
_Bool x = (_Bool)__builtin_amdgcn_processor_is("gfx900"); // OK
if (__builtin_amdgcn_processor_is("gfx900")) // Implicit cast to _Bool
return;
for (; __builtin_amdgcn_processor_is("gfx900");) // Implicit cast to _Bool
break;
while (__builtin_amdgcn_processor_is("gfx900")) // Implicit cast to _Bool
break;
do {
break;
} while (__builtin_amdgcn_processor_is("gfx900")); // Implicit cast to _Bool
__builtin_amdgcn_processor_is("gfx900") ? x : !x;
}
```
The boolean interpretation of the predicate values returned by the builtins:
- indicates whether the current target matches the argument; the argument MUST
be a string literal and a valid AMDGPU target
- indicates whether the builtin function passed as the argument can be invoked
by the current target; the argument MUST be either a generic or AMDGPU
specific builtin name
When invoked while compiling for a concrete target, the builtins are evaluated
early by Clang, and never produce any CodeGen effects / have no observable
side-effects in IR. Conversely, when compiling for AMDGCN flavoured SPIR-v,
which is an abstract target, a series of specialization constants are implicitly
created, in correspondence with the predicates. These predicates get resolved
when finalizing the compilation process for a concrete target, and shall reflect
the latter's identity and features. Thus, it is possible to author high-level
code, in e.g. HIP, that is target adaptive in a dynamic fashion, contrary to
macro based mechanisms.
#### \_\_builtin_amdgcn_ballot_w{32,64}
`__builtin_amdgcn_ballot_w{32,64}` returns a bitmask that contains its
boolean argument as a bit for every lane of the current wave that is currently
active (i.e., that is converged with the executing thread), and a 0 bit for
every lane that is not active.
The result is uniform, i.e., it is the same in every active thread of the wave.
#### \_\_builtin_amdgcn_inverse_ballot_w{32,64}
Given a wave-uniform bitmask, `__builtin_amdgcn_inverse_ballot_w{32,64}(mask)`
returns the bit at the position of the current lane. It is almost equivalent to
`(mask & (1 << lane_id)) != 0`, except that its behavior is only defined if
the given mask has the same value for all active lanes of the current wave.
#### \_\_builtin_amdgcn_av\_{load,store}\_b128
Signature:
```c
typedef __attribute__((__vector_size__(4 * sizeof(unsigned int)))) unsigned int v4u;
v4u __builtin_amdgcn_av_load_b128(v4u *src, int scope);
void __builtin_amdgcn_av_store_b128(v4u *dst, v4u data, int scope);
```
Load or store a vector of 4 unsigned integers from or to memory with cache
behavior specified by `scope`, which is one of the `__MEMORY_SCOPE_*` macros
defined for {ref}`scoped atomic builtins <langext-c11-atomic>`.
The pointer argument must point to the global or generic address space.
These builtins are supported on gfx9, gfx10, gfx11, and gfx12 targets.
They map to the LLVM intrinsics `llvm.amdgcn.av.load.b128` and
`llvm.amdgcn.av.store.b128` documented in [User Guide for AMDGPU Backend](https://llvm.org/docs/AMDGPUUsage.html).
### ARM/AArch64 Language Extensions
#### Memory Barrier Intrinsics
Clang implements the `__dmb`, `__dsb` and `__isb` intrinsics as defined
in the [Arm C Language Extensions](https://github.com/ARM-software/acle/releases).
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.
#### Pointer Authentication
See {doc}`PointerAuthentication`.
### X86/X86-64 Language Extensions
The X86 backend has these language extensions:
#### Memory references to specified segments
Annotating a pointer with address space #256 causes it to be code generated
relative to the X86 GS segment register, address space #257 causes it to be
relative to the X86 FS segment, and address space #258 causes it to be
relative to the X86 SS segment. Note that this is a very very low-level
feature that should only be used if you know what you're doing (for example in
an OS kernel).
Here is an example:
```c++
#define GS_RELATIVE __attribute__((address_space(256)))
int foo(int GS_RELATIVE *P) {
return *P;
}
```
Which compiles to (on X86-32):
```gas
_foo:
movl 4(%esp), %eax
movl %gs:(%eax), %eax
ret
```
You can also use the GCC compatibility macros `__seg_fs` and `__seg_gs` for
the same purpose. The preprocessor symbols `__SEG_FS` and `__SEG_GS`
indicate their support.
### PowerPC Language Extensions
#### Set the Floating Point Rounding Mode
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.
```c++
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)`.
#### PowerPC cache builtins
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**:
```c
void __dcbf(const void* addr); /* Data Cache Block Flush */
```
**Example of Use**:
```c
int a = 1;
__builtin_dcbf (&a);
```
### z/OS Language Extensions
#### z/OS builtins
z/OS supports builtins for compare-and-swap operations that generate the
corresponding z/OS assembly instructions (CS, CSG, CDSG). These builtins compare
the value pointed to by oldptr to the value pointed to by curptr. If they are
equal, the value pointed to by newword (or newword itself for `__cs`) is
copied into the location pointed to by curptr. If they are unequal, the value
pointed to by curptr is copied into the location pointed to by oldptr.
The builtins return 0 if the values are equal, or 1 if they are unequal.
- `int __cs(unsigned int *oldptr, unsigned int *curptr, unsigned int newword)`
Generates a 4-byte compare-and-swap using the CS instruction.
Use `__cs` instead of `__cs1` when the newword arg is an r-value instead
of an l-value.
- `int __cs1(void *oldptr, void *curptr, void *newword)`
Generates a 4-byte compare-and-swap using the CS instruction.
- `int __csg(void *oldptr, void *curptr, void *newword)`
Generates an 8-byte compare-and-swap using the CSG instruction.
