LanguageExtensions.rst

to determine if support for compile-time assertions using ``_Static_assert`` is
enabled.

C11 ``_Thread_local``
^^^^^^^^^^^^^^^^^^^^^

Use ``__has_feature(c_thread_local)`` or ``__has_extension(c_thread_local)``
to determine if support for ``_Thread_local`` variables is enabled.

Modules
-------

Use ``__has_feature(modules)`` to determine if Modules have been enabled.
For example, compiling code with ``-fmodules`` enables the use of Modules.

More information could be found `here <http://clang.llvm.org/docs/Modules.html>`_.

Checks for Type Trait Primitives
================================

Type trait primitives are special builtin constant expressions that can be used
by the standard C++ library to facilitate or simplify the implementation of
user-facing type traits in the <type_traits> header.

They are not intended to be used directly by user code because they are
implementation-defined and subject to change -- as such they're tied closely to
the supported set of system headers, currently:

* LLVM's own libc++
* GNU libstdc++
* The Microsoft standard C++ library

Clang supports the `GNU C++ type traits
<http://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html>`_ and a subset of the
`Microsoft Visual C++ Type traits
<http://msdn.microsoft.com/en-us/library/ms177194(v=VS.100).aspx>`_.

Feature detection is supported only for some of the primitives at present. User
code should not use these checks because they bear no direct relation to the
actual set of type traits supported by the C++ standard library.

For type trait ``__X``, ``__has_extension(X)`` indicates the presence of the
type trait primitive in the compiler. A simplistic usage example as might be
seen in standard C++ headers follows:

.. code-block:: c++

  #if __has_extension(is_convertible_to)
  template<typename From, typename To>
  struct is_convertible_to {
    static const bool value = __is_convertible_to(From, To);
  };
  #else
  // Emulate type trait for compatibility with other compilers.
  #endif

The following type trait primitives are supported by Clang:

* ``__has_nothrow_assign`` (GNU, Microsoft)
* ``__has_nothrow_copy`` (GNU, Microsoft)
* ``__has_nothrow_constructor`` (GNU, Microsoft)
* ``__has_trivial_assign`` (GNU, Microsoft)
* ``__has_trivial_copy`` (GNU, Microsoft)
* ``__has_trivial_constructor`` (GNU, Microsoft)
* ``__has_trivial_destructor`` (GNU, Microsoft)
* ``__has_virtual_destructor`` (GNU, Microsoft)
* ``__is_abstract`` (GNU, Microsoft)
* ``__is_aggregate`` (GNU, Microsoft)
* ``__is_base_of`` (GNU, Microsoft)
* ``__is_class`` (GNU, Microsoft)
* ``__is_convertible_to`` (Microsoft)
* ``__is_empty`` (GNU, Microsoft)
* ``__is_enum`` (GNU, Microsoft)
* ``__is_interface_class`` (Microsoft)
* ``__is_pod`` (GNU, Microsoft)
* ``__is_polymorphic`` (GNU, Microsoft)
* ``__is_union`` (GNU, Microsoft)
* ``__is_literal(type)``: Determines whether the given type is a literal type
* ``__is_final``: Determines whether the given type is declared with a
  ``final`` class-virt-specifier.
* ``__underlying_type(type)``: Retrieves the underlying type for a given
  ``enum`` type.  This trait is required to implement the C++11 standard
  library.
* ``__is_trivially_assignable(totype, fromtype)``: Determines whether a value
  of type ``totype`` can be assigned to from a value of type ``fromtype`` such
  that no non-trivial functions are called as part of that assignment.  This
  trait is required to implement the C++11 standard library.
* ``__is_trivially_constructible(type, argtypes...)``: Determines whether a
  value of type ``type`` can be direct-initialized with arguments of types
  ``argtypes...`` such that no non-trivial functions are called as part of
  that initialization.  This trait is required to implement the C++11 standard
  library.
* ``__is_destructible`` (MSVC 2013)
* ``__is_nothrow_destructible`` (MSVC 2013)
* ``__is_nothrow_assignable`` (MSVC 2013, clang)
* ``__is_constructible`` (MSVC 2013, clang)
* ``__is_nothrow_constructible`` (MSVC 2013, clang)
* ``__is_assignable`` (MSVC 2015, clang)

Blocks
======

The syntax and high level language feature description is in
:doc:`BlockLanguageSpec<BlockLanguageSpec>`. Implementation and ABI details for
the clang implementation are in :doc:`Block-ABI-Apple<Block-ABI-Apple>`.