- `int __cds1(void *oldptr, void *curptr, void *newword)`
Generates an 8-byte compare-and-swap using the CSG instruction.
- `int __cdsg(void *oldptr, void *curptr, void *newword)`
Generates a 16-byte compare-and-swap using the CDSG instruction.
## Extensions for Static Analysis
Clang supports additional attributes that are useful for documenting program
invariants and rules for static analysis tools, such as the [Clang Static
Analyzer](https://clang-analyzer.llvm.org/). These attributes are documented
in the analyzer's {doc}`list of annotations for analysis <analyzer/user-docs/Annotations>`.
## Extensions for Dynamic Analysis
Use `__has_feature(address_sanitizer)` to check if the code is being built
with {doc}`AddressSanitizer`.
Use `__has_feature(thread_sanitizer)` to check if the code is being built
with {doc}`ThreadSanitizer`.
Use `__has_feature(memory_sanitizer)` to check if the code is being built
with {doc}`MemorySanitizer`.
Use `__has_feature(dataflow_sanitizer)` to check if the code is being built
with {doc}`DataFlowSanitizer`.
Use `__has_feature(safe_stack)` to check if the code is being built
with {doc}`SafeStack`.
## Extensions for selectively disabling optimization
Clang provides a mechanism for selectively disabling optimizations in functions
and methods.
To disable optimizations in a single function definition, the GNU-style or C++11
non-standard attribute `optnone` can be used.
```c++
// The following functions will not be optimized.
// GNU-style attribute
__attribute__((optnone)) int foo() {
// ... code
}
// C++11 attribute
[[clang::optnone]] int bar() {
// ... code
}
```
To facilitate disabling optimization for a range of function definitions, a
range-based pragma is provided. Its syntax is `#pragma clang optimize`
followed by `off` or `on`.
All function definitions in the region between an `off` and the following
`on` will be decorated with the `optnone` attribute unless doing so would
conflict with explicit attributes already present on the function (e.g., the
ones that control inlining).
```c++
#pragma clang optimize off
// This function will be decorated with optnone.
int foo() {
// ... code
}
// optnone conflicts with always_inline, so bar() will not be decorated.
__attribute__((always_inline)) int bar() {
// ... code
}
#pragma clang optimize on
```
If no `on` is found to close an `off` region, the end of the region is the
end of the compilation unit.
Note that a stray `#pragma clang optimize on` does not selectively enable
additional optimizations when compiling at low optimization levels. This feature
can only be used to selectively disable optimizations.
The pragma has an effect on functions only at the point of their definition; for
function templates, this means that the state of the pragma at the point of an
instantiation is not necessarily relevant. Consider the following example:
```c++
template<typename T> T twice(T t) {
return 2 * t;
}
#pragma clang optimize off
template<typename T> T thrice(T t) {
return 3 * t;
}
int container(int a, int b) {
return twice(a) + thrice(b);
}
#pragma clang optimize on
```
In this example, the definition of the template function `twice` is outside
the pragma region, whereas the definition of `thrice` is inside the region.
The `container` function is also in the region and will not be optimized, but
it causes the instantiation of `twice` and `thrice` with an `int` type; of
these two instantiations, `twice` will be optimized (because its definition
was outside the region) and `thrice` will not be optimized.
Clang also implements MSVC's range-based pragma,
`#pragma optimize("[optimization-list]", on | off)`. At the moment, Clang only
supports an empty optimization list, whereas MSVC supports the arguments, `s`,
`g`, `t`, and `y`. Currently, the implementation of `pragma optimize` behaves
the same as `#pragma clang optimize`. All functions
between `off` and `on` will be decorated with the `optnone` attribute.
```c++
#pragma optimize("", off)
// This function will be decorated with optnone.
void f1() {}
#pragma optimize("", on)
// This function will be optimized with whatever was specified on
// the commandline.
void f2() {}
// This will warn with Clang's current implementation.
#pragma optimize("g", on)
void f3() {}
```
For MSVC, an empty optimization list and `off` parameter will turn off
all optimizations, `s`, `g`, `t`, and `y`. An empty optimization and
`on` parameter will reset the optimizations to the ones specified on the
commandline.
```{list-table} Parameters (unsupported by Clang)
:header-rows: 1
* - Parameter
- Type of optimization
* - g
- Deprecated
* - s or t
- Short or fast sequences of machine code
* - y
- Enable frame pointers
```
(langext-loop-hint-optimizations)=
## Extensions for loop hint optimizations
The `#pragma clang loop` directive is used to specify hints for optimizing the
subsequent for, while, do-while, or c++11 range-based for loop. The directive
provides options for vectorization, interleaving, predication, unrolling and
distribution. Loop hints can be specified before any loop and will be ignored if
the optimization is not safe to apply.
There are loop hints that control transformations (e.g., vectorization, loop
unrolling) and there are loop hints that set transformation options (e.g.
`vectorize_width`, `unroll_count`). Pragmas setting transformation options
imply the transformation is enabled, as if it were enabled via the corresponding
transformation pragma (e.g., `vectorize(enable)`). If the transformation is
disabled (e.g., `vectorize(disable)`), that takes precedence over
transformations option pragmas implying that transformation.
### Vectorization, Interleaving, and Predication
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.
```c++
#pragma clang loop vectorize(enable)
#pragma clang loop interleave(enable)
for(...) {
...
}
```
The vector width is specified by
`vectorize_width(_value_[, fixed|scalable])`, where \_value\_ is a positive
integer and the type of vectorization can be specified with an optional
second parameter. The default for the second parameter is 'fixed' and
refers to fixed width vectorization, whereas 'scalable' indicates the
compiler should use scalable vectors instead. Another use of vectorize_width
is `vectorize_width(fixed|scalable)` where the user can hint at the type
of vectorization to use without specifying the exact width. In both variants
of the pragma the vectorizer may decide to fall back on fixed width
vectorization if the target does not support scalable vectors.
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.
```c++
#pragma clang loop vectorize_width(2)
#pragma clang loop interleave_count(2)
for(...) {
...
}
```
Specifying a width/count of 1 disables the optimization, and is equivalent to
`vectorize(disable)` or `interleave(disable)`.
Vector predication is enabled by `vectorize_predicate(enable)`, for example:
```c++
#pragma clang loop vectorize(enable)
#pragma clang loop vectorize_predicate(enable)
for(...) {
...
}
```
This predicates (masks) all instructions in the loop, which allows the scalar
remainder loop (the tail) to be folded into the main vectorized loop. This
might be more efficient when vector predication is efficiently supported by the
target platform.
### Loop Unrolling
Unrolling a loop reduces the loop control overhead and exposes more
opportunities for ILP. Loops can be fully or partially unrolled. Full unrolling
eliminates the loop and replaces it with an enumerated sequence of loop
iterations. Full unrolling is only possible if the loop trip count is known at
compile time. Partial unrolling replicates the loop body within the loop and
reduces the trip count.
If `unroll(enable)` is specified the unroller will attempt to fully unroll the
loop if the trip count is known at compile time. If the fully unrolled code size
is greater than an internal limit the loop will be partially unrolled up to this
limit. If the trip count is not known at compile time the loop will be partially
unrolled with a heuristically chosen unroll factor.
```c++
#pragma clang loop unroll(enable)
for(...) {
...
}
```
If `unroll(full)` is specified the unroller will attempt to fully unroll the
loop if the trip count is known at compile time identically to
`unroll(enable)`. However, with `unroll(full)` the loop will not be unrolled
if the loop count is not known at compile time.
```c++
#pragma clang loop unroll(full)
for(...) {
...
}
```
The unroll count can be specified explicitly with `unroll_count(_value_)` where
\_value\_ is a positive integer. If this value is greater than the trip count the
loop will be fully unrolled. Otherwise the loop is partially unrolled subject
to the same code size limit as with `unroll(enable)`.
```c++
#pragma clang loop unroll_count(8)
for(...) {
...
}
```
Unrolling of a loop can be prevented by specifying `unroll(disable)`.
Loop unroll parameters can be controlled by options
`-mllvm -unroll-count=n` and `-mllvm -pragma-unroll-threshold=n`.
### Loop Distribution
Loop Distribution allows splitting a loop into multiple loops. This is
beneficial for example when the entire loop cannot be vectorized but some of the
resulting loops can.
If `distribute(enable))` is specified and the loop has memory dependencies
that inhibit vectorization, the compiler will attempt to isolate the offending
operations into a new loop. This optimization is not enabled by default, only
loops marked with the pragma are considered.
```c++
#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.
### Disable Loop Invariant Code Motion
Loop Invariant Code Motion (LICM) moves loop invariant code outside of the loop.
If `licm(disable))` is specified, compiler will skip LICM on the specific loop.
```c++
#pragma clang loop licm(disable)
while (i < Length) {
List[i] = A[x] * i * 2;
i++;
}
```
The load for A[x] is loop invariant, it will not be hoisted out of the loop
when LICM is disabled.
### Additional Information
For convenience multiple loop hints can be specified on a single line.
```c++
#pragma clang loop vectorize_width(4) interleave_count(8)
for(...) {
...
}
```
If an optimization cannot be applied any hints that apply to it will be ignored.
For example, the hint `vectorize_width(4)` is ignored if the loop is not
proven safe to vectorize. To identify and diagnose optimization issues use
`-Rpass`, `-Rpass-missed`, and `-Rpass-analysis` command line options. See the
user guide for details.
## Extensions to specify floating-point flags
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, the following settings can be controlled with this pragma:
`#pragma clang fp reassociate` allows control over the reassociation
of floating point expressions. When enabled, this pragma allows the expression
`x + (y + z)` to be reassociated as `(x + y) + z`.
Reassociation can also occur across multiple statements.
This pragma can be used to disable reassociation when it is otherwise
enabled for the translation unit with the `-fassociative-math` flag.
The pragma can take two values: `on` and `off`.
```c++
float f(float x, float y, float z)
{
// Enable floating point reassociation across statements
#pragma clang fp reassociate(on)
float t = x + y;
float v = t + z;
}
```
`#pragma clang fp reciprocal` allows control over using reciprocal
approximations in floating point expressions. When enabled, this
pragma allows the expression `x / y` to be approximated as `x *
(1.0 / y)`. This pragma can be used to disable reciprocal
approximation when it is otherwise enabled for the translation unit
with the `-freciprocal-math` flag or other fast-math options. The
pragma can take two values: `on` and `off`.
```c++
float f(float x, float y)
{
// Enable floating point reciprocal approximation
#pragma clang fp reciprocal(on)
return x / y;
}
```
`#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 fusion
in cases when the language standard does not make this possible (e.g., across
statements in C).
```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-honor-pragmas` flag.
Note that `-ffp-contract=fast` will override pragmas to fuse multiply and
addition across statements regardless of any controlling pragmas.
`#pragma clang fp exceptions` specifies floating point exception behavior. It
may take one of the values: `ignore`, `maytrap` or `strict`. Meaning of
these values is same as for [constrained floating point intrinsics](http://llvm.org/docs/LangRef.html#constrained-floating-point-intrinsics).
```c++
{
// Preserve floating point exceptions
#pragma clang fp exceptions(strict)
z = x + y;
if (fetestexcept(FE_OVERFLOW))
...
}
```
A `#pragma clang fp` pragma may contain any number of options:
```c++
void func(float *dest, float a, float b) {
#pragma clang fp exceptions(maytrap) contract(fast) reassociate(on)
...