Query for this feature with ``__has_extension(blocks)``.

Objective-C Features
====================

Related result types
--------------------

According to Cocoa conventions, Objective-C methods with certain names
("``init``", "``alloc``", etc.) always return objects that are an instance of
the receiving class's type.  Such methods are said to have a "related result
type", meaning that a message send to one of these methods will have the same
static type as an instance of the receiver class.  For example, given the
following classes:

.. code-block:: objc

  @interface NSObject
  + (id)alloc;
  - (id)init;
  @end

  @interface NSArray : NSObject
  @end

and this common initialization pattern

.. code-block:: objc

  NSArray *array = [[NSArray alloc] init];

the type of the expression ``[NSArray alloc]`` is ``NSArray*`` because
``alloc`` implicitly has a related result type.  Similarly, the type of the
expression ``[[NSArray alloc] init]`` is ``NSArray*``, since ``init`` has a
related result type and its receiver is known to have the type ``NSArray *``.
If neither ``alloc`` nor ``init`` had a related result type, the expressions
would have had type ``id``, as declared in the method signature.

A method with a related result type can be declared by using the type
``instancetype`` as its result type.  ``instancetype`` is a contextual keyword
that is only permitted in the result type of an Objective-C method, e.g.

.. code-block:: objc

  @interface A
  + (instancetype)constructAnA;
  @end

The related result type can also be inferred for some methods.  To determine
whether a method has an inferred related result type, the first word in the
camel-case selector (e.g., "``init``" in "``initWithObjects``") is considered,
and the method will have a related result type if its return type is compatible
with the type of its class and if:

* the first word is "``alloc``" or "``new``", and the method is a class method,
  or

* the first word is "``autorelease``", "``init``", "``retain``", or "``self``",
  and the method is an instance method.

If a method with a related result type is overridden by a subclass method, the
subclass method must also return a type that is compatible with the subclass
type.  For example:

.. code-block:: objc

  @interface NSString : NSObject
  - (NSUnrelated *)init; // incorrect usage: NSUnrelated is not NSString or a superclass of NSString
  @end

Related result types only affect the type of a message send or property access
via the given method.  In all other respects, a method with a related result
type is treated the same way as method that returns ``id``.

Use ``__has_feature(objc_instancetype)`` to determine whether the
``instancetype`` contextual keyword is available.

Automatic reference counting
----------------------------

Clang provides support for :doc:`automated reference counting
<AutomaticReferenceCounting>` in Objective-C, which eliminates the need
for manual ``retain``/``release``/``autorelease`` message sends.  There are two
feature macros associated with automatic reference counting:
``__has_feature(objc_arc)`` indicates the availability of automated reference
counting in general, while ``__has_feature(objc_arc_weak)`` indicates that
automated reference counting also includes support for ``__weak`` pointers to
Objective-C objects.

.. _objc-fixed-enum:

Enumerations with a fixed underlying type
-----------------------------------------

Clang provides support for C++11 enumerations with a fixed underlying type
within Objective-C.  For example, one can write an enumeration type as:

.. code-block:: c++

  typedef enum : unsigned char { Red, Green, Blue } Color;

This specifies that the underlying type, which is used to store the enumeration
value, is ``unsigned char``.

Use ``__has_feature(objc_fixed_enum)`` to determine whether support for fixed
underlying types is available in Objective-C.