}
```
`#pragma clang fp eval_method` allows floating-point behavior to be specified
for a section of the source code. This pragma can appear at file or namespace
scope, or at the start of a compound statement (excluding comments).
The pragma is active within the scope of the compound statement.
When `pragma clang fp eval_method(source)` is enabled, the section of code
governed by the pragma behaves as though the command-line option
`-ffp-eval-method=source` is enabled. Rounds intermediate results to
source-defined precision.
When `pragma clang fp eval_method(double)` is enabled, the section of code
governed by the pragma behaves as though the command-line option
`-ffp-eval-method=double` is enabled. Rounds intermediate results to
`double` precision.
When `pragma clang fp eval_method(extended)` is enabled, the section of code
governed by the pragma behaves as though the command-line option
`-ffp-eval-method=extended` is enabled. Rounds intermediate results to
target-dependent `long double` precision. In Win32 programming, for instance,
the long double data type maps to the double, 64-bit precision data type.
The full syntax this pragma supports is
`#pragma clang fp eval_method(source|double|extended)`.
```c++
for(...) {
// The compiler will use long double as the floating-point evaluation
// method.
#pragma clang fp eval_method(extended)
a = b[i] * c[i] + e;
}
```
Note: `math.h` defines the typedefs `float_t` and `double_t` based on the active
evaluation method at the point where the header is included, not where the
typedefs are used. Because of this, it is unwise to combine these typedefs with
`#pragma clang fp eval_method`. To catch obvious bugs, Clang will emit an
error for any references to these typedefs within the scope of this pragma;
however, this is not a fool-proof protection, and programmers must take care.
The `#pragma float_control` pragma allows precise floating-point
semantics and floating-point exception behavior to be specified
for a section of the source code. This pragma can only appear at file or
namespace scope, within a language linkage specification or at the start of a
compound statement (excluding comments). When used within a compound statement,
the pragma is active within the scope of the compound statement. This pragma
is modeled after a Microsoft pragma with the same spelling and syntax. For
pragmas specified at file or namespace scope, or within a language linkage
specification, a stack is supported so that the `pragma float_control`
settings can be pushed or popped.
When `pragma float_control(precise, on)` is enabled, the section of code
governed by the pragma uses precise floating point semantics, effectively
`-ffast-math` is disabled and `-ffp-contract=on`
(fused multiply add) is enabled. This pragma enables `-fmath-errno`.
When `pragma float_control(precise, off)` is enabled, unsafe-floating point
optimizations are enabled in the section of code governed by the pragma.
Effectively `-ffast-math` is enabled and `-ffp-contract=fast`. This pragma
disables `-fmath-errno`.
When `pragma float_control(except, on)` is enabled, the section of code
governed by the pragma behaves as though the command-line option
`-ffp-exception-behavior=strict` is enabled,
when `pragma float_control(except, off)` is enabled, the section of code
governed by the pragma behaves as though the command-line option
`-ffp-exception-behavior=ignore` is enabled.
The full syntax this pragma supports is
`float_control(except|precise, on|off [, push])` and
`float_control(push|pop)`.
The `push` and `pop` forms, including using `push` as the optional
third argument, can only occur at file scope.
```c++
for(...) {
// This block will be compiled with -fno-fast-math and -ffp-contract=on
#pragma float_control(precise, on)
a = b[i] * c[i] + e;
}
```
(langext-atomic-code-generation)=
## Extensions for controlling atomic code generation
The `[[clang::atomic]]` statement attribute enables users to control how
atomic operations are lowered in LLVM IR by conveying additional metadata to
the backend. The primary goal is to allow users to specify certain options,
like whether the affected atomic operations might be used with specific types of memory or
whether to ignore denormal mode correctness in floating-point operations,
without affecting the correctness of code that does not rely on these properties.
In LLVM, lowering of atomic operations (e.g., `atomicrmw`) can differ based
on the target's capabilities. Some backends support native atomic instructions
only for certain operation types or alignments, or only in specific memory
regions. Likewise, floating-point atomic instructions may or may not respect
IEEE denormal requirements. When the user is unconcerned about denormal-mode
compliance (for performance reasons) or knows that certain atomic operations
will not be performed on a particular type of memory, extra hints are needed to
tell the backend how to proceed.
A classic example is an architecture where floating-point atomic add does not
fully conform to IEEE denormal-mode handling. If the user does not mind ignoring
that aspect, they would prefer to emit a faster hardware atomic instruction,
rather than a fallback or CAS loop. Conversely, on certain GPUs (e.g., AMDGPU),
memory accessed via PCIe may only support a subset of atomic operations. To ensure
correct and efficient lowering, the compiler must know whether the user needs
the atomic operations to work with that type of memory.
The allowed atomic attribute values are now `remote_memory`, `fine_grained_memory`,
and `ignore_denormal_mode`, each optionally prefixed with `no_`. The meanings
are as follows:
- `remote_memory` means atomic operations may be performed on remote
memory, i.e., memory accessed through off-chip interconnects (e.g., PCIe).
On ROCm platforms using HIP, remote memory refers to memory accessed via
PCIe and is subject to specific atomic operation support. See
[ROCm PCIe Atomics](https://rocm.docs.amd.com/en/latest/conceptual/pcie-atomics.html) for further details. Prefixing with `no_remote_memory` indicates that
atomic operations should not be performed on remote memory.
- `fine_grained_memory` means atomic operations may be performed on fine-grained
memory, i.e., memory regions that support fine-grained coherence, where updates to
memory are visible to other parts of the system even while modifications are ongoing.
For example, in HIP, fine-grained coherence ensures that host and device share
up-to-date data without explicit synchronization (see
[HIP Definition](https://rocm.docs.amd.com/projects/HIP/en/docs-6.3.3/how-to/hip_runtime_api/memory_management/coherence_control.html#coherence-control)).