Interoperability with C++11 lambdas
-----------------------------------

Clang provides interoperability between C++11 lambdas and blocks-based APIs, by
permitting a lambda to be implicitly converted to a block pointer with the
corresponding signature.  For example, consider an API such as ``NSArray``'s
array-sorting method:

.. code-block:: objc

  - (NSArray *)sortedArrayUsingComparator:(NSComparator)cmptr;

``NSComparator`` is simply a typedef for the block pointer ``NSComparisonResult
(^)(id, id)``, and parameters of this type are generally provided with block
literals as arguments.  However, one can also use a C++11 lambda so long as it
provides the same signature (in this case, accepting two parameters of type
``id`` and returning an ``NSComparisonResult``):

.. code-block:: objc

  NSArray *array = @[@"string 1", @"string 21", @"string 12", @"String 11",
                     @"String 02"];
  const NSStringCompareOptions comparisonOptions
    = NSCaseInsensitiveSearch | NSNumericSearch |
      NSWidthInsensitiveSearch | NSForcedOrderingSearch;
  NSLocale *currentLocale = [NSLocale currentLocale];
  NSArray *sorted
    = [array sortedArrayUsingComparator:[=](id s1, id s2) -> NSComparisonResult {
               NSRange string1Range = NSMakeRange(0, [s1 length]);
               return [s1 compare:s2 options:comparisonOptions
               range:string1Range locale:currentLocale];
       }];
  NSLog(@"sorted: %@", sorted);

This code relies on an implicit conversion from the type of the lambda
expression (an unnamed, local class type called the *closure type*) to the
corresponding block pointer type.  The conversion itself is expressed by a
conversion operator in that closure type that produces a block pointer with the
same signature as the lambda itself, e.g.,

.. code-block:: objc

  operator NSComparisonResult (^)(id, id)() const;

This conversion function returns a new block that simply forwards the two
parameters to the lambda object (which it captures by copy), then returns the
result.  The returned block is first copied (with ``Block_copy``) and then
autoreleased.  As an optimization, if a lambda expression is immediately
converted to a block pointer (as in the first example, above), then the block
is not copied and autoreleased: rather, it is given the same lifetime as a
block literal written at that point in the program, which avoids the overhead
of copying a block to the heap in the common case.

The conversion from a lambda to a block pointer is only available in
Objective-C++, and not in C++ with blocks, due to its use of Objective-C memory
management (autorelease).

Object Literals and Subscripting
--------------------------------

Clang provides support for :doc:`Object Literals and Subscripting
<ObjectiveCLiterals>` in Objective-C, which simplifies common Objective-C
programming patterns, makes programs more concise, and improves the safety of
container creation.  There are several feature macros associated with object
literals and subscripting: ``__has_feature(objc_array_literals)`` tests the
availability of array literals; ``__has_feature(objc_dictionary_literals)``
tests the availability of dictionary literals;
``__has_feature(objc_subscripting)`` tests the availability of object
subscripting.

Objective-C Autosynthesis of Properties
---------------------------------------

Clang provides support for autosynthesis of declared properties.  Using this
feature, clang provides default synthesis of those properties not declared
@dynamic and not having user provided backing getter and setter methods.
``__has_feature(objc_default_synthesize_properties)`` checks for availability
of this feature in version of clang being used.

.. _langext-objc-retain-release:

Objective-C retaining behavior attributes
-----------------------------------------

In Objective-C, functions and methods are generally assumed to follow the
`Cocoa Memory Management 
<http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html>`_
conventions for ownership of object arguments and
return values. However, there are exceptions, and so Clang provides attributes
to allow these exceptions to be documented. This are used by ARC and the
`static analyzer <http://clang-analyzer.llvm.org>`_ Some exceptions may be
better described using the ``objc_method_family`` attribute instead.

**Usage**: The ``ns_returns_retained``, ``ns_returns_not_retained``,
``ns_returns_autoreleased``, ``cf_returns_retained``, and
``cf_returns_not_retained`` attributes can be placed on methods and functions
that return Objective-C or CoreFoundation objects. They are commonly placed at
the end of a function prototype or method declaration:

.. code-block:: objc

  id foo() __attribute__((ns_returns_retained));

  - (NSString *)bar:(int)x __attribute__((ns_returns_retained));

The ``*_returns_retained`` attributes specify that the returned object has a +1
retain count.  The ``*_returns_not_retained`` attributes specify that the return
object has a +0 retain count, even if the normal convention for its selector
would be +1.  ``ns_returns_autoreleased`` specifies that the returned object is
+0, but is guaranteed to live at least as long as the next flush of an
autorelease pool.

**Usage**: The ``ns_consumed`` and ``cf_consumed`` attributes can be placed on
an parameter declaration; they specify that the argument is expected to have a
+1 retain count, which will be balanced in some way by the function or method.
The ``ns_consumes_self`` attribute can only be placed on an Objective-C
method; it specifies that the method expects its ``self`` parameter to have a
+1 retain count, which it will balance in some way.

.. code-block:: objc

  void foo(__attribute__((ns_consumed)) NSString *string);

  - (void) bar __attribute__((ns_consumes_self));
  - (void) baz:(id) __attribute__((ns_consumed)) x;

Further examples of these attributes are available in the static analyzer's `list of annotations for analysis
<http://clang-analyzer.llvm.org/annotations.html#cocoa_mem>`_.

Query for these features with ``__has_attribute(ns_consumed)``,
``__has_attribute(ns_returns_retained)``, etc.

Objective-C @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 ``-mmacosx-version-min=`` /
``-miphoneos-version-min=``.

Before LLVM 5.0, when calling a function that exists only in the OS that's
newer than the target OS (as determined by the minimum deployment version),
programmers had to carefully check if the function exists at runtime, using
null checks for weakly-linked C functions, ``+class`` for Objective-C classes,
and ``-respondsToSelector:`` or ``+instancesRespondToSelector:`` for
Objective-C methods.  If such a check was missed, the program would compile
fine, run fine on newer systems, but crash on older systems.

As of LLVM 5.0, ``-Wunguarded-availability`` uses the `availability attributes
<http://clang.llvm.org/docs/AttributeReference.html#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:

.. code-block:: objc

  void my_fun(NSSomeClass* var) {
    // If fancyNewMethod was added in e.g. macOS 10.12, but the code is
    // built with -mmacosx-version-min=10.11, then this unconditional call
    // will emit a -Wunguarded-availability warning:
    [var fancyNewMethod];
  }

To fix the warning and to avoid the crash on macOS 10.11, wrap it in
``if(@available())``:

.. code-block:: 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()``:

.. code-block:: 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
<http://clang.llvm.org/docs/AttributeReference.html#availability>`_ to it,
which will also suppress the warning and require that calls to my_fun() are
checked:

.. code-block:: 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:

.. code-block:: c++

  #include <math.h>
  #include <complex.h>
  complex float x = { 1.0f, INFINITY }; // Init to (1, Inf)

This construct is useful because there is no way to separately initialize the
real and imaginary parts of a complex variable in standard C, given that clang
does not support ``_Imaginary``.  (Clang also supports the ``__real__`` and
``__imag__`` extensions from gcc, which help in some cases, but are not usable
in static initializers.)

Note that this extension does not allow eliding the braces; the meaning of the
following two lines is different:

.. code-block:: c++

  complex float x[] = { { 1.0f, 1.0f } }; // [0] = (1, 1)
  complex float x[] = { 1.0f, 1.0f }; // [0] = (1, 0), [1] = (1, 0)

This extension also works in C++ mode, as far as that goes, but does not apply
to the C++ ``std::complex``.  (In C++11, list initialization allows the same
syntax to be used with ``std::complex`` with the same meaning.)

Builtin Functions
=================

Clang supports a number of builtin library functions with the same syntax as
GCC, including things like ``__builtin_nan``, ``__builtin_constant_p``,
``__builtin_choose_expr``, ``__builtin_types_compatible_p``,
``__builtin_assume_aligned``, ``__sync_fetch_and_add``, etc.  In addition to
the GCC builtins, Clang supports a number of builtins that GCC does not, which
are listed here.

Please note that Clang does not and will not support all of the GCC builtins
for vector operations.  Instead of using builtins, you should use the functions
defined in target-specific header files like ``<xmmintrin.h>``, which define
portable wrappers for these.  Many of the Clang versions of these functions are
implemented directly in terms of :ref:`extended vector support
<langext-vectors>` instead of builtins, in order to reduce the number of
builtins that we need to implement.

``__builtin_assume``
------------------------------

``__builtin_assume`` is used to provide the optimizer with a boolean
invariant that is defined to be true.