Similarly, OpenCL 2.0 provides fine-grained synchronization in shared virtual memory
allocations, allowing concurrent modifications by host and device (see
[OpenCL 2.0 Overview](https://www.intel.com/content/www/us/en/developer/articles/technical/opencl-20-shared-virtual-memory-overview.html)).
Prefixing with `no_fine_grained_memory` indicates that atomic operations should not
be performed on fine-grained memory.
- `ignore_denormal_mode` means that atomic operations are allowed to ignore
correctness for denormal mode in floating-point operations, potentially improving
performance on architectures that handle denormals inefficiently. The negated form,
if specified as `no_ignore_denormal_mode`, would enforce strict denormal mode
correctness.
Any unspecified option is inherited from the global defaults, which can be set
by a compiler flag or the target's built-in defaults.
Within the same atomic attribute, duplicate and conflicting values are accepted,
and the last of any conflicting values wins. Multiple atomic attributes are
allowed for the same compound statement, and the last atomic attribute wins.
Without any atomic metadata, LLVM IR defaults to conservative settings for
correctness: atomic operations enforce denormal mode correctness and are assumed
to potentially use remote and fine-grained memory (i.e., the equivalent of
`remote_memory`, `fine_grained_memory`, and `no_ignore_denormal_mode`).
The attribute may be applied only to a compound statement and looks like:
```c++
[[clang::atomic(remote_memory, fine_grained_memory, ignore_denormal_mode)]]
{
// Atomic instructions in this block carry extra metadata reflecting
// these user-specified options.
}
```
A new compiler option now globally sets the defaults for these atomic-lowering
options. The command-line format has changed to:
```console
$ clang -fatomic-remote-memory -fno-atomic-fine-grained-memory -fatomic-ignore-denormal-mode file.cpp
```
Each option has a corresponding flag:
`-fatomic-remote-memory` / `-fno-atomic-remote-memory`,
`-fatomic-fine-grained-memory` / `-fno-atomic-fine-grained-memory`,
and `-fatomic-ignore-denormal-mode` / `-fno-atomic-ignore-denormal-mode`.
Code using the `[[clang::atomic]]` attribute can then selectively override
the command-line defaults on a per-block basis. For instance:
```c++
// Suppose the global defaults assume:
// remote_memory, fine_grained_memory, and no_ignore_denormal_mode
// (for conservative correctness)
void example() {
// Locally override the settings: disable remote_memory and enable
// fine_grained_memory.
[[clang::atomic(no_remote_memory, fine_grained_memory)]]
{
// In this block:
// - Atomic operations are not performed on remote memory.
// - Atomic operations are performed on fine-grained memory.
// - The setting for denormal mode remains as the global default
// (typically no_ignore_denormal_mode, enforcing strict denormal mode correctness).
// ...
}
}
```
Function bodies do not accept statement attributes, so this will not work:
```c++
void func() [[clang::atomic(remote_memory)]] { // Wrong: applies to function type
}
```
Use the attribute on a compound statement within the function:
```c++
void func() {
[[clang::atomic(remote_memory)]]
{
// Atomic operations in this block carry the specified metadata.
}
}
```
The `[[clang::atomic]]` attribute affects only the code generation of atomic
instructions within the annotated compound statement. Clang attaches target-specific
metadata to those atomic instructions in the emitted LLVM IR to guide backend lowering.
This metadata is fixed at the Clang code generation phase and is not modified by later
LLVM passes (such as function inlining).
For example, consider:
```cpp
inline void func() {
[[clang::atomic(remote_memory)]]
{
// Atomic instructions lowered with metadata.
}
}
void foo() {
[[clang::atomic(no_remote_memory)]]
{
func(); // Inlined by LLVM, but the metadata from 'func()' remains unchanged.
}
}
```
Although current usage focuses on AMDGPU, the mechanism is general. Other
backends can ignore or implement their own responses to these flags if desired.
If a target does not understand or enforce these hints, the IR remains valid,
and the resulting program is still correct (although potentially less optimized
for that user's needs).
## Specifying an attribute for multiple declarations (#pragma clang attribute)
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:
```c++
#pragma clang attribute push (__attribute__((annotate("custom"))), apply_to = function)
void function(); // The function now has the annotate("custom") attribute
#pragma clang attribute pop
```
The attributes can also be written using the C++11 style syntax:
```c++
#pragma clang attribute push ([[noreturn]], apply_to = function)
void function(); // The function now has the [[noreturn]] attribute
#pragma clang attribute pop
```
The `__declspec` style syntax is also supported:
```c++
#pragma clang attribute push (__declspec(dllexport), apply_to = function)
void function(); // The function now has the __declspec(dllexport) attribute
#pragma clang attribute pop
```
A single push directive can contain multiple attributes, however,
only one syntax style can be used within a single directive:
```c++
#pragma clang attribute push ([[noreturn, noinline]], apply_to = function)
void function1(); // The function now has the [[noreturn]] and [[noinline]] attributes
#pragma clang attribute pop
#pragma clang attribute push (__attribute((noreturn, noinline)), apply_to = function)
void function2(); // The function now has the __attribute((noreturn)) and __attribute((noinline)) attributes
#pragma clang attribute pop
```
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:
```c++
#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_END
```
Without 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)`.
### Subject Match Rules
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`:
```c++
#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 pop
```
Multiple 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`. Failing to
specify a rule within the `any` rule results in an error.
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 to `typedef` declarations
and C++11 type aliases.
- `record`: Can be used to apply attributes to `struct`, `class`, and
`union` declarations.
- `record(unless(is_union))`: Can be used to apply attributes only to
`struct` and `class` declarations.
- `enum`: Can 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_local)`: Can be used to apply attributes to local 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 to `namespace` declarations.