**Syntax**:

.. code-block:: c++

  __builtin_assume(bool)

**Example of Use**:

.. code-block:: c++

  int foo(int x) {
    __builtin_assume(x != 0);

    // The optimizer may short-circuit this check using the invariant.
    if (x == 0)
      return do_something();

    return do_something_else();
  }

**Description**:

The boolean argument to this function is defined to be true. The optimizer may
analyze the form of the expression provided as the argument and deduce from
that information used to optimize the program. If the condition is violated
during execution, the behavior is undefined. The argument itself is never
evaluated, so any side effects of the expression will be discarded.

Query for this feature with ``__has_builtin(__builtin_assume)``.

``__builtin_readcyclecounter``
------------------------------

``__builtin_readcyclecounter`` is used to access the cycle counter register (or
a similar low-latency, high-accuracy clock) on those targets that support it.

**Syntax**:

.. code-block:: c++

  __builtin_readcyclecounter()

**Example of Use**:

.. code-block:: c++

  unsigned long long t0 = __builtin_readcyclecounter();
  do_something();
  unsigned long long t1 = __builtin_readcyclecounter();
  unsigned long long cycles_to_do_something = t1 - t0; // assuming no overflow

**Description**:

The ``__builtin_readcyclecounter()`` builtin returns the cycle counter value,
which may be either global or process/thread-specific depending on the target.
As the backing counters often overflow quickly (on the order of seconds) this
should only be used for timing small intervals.  When not supported by the
target, the return value is always zero.  This builtin takes no arguments and
produces an unsigned long long result.

Query for this feature with ``__has_builtin(__builtin_readcyclecounter)``. Note
that even if present, its use may depend on run-time privilege or other OS
controlled state.

.. _langext-__builtin_shufflevector:

``__builtin_shufflevector``
---------------------------

``__builtin_shufflevector`` is used to express generic vector
permutation/shuffle/swizzle operations.  This builtin is also very important
for the implementation of various target-specific header files like
``<xmmintrin.h>``.

**Syntax**:

.. code-block:: c++

  __builtin_shufflevector(vec1, vec2, index1, index2, ...)

**Examples**:

.. code-block:: c++

  // identity operation - return 4-element vector v1.
  __builtin_shufflevector(v1, v1, 0, 1, 2, 3)

  // "Splat" element 0 of V1 into a 4-element result.
  __builtin_shufflevector(V1, V1, 0, 0, 0, 0)

  // Reverse 4-element vector V1.
  __builtin_shufflevector(V1, V1, 3, 2, 1, 0)

  // Concatenate every other element of 4-element vectors V1 and V2.
  __builtin_shufflevector(V1, V2, 0, 2, 4, 6)

  // Concatenate every other element of 8-element vectors V1 and V2.
  __builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)

  // Shuffle v1 with some elements being undefined
  __builtin_shufflevector(v1, v1, 3, -1, 1, -1)

**Description**:

The first two arguments to ``__builtin_shufflevector`` are vectors that have
the same element type.  The remaining arguments are a list of integers that
specify the elements indices of the first two vectors that should be extracted
and returned in a new vector.  These element indices are numbered sequentially
starting with the first vector, continuing into the second vector.  Thus, if
``vec1`` is a 4-element vector, index 5 would refer to the second element of
``vec2``. An index of -1 can be used to indicate that the corresponding element
in the returned vector is a don't care and can be optimized by the backend.

The result of ``__builtin_shufflevector`` is a vector with the same element
type as ``vec1``/``vec2`` but that has an element count equal to the number of
indices specified.

Query for this feature with ``__has_builtin(__builtin_shufflevector)``.

.. _langext-__builtin_convertvector:

``__builtin_convertvector``
---------------------------

``__builtin_convertvector`` is used to express generic vector
type-conversion operations. The input vector and the output vector
type must have the same number of elements.