- `objc_interface`: Can be used to apply attributes to `@interface`
declarations.
- `objc_protocol`: Can be used to apply attributes to `@protocol`
declarations.
- `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 `@property`
declarations.
- `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.
### Supported Attributes
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.
## Specifying section names for global objects (#pragma clang section)
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:
```c++
#pragma clang section bss="myBSS" data="myData" rodata="myRodata" relro="myRelro" text="myText"
```
The section names can be reverted back to default name by supplying an empty
string to the section kind, for example:
```c++
#pragma clang section bss="" data="" text="" rodata="" relro=""
```
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, Mach-O and COFF 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 section` directive does not 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 section` directive, is applied to that global.
## Specifying Linker Options on ELF Targets
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.
## Embedding Copyright Information on AIX
Clang supports the `#pragma comment(copyright, "string")` directive for AIX
targets. This directive embeds a copyright or identifying string into the
compiled object file. The string is included in the final executable or shared
library and loaded into memory at program runtime. The directive is ignored on
non-AIX targets.
```c
#pragma comment(copyright, "string-literal")
```
The second argument is an ordinary string literal. Concatenated ordinary string
literals are also accepted. The directive is intended to appear at file scope;
Clang treats it as being at file scope when it appears within other scopes.
### Interaction with C++20 Modules
When `#pragma comment(copyright, ...)` appears in a C++20 module interface
unit, the copyright string is embedded only in the object file compiled from
that interface unit. Importing TUs do not re-emit the string.
(langext-evaluating-object-size)=
## Evaluating Object Size
Clang supports the builtins `__builtin_object_size` and
`__builtin_dynamic_object_size`. The semantics are compatible with GCC's
builtins of the same names, but the details are slightly different.
```c
size_t __builtin_[dynamic_]object_size(const void *ptr, int type)
```
Returns the number of accessible bytes `n` past `ptr`. The value returned
depends on `type`, which is required to be an integer constant between 0 and
3:
- If `type & 2 == 0`, the least `n` is returned such that accesses to
`(const char*)ptr + n` and beyond are known to be out of bounds. This is
`(size_t)-1` if no better bound is known.
- If `type & 2 == 2`, the greatest `n` is returned such that accesses to
`(const char*)ptr + i` are known to be in bounds, for 0 \<= `i` < `n`.
This is `(size_t)0` if no better bound is known.
```c
char small[10], large[100];
bool cond;
// Returns 100: writes of more than 100 bytes are known to be out of bounds.
int n100 = __builtin_object_size(cond ? small : large, 0);
// Returns 10: writes of 10 or fewer bytes are known to be in bounds.
int n10 = __builtin_object_size(cond ? small : large, 2);
```
- If `type & 1 == 0`, pointers are considered to be in bounds if they point
into the same storage as `ptr` -- that is, the same stack object, global
variable, or heap allocation.
- If `type & 1 == 1`, pointers are considered to be in bounds if they point
to the same subobject that `ptr` points to. If `ptr` points to an array
element, other elements of the same array, but not of enclosing arrays, are
considered in bounds.
```c
struct X { char a, b, c; } x;
static_assert(__builtin_object_size(&x, 0) == 3);
static_assert(__builtin_object_size(&x.b, 0) == 2);
static_assert(__builtin_object_size(&x.b, 1) == 1);
```
```c
char a[10][10][10];
static_assert(__builtin_object_size(&a, 1) == 1000);
static_assert(__builtin_object_size(&a[1], 1) == 900);
static_assert(__builtin_object_size(&a[1][1], 1) == 90);
static_assert(__builtin_object_size(&a[1][1][1], 1) == 9);
```
The values returned by this builtin are a best effort conservative approximation
of the correct answers. When `type & 2 == 0`, the true value is less than or
equal to the value returned by the builtin, and when `type & 2 == 1`, the true
value is greater than or equal to the value returned by the builtin.
For `__builtin_object_size`, the value is determined entirely at compile time.
With optimization enabled, better results will be produced, especially when the
call to `__builtin_object_size` is in a different function from the formation
of the pointer. Unlike in GCC, enabling optimization in Clang does not allow
more information about subobjects to be determined, so the `type & 1 == 1`
case will often give imprecise results when used across a function call boundary
even when optimization is enabled.
[The pass_object_size and pass_dynamic_object_size attributes](AttributeReference.md#pass-object-size-pass-dynamic-object-size)
can be used to invisibly pass the object size for a pointer parameter alongside
the pointer in a function call. This allows more precise object sizes to be
determined both when building without optimizations and in the `type & 1 == 1`
case.
For `__builtin_dynamic_object_size`, the result is not limited to being a
compile time constant. Instead, a small amount of runtime evaluation is
permitted to determine the size of the object, in order to give a more precise
result. `__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:
```c
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.
## Deprecating Macros
Clang supports the pragma `#pragma clang deprecated`, which can be used to
provide deprecation warnings for macro uses. For example:
```c
#define MIN(x, y) x < y ? x : y
#pragma clang deprecated(MIN, "use std::min instead")
int min(int a, int b) {
return MIN(a, b); // warning: MIN is deprecated: use std::min instead
}
```
`#pragma clang deprecated` should be preferred for this purpose over
`#pragma GCC warning` because the warning can be controlled with
`-Wdeprecated`.
## Restricted Expansion Macros
Clang supports the pragma `#pragma clang restrict_expansion`, which can be
used restrict macro expansion in headers. This can be valuable when providing
headers with ABI stability requirements. Any expansion of the annotated macro
processed by the preprocessor after the `#pragma` annotation will log a
warning. Redefining the macro or undefining the macro will not be diagnosed, nor
will expansion of the macro within the main source file. For example:
```c
#define TARGET_ARM 1
#pragma clang restrict_expansion(TARGET_ARM, "<reason>")
/// Foo.h
struct Foo {
#if TARGET_ARM // warning: TARGET_ARM is marked unsafe in headers: <reason>
uint32_t X;
#else
uint64_t X;
#endif
};
/// main.c
#include "foo.h"
#if TARGET_ARM // No warning in main source file
X_TYPE uint32_t
#else
X_TYPE uint64_t
#endif
```
This warning is controlled by `-Wpedantic-macros`.