**Syntax**:

.. code-block:: c++

  __builtin_convertvector(src_vec, dst_vec_type)

**Examples**:

.. code-block:: c++

  typedef double vector4double __attribute__((__vector_size__(32)));
  typedef float  vector4float  __attribute__((__vector_size__(16)));
  typedef short  vector4short  __attribute__((__vector_size__(8)));
  vector4float vf; vector4short vs;

  // convert from a vector of 4 floats to a vector of 4 doubles.
  __builtin_convertvector(vf, vector4double)
  // equivalent to:
  (vector4double) { (double) vf[0], (double) vf[1], (double) vf[2], (double) vf[3] }

  // convert from a vector of 4 shorts to a vector of 4 floats.
  __builtin_convertvector(vs, vector4float)
  // equivalent to:
  (vector4float) { (float) vs[0], (float) vs[1], (float) vs[2], (float) vs[3] }

**Description**:

The first argument to ``__builtin_convertvector`` is a vector, and the second
argument is a vector type with the same number of elements as the first
argument.

The result of ``__builtin_convertvector`` is a vector with the same element
type as the second argument, with a value defined in terms of the action of a
C-style cast applied to each element of the first argument.

Query for this feature with ``__has_builtin(__builtin_convertvector)``.

``__builtin_bitreverse``
------------------------

* ``__builtin_bitreverse8``
* ``__builtin_bitreverse16``
* ``__builtin_bitreverse32``
* ``__builtin_bitreverse64``

**Syntax**:

.. code-block:: c++

     __builtin_bitreverse32(x)

**Examples**:

.. code-block:: 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``.

``__builtin_unreachable``
-------------------------

``__builtin_unreachable`` is used to indicate that a specific point in the
program cannot be reached, even if the compiler might otherwise think it can.
This is useful to improve optimization and eliminates certain warnings.  For
example, without the ``__builtin_unreachable`` in the example below, the
compiler assumes that the inline asm can fall through and prints a "function
declared '``noreturn``' should not return" warning.

**Syntax**:

.. code-block:: c++

    __builtin_unreachable()

**Example of use**:

.. code-block:: c++

  void myabort(void) __attribute__((noreturn));
  void myabort(void) {
    asm("int3");
    __builtin_unreachable();
  }

**Description**:

The ``__builtin_unreachable()`` builtin has completely undefined behavior.
Since it has undefined behavior, it is a statement that it is never reached and
the optimizer can take advantage of this to produce better code.  This builtin
takes no arguments and produces a void result.

Query for this feature with ``__has_builtin(__builtin_unreachable)``.

``__builtin_unpredictable``
---------------------------

``__builtin_unpredictable`` is used to indicate that a branch condition is
unpredictable by hardware mechanisms such as branch prediction logic.

**Syntax**:

.. code-block:: c++

    __builtin_unpredictable(long long)

**Example of use**:

.. code-block:: c++

  if (__builtin_unpredictable(x > 0)) {
     foo();
  }

**Description**:

The ``__builtin_unpredictable()`` builtin is expected to be used with control
flow conditions such as in ``if`` and ``switch`` statements.

Query for this feature with ``__has_builtin(__builtin_unpredictable)``.

``__sync_swap``
---------------

``__sync_swap`` is used to atomically swap integers or pointers in memory.

**Syntax**:

.. code-block:: c++

  type __sync_swap(type *ptr, type value, ...)

**Example of Use**:

.. code-block:: c++

  int old_value = __sync_swap(&value, new_value);

**Description**:

The ``__sync_swap()`` builtin extends the existing ``__sync_*()`` family of
atomic intrinsics to allow code to atomically swap the current value with the
new value.  More importantly, it helps developers write more efficient and
correct code by avoiding expensive loops around
``__sync_bool_compare_and_swap()`` or relying on the platform specific
implementation details of ``__sync_lock_test_and_set()``.  The
``__sync_swap()`` builtin is a full barrier.

``__builtin_addressof``
-----------------------

``__builtin_addressof`` performs the functionality of the built-in ``&``
operator, ignoring any ``operator&`` overload.  This is useful in constant
expressions in C++11, where there is no other way to take the address of an
object that overloads ``operator&``.

**Example of use**:

.. code-block:: c++

  template<typename T> constexpr T *addressof(T &value) {
    return __builtin_addressof(value);
  }

``__builtin_operator_new`` and ``__builtin_operator_delete``
------------------------------------------------------------

``__builtin_operator_new`` allocates memory just like a non-placement non-class
*new-expression*. This is exactly like directly calling the normal
non-placement ``::operator new``, except that it allows certain optimizations
that the C++ standard does not permit for a direct function call to
``::operator new`` (in particular, removing ``new`` / ``delete`` pairs and
merging allocations).