## Final Macros
Clang supports the pragma `#pragma clang final`, which can be used to
mark macros as final, meaning they cannot be undef'd or re-defined. For example:
```c
#define FINAL_MACRO 1
#pragma clang final(FINAL_MACRO)
#define FINAL_MACRO // warning: FINAL_MACRO is marked final and should not be redefined
#undef FINAL_MACRO // warning: FINAL_MACRO is marked final and should not be undefined
```
This is useful for enforcing system-provided macros that should not be altered
in user headers or code. This is controlled by `-Wpedantic-macros`. Final
macros will always warn on redefinition, including situations with identical
bodies and in system headers.
## Line Control
Clang supports an extension for source line control, which takes the
form of a preprocessor directive starting with an unsigned integral
constant. In addition to the standard `#line` directive, this form
allows control of an include stack and header file type, which is used
in issuing diagnostics. These lines are emitted in preprocessed
output.
```c
# <line:number> <filename:string> <header-type:numbers>
```
The filename is optional, and if unspecified indicates no change in
source filename. The header-type is an optional, whitespace-delimited,
sequence of magic numbers as follows.
- `1:` Push the current source file name onto the include stack and
enter a new file.
- `2`: Pop the include stack and return to the specified file. If
the filename is `""`, the name popped from the include stack is
used. Otherwise there is no requirement that the specified filename
matches the current source when originally pushed.
- `3`: Enter a system-header region. System headers often contain
implementation-specific source that would normally emit a diagnostic.
- `4`: Enter an implicit `extern "C"` region. This is not required on
modern systems where system headers are C++-aware.
At most a single `1` or `2` can be present, and values must be in
ascending order.
Examples are:
```c
# 57 // Advance (or return) to line 57 of the current source file
# 57 "frob" // Set to line 57 of "frob"
# 1 "foo.h" 1 // Enter "foo.h" at line 1
# 59 "main.c" 2 // Leave current include and return to "main.c"
# 1 "/usr/include/stdio.h" 1 3 // Enter a system header
# 60 "" 2 // return to "main.c"
# 1 "/usr/ancient/header.h" 1 4 // Enter an implicit extern "C" header
```
## Intrinsics Support within Constant Expressions
The following builtin intrinsics can be used in constant expressions:
- `__builtin_addcb`
- `__builtin_addcs`
- `__builtin_addc`
- `__builtin_addcl`
- `__builtin_addcll`
- `__builtin_bitreverse8`
- `__builtin_bitreverse16`
- `__builtin_bitreverse32`
- `__builtin_bitreverse64`
- `__builtin_bswap16`
- `__builtin_bswap32`
- `__builtin_bswap64`
- `__builtin_clrsb`
- `__builtin_clrsbl`
- `__builtin_clrsbll`
- `__builtin_clz`
- `__builtin_clzl`
- `__builtin_clzll`
- `__builtin_clzs`
- `__builtin_clzg`
- `__builtin_ctz`
- `__builtin_ctzl`
- `__builtin_ctzll`
- `__builtin_ctzs`
- `__builtin_ctzg`
- `__builtin_ffs`
- `__builtin_ffsl`
- `__builtin_ffsll`
- `__builtin_fmax`
- `__builtin_fmin`
- `__builtin_fpclassify`
- `__builtin_inf`
- `__builtin_isinf`
- `__builtin_isinf_sign`
- `__builtin_isfinite`
- `__builtin_isnan`
- `__builtin_isnormal`
- `__builtin_nan`
- `__builtin_nans`
- `__builtin_parity`
- `__builtin_parityl`
- `__builtin_parityll`
- `__builtin_popcount`
- `__builtin_popcountl`
- `__builtin_popcountll`
- `__builtin_popcountg`
- `__builtin_rotateleft8`
- `__builtin_rotateleft16`
- `__builtin_rotateleft32`
- `__builtin_rotateleft64`
- `__builtin_rotateright8`
- `__builtin_rotateright16`
- `__builtin_rotateright32`
- `__builtin_rotateright64`
- `__builtin_subcb`
- `__builtin_subcs`
- `__builtin_subc`
- `__builtin_subcl`
- `__builtin_subcll`
The following x86-specific intrinsics can be used in constant expressions:
- `_addcarry_u32`
- `_addcarry_u64`
- `_bit_scan_forward`
- `_bit_scan_reverse`
- `__bsfd`
- `__bsfq`
- `__bsrd`
- `__bsrq`
- `__bswap`
- `__bswapd`
- `__bswap64`
- `__bswapq`
- `_castf32_u32`
- `_castf64_u64`
- `_castu32_f32`
- `_castu64_f64`
- `__lzcnt16`
- `__lzcnt`
- `__lzcnt64`
- `_mm_popcnt_u32`
- `_mm_popcnt_u64`
- `_popcnt32`
- `_popcnt64`
- `__popcntd`
- `__popcntq`
- `__popcnt16`
- `__popcnt`
- `__popcnt64`
- `__rolb`
- `__rolw`
- `__rold`
- `__rolq`
- `__rorb`
- `__rorw`
- `__rord`
- `__rorq`
- `_rotl`
- `_rotr`
- `_rotwl`
- `_rotwr`
- `_lrotl`
- `_lrotr`
- `_subborrow_u32`
- `_subborrow_u64`
## Debugging the Compiler
Clang supports a number of pragma directives that help debugging the compiler itself.
Syntax is the following: `#pragma clang __debug <command> <arguments>`.
Note, all of debugging pragmas are subject to change.
### `dump`
Accepts either a single identifier or an expression. When a single identifier is passed,
the lookup results for the identifier are printed to `stderr`. When an expression is passed,
the AST for the expression is printed to `stderr`. The expression is an unevaluated operand,
so things like overload resolution and template instantiations are performed,
but the expression has no runtime effects.
Type- and value-dependent expressions are not supported yet.
This facility is designed to aid with testing name lookup machinery.
## Predefined Macros
### `__GCC_DESTRUCTIVE_SIZE` and `__GCC_CONSTRUCTIVE_SIZE`
Specify the mimum offset between two objects to avoid false sharing and the
maximum size of contiguous memory to promote true sharing, respectively. These
macros are predefined in all C and C++ language modes, but can be redefined on
the command line with `-D` to specify different values as needed or can be
undefined on the command line with `-U` to disable support for the feature.
**Note: the values the macros expand to are not guaranteed to be stable. They
are are affected by architectures and CPU tuning flags, can change between
releases of Clang and will not match the values defined by other compilers such
as GCC.**
Compiling different TUs depending on these flags (including use of
`std::hardware_constructive_interference` or
`std::hardware_destructive_interference`) with different compilers, macro
definitions, or architecture flags will lead to ODR violations and should be
avoided.
## `#embed` Parameters
### `clang::offset`
The `clang::offset` embed parameter may appear zero or one time in the
embed parameter sequence. Its preprocessor argument clause shall be present and
have the form:
```text
( constant-expression )
```
and shall be an integer constant expression. The integer constant expression
shall not evaluate to a value less than 0. The token `defined` shall not
appear within the constant expression.
The offset will be used when reading the contents of the embedded resource to
specify the starting offset to begin embedding from. The resources is treated
as being empty if the specified offset is larger than the number of bytes in
the resource. The offset will be applied *before* any `limit` parameters are
applied.
## Union and aggregate initialization in C
In C23 (N2900), when an object is initialized from initializer `= {}`, all
elements of arrays, all members of structs, and the first members of unions are
empty-initialized recursively. In addition, all padding bits are initialized to
zero.
Clang guarantees the following behaviors:
- `1:` Clang supports initializer `= {}` mentioned above in all C
standards.
- `2:` When unions are initialized from initializer `= {}`, bytes outside
of the first members of unions are also initialized to zero.
- `3:` When unions, structures and arrays are initialized from initializer
`= { initializer-list }`, all members not explicitly initialized in
the initializer list are empty-initialized recursively. In addition, all
padding bits are initialized to zero.
Currently, the above extension only applies to C source code, not C++.
## Empty Objects in C
The declaration of a structure or union type which has no named members is
undefined behavior (C23 and earlier) or implementation-defined behavior (C2y).
Clang allows the declaration of a structure or union type with no named members
in all C language modes. `sizeof` for such a type returns `0`, which is
different behavior than in C++ (where the size of such an object is typically
`1`).
## Qualified function types in C
Declaring a function with a qualified type in C is undefined behavior (C23 and
earlier) or implementation-defined behavior (C2y). Clang allows a function type
to be specified with the `const` and `volatile` qualifiers, but ignores the
qualifications.
```c
typedef int f(void);
const volatile f func; // Qualifier on function type has no effect.
```
Note, Clang does not allow an `_Atomic` function type because
of explicit constraints against atomically qualified (arrays and) function
types.
## Underspecified Object Declarations in C
C23 introduced the notion of [underspecified object declarations](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3006.htm)
(note, the final standards text is different from WG14 N3006 due to changes
during national body comment review). When an object is declared with the
`constexpr` storage class specifier or has a deduced type (with the `auto`
specifier), it is said to be "underspecified". Underspecified declarations have
different requirements than non-underspecified declarations. In particular, the
identifier being declared cannot be used in its initialization. e.g.,
```c
auto x = x; // Invalid
constexpr int y = y; // Invalid
```
The standard leaves it implementation-defined whether an underspecified
declaration may introduce additional identifiers as part of the declaration.
Clang allows additional identifiers to be declared in the following cases:
- A compound literal may introduce a new type. e.g.,
```c
auto x = (struct S { int x, y; }){ 1, 2 }; // Accepted by Clang
constexpr int i = (struct T { int x; }){ 1 }.x; // Accepted by Clang
```
- The type specifier for a `constexpr` declaration may define a new type.
e.g.,
```c
constexpr struct S { int x; } s = { 1 }; // Accepted by Clang
```
- A function declarator may be declared with parameters, including parameters
which introduce a new type. e.g.,
```c
constexpr int (*fp)(int x) = nullptr; // Accepted by Clang
auto f = (void (*)(struct S { int x; } s))nullptr; // Accepted by Clang
```
- The initializer may contain a GNU statement expression which defines new
types or objects. e.g.,
```c
constexpr int i = ({ // Accepted by Clang
constexpr int x = 12;
constexpr struct S { int x; } s = { x };
s.x;
});
auto x = ({ struct S { int x; } s = { 0 }; s; }); // Accepted by Clang
```
Clang intentionally does not implement the changed scoping rules from C23
for underspecified declarations. Doing so would significantly complicate the
implementation in order to get reasonable diagnostic behavior and also means
Clang fails to reject some code that should be rejected. e.g.,
```c
// This should be rejected because 'x' is not in scope within the initializer
// of an underspecified declaration. Clang accepts because it treats the scope
// of the identifier as beginning immediately after the declarator, same as with
// a non-underspecified declaration.
constexpr int x = sizeof(x);
// Clang rejects this code with a diagnostic about using the variable within its
// own initializer rather than rejecting the code with an undeclared identifier
// diagnostic.
auto x = x;
```