Likewise, ``__builtin_operator_delete`` deallocates memory just like a
non-class *delete-expression*, and is exactly like directly calling the normal
``::operator delete``, except that it permits optimizations. Only the unsized
form of ``__builtin_operator_delete`` is currently available.

These builtins are intended for use in the implementation of ``std::allocator``
and other similar allocation libraries, and are only available in C++.

Multiprecision Arithmetic Builtins
----------------------------------

Clang provides a set of builtins which expose multiprecision arithmetic in a
manner amenable to C. They all have the following form:

.. code-block:: c

  unsigned x = ..., y = ..., carryin = ..., carryout;
  unsigned sum = __builtin_addc(x, y, carryin, &carryout);

Thus one can form a multiprecision addition chain in the following manner:

.. code-block:: c

  unsigned *x, *y, *z, carryin=0, carryout;
  z[0] = __builtin_addc(x[0], y[0], carryin, &carryout);
  carryin = carryout;
  z[1] = __builtin_addc(x[1], y[1], carryin, &carryout);
  carryin = carryout;
  z[2] = __builtin_addc(x[2], y[2], carryin, &carryout);
  carryin = carryout;
  z[3] = __builtin_addc(x[3], y[3], carryin, &carryout);

The complete list of builtins are:

.. code-block:: c

  unsigned char      __builtin_addcb (unsigned char x, unsigned char y, unsigned char carryin, unsigned char *carryout);
  unsigned short     __builtin_addcs (unsigned short x, unsigned short y, unsigned short carryin, unsigned short *carryout);
  unsigned           __builtin_addc  (unsigned x, unsigned y, unsigned carryin, unsigned *carryout);
  unsigned long      __builtin_addcl (unsigned long x, unsigned long y, unsigned long carryin, unsigned long *carryout);
  unsigned long long __builtin_addcll(unsigned long long x, unsigned long long y, unsigned long long carryin, unsigned long long *carryout);
  unsigned char      __builtin_subcb (unsigned char x, unsigned char y, unsigned char carryin, unsigned char *carryout);
  unsigned short     __builtin_subcs (unsigned short x, unsigned short y, unsigned short carryin, unsigned short *carryout);
  unsigned           __builtin_subc  (unsigned x, unsigned y, unsigned carryin, unsigned *carryout);
  unsigned long      __builtin_subcl (unsigned long x, unsigned long y, unsigned long carryin, unsigned long *carryout);
  unsigned long long __builtin_subcll(unsigned long long x, unsigned long long y, unsigned long long carryin, unsigned long long *carryout);

Checked Arithmetic Builtins
---------------------------

Clang provides a set of builtins that implement checked arithmetic for security
critical applications in a manner that is fast and easily expressable in C. As
an example of their usage:

.. code-block:: c

  errorcode_t security_critical_application(...) {
    unsigned x, y, result;
    ...
    if (__builtin_mul_overflow(x, y, &result))
      return kErrorCodeHackers;
    ...
    use_multiply(result);
    ...
  }

Clang provides the following checked arithmetic builtins:

.. code-block:: c

  bool __builtin_add_overflow   (type1 x, type2 y, type3 *sum);
  bool __builtin_sub_overflow   (type1 x, type2 y, type3 *diff);
  bool __builtin_mul_overflow   (type1 x, type2 y, type3 *prod);
  bool __builtin_uadd_overflow  (unsigned x, unsigned y, unsigned *sum);
  bool __builtin_uaddl_overflow (unsigned long x, unsigned long y, unsigned long *sum);
  bool __builtin_uaddll_overflow(unsigned long long x, unsigned long long y, unsigned long long *sum);
  bool __builtin_usub_overflow  (unsigned x, unsigned y, unsigned *diff);
  bool __builtin_usubl_overflow (unsigned long x, unsigned long y, unsigned long *diff);
  bool __builtin_usubll_overflow(unsigned long long x, unsigned long long y, unsigned long long *diff);
  bool __builtin_umul_overflow  (unsigned x, unsigned y, unsigned *prod);
  bool __builtin_umull_overflow (unsigned long x, unsigned long y, unsigned long *prod);
  bool __builtin_umulll_overflow(unsigned long long x, unsigned long long y, unsigned long long *prod);
  bool __builtin_sadd_overflow  (int x, int y, int *sum);
  bool __builtin_saddl_overflow (long x, long y, long *sum);
  bool __builtin_saddll_overflow(long long x, long long y, long long *sum);
  bool __builtin_ssub_overflow  (int x, int y, int *diff);
  bool __builtin_ssubl_overflow (long x, long y, long *diff);
  bool __builtin_ssubll_overflow(long long x, long long y, long long *diff);
  bool __builtin_smul_overflow  (int x, int y, int *prod);
  bool __builtin_smull_overflow (long x, long y, long *prod);
  bool __builtin_smulll_overflow(long long x, long long y, long long *prod);

Each builtin performs the specified mathematical operation on the
first two arguments and stores the result in the third argument.  If
possible, the result will be equal to mathematically-correct result
and the builtin will return 0.  Otherwise, the builtin will return
1 and the result will be equal to the unique value that is equivalent
to the mathematically-correct result modulo two raised to the *k*
power, where *k* is the number of bits in the result type.  The
behavior of these builtins is well-defined for all argument values.

The first three builtins work generically for operands of any integer type,
including boolean types.  The operands need not have the same type as each
other, or as the result.  The other builtins may implicitly promote or
convert their operands before performing the operation.

Query for this feature with ``__has_builtin(__builtin_add_overflow)``, etc.

Floating point builtins
---------------------------------------

``__builtin_canonicalize``
--------------------------

.. code-block:: 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
<http://llvm.org/docs/LangRef.html#llvm-canonicalize-intrinsic>`_ for
more information on the semantics.

String builtins
---------------

Clang provides constant expression evaluation support for builtins forms of
the following functions from the C standard library ``<string.h>`` header:

* ``memchr``
* ``memcmp``
* ``strchr``
* ``strcmp``
* ``strlen``
* ``strncmp``
* ``wcschr``
* ``wcscmp``
* ``wcslen``
* ``wcsncmp``
* ``wmemchr``
* ``wmemcmp``

In each case, the builtin form has the name of the C library function prefixed
by ``__builtin_``. Example:

.. code-block:: c

  void *p = __builtin_memchr("foobar", 'b', 5);

In addition to the above, one further builtin is provided:

.. code-block:: 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).

Support for constant expression evaluation for the above builtins be detected
with ``__has_feature(cxx_constexpr_string_builtins)``.

.. _langext-__c11_atomic:

__c11_atomic builtins
---------------------

Clang provides a set of builtins which are intended to be used to implement
C11's ``<stdatomic.h>`` header.  These builtins provide the semantics of the
``_explicit`` form of the corresponding C11 operation, and are named with a
``__c11_`` prefix.  The supported operations, and the differences from
the corresponding C11 operations, are:

* ``__c11_atomic_init``
* ``__c11_atomic_thread_fence``
* ``__c11_atomic_signal_fence``
* ``__c11_atomic_is_lock_free`` (The argument is the size of the
  ``_Atomic(...)`` object, instead of its address)
* ``__c11_atomic_store``
* ``__c11_atomic_load``
* ``__c11_atomic_exchange``
* ``__c11_atomic_compare_exchange_strong``
* ``__c11_atomic_compare_exchange_weak``
* ``__c11_atomic_fetch_add``
* ``__c11_atomic_fetch_sub``
* ``__c11_atomic_fetch_and``
* ``__c11_atomic_fetch_or``
* ``__c11_atomic_fetch_xor``

The macros ``__ATOMIC_RELAXED``, ``__ATOMIC_CONSUME``, ``__ATOMIC_ACQUIRE``,
``__ATOMIC_RELEASE``, ``__ATOMIC_ACQ_REL``, and ``__ATOMIC_SEQ_CST`` are
provided, with values corresponding to the enumerators of C11's