AttrDocs.td

//==--- AttrDocs.td - Attribute documentation ----------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===---------------------------------------------------------------------===//

// To test that the documentation builds cleanly, you must run clang-tblgen to
// convert the .td file into a .rst file, and then run sphinx to convert the
// .rst file into an HTML file. After completing testing, you should revert the
// generated .rst file so that the modified version does not get checked in to
// version control.
//
// To run clang-tblgen to generate the .rst file:
// clang-tblgen -gen-attr-docs -I <root>/llvm/tools/clang/include
//   <root>/llvm/tools/clang/include/clang/Basic/Attr.td -o
//   <root>/llvm/tools/clang/docs/AttributeReference.rst
//
// To run sphinx to generate the .html files (note that sphinx-build must be
// available on the PATH):
// Windows (from within the clang\docs directory):
//   make.bat html
// Non-Windows (from within the clang\docs directory):
//   sphinx-build -b html _build/html

def GlobalDocumentation {
  code Intro =[{..
  -------------------------------------------------------------------
  NOTE: This file is automatically generated by running clang-tblgen
  -gen-attr-docs. Do not edit this file by hand!!
  -------------------------------------------------------------------

===================
Attributes in Clang
===================
.. contents::
   :local:

.. |br| raw:: html

  <br/>

Introduction
============

This page lists the attributes currently supported by Clang.
}];
}

def SectionDocs : Documentation {
  let Category = DocCatVariable;
  let Content = [{
The ``section`` attribute allows you to specify a specific section a
global variable or function should be in after translation.
  }];
  let Heading = "section, __declspec(allocate)";
}

def UsedDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
This attribute, when attached to a function or variable definition, indicates
that there may be references to the entity which are not apparent in the source
code.  For example, it may be referenced from inline ``asm``, or it may be
found through a dynamic symbol or section lookup.

The compiler must emit the definition even if it appears to be unused, and it
must not apply optimizations which depend on fully understanding how the entity
is used.

Whether this attribute has any effect on the linker depends on the target and
the linker. Most linkers support the feature of section garbage collection
(``--gc-sections``), also known as "dead stripping" (``ld64 -dead_strip``) or
discarding unreferenced sections (``link.exe /OPT:REF``). On COFF and Mach-O
targets (Windows and Apple platforms), the `used` attribute prevents symbols
from being removed by linker section GC. On ELF targets, it has no effect on its
own, and the linker may remove the definition if it is not otherwise referenced.
This linker GC can be avoided by also adding the ``retain`` attribute.  Note
that ``retain`` requires special support from the linker; see that attribute's
documentation for further information.
  }];
}

def RetainDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
This attribute, when attached to a function or variable definition, prevents
section garbage collection in the linker. It does not prevent other discard
mechanisms, such as archive member selection, and COMDAT group resolution.

If the compiler does not emit the definition, e.g. because it was not used in
the translation unit or the compiler was able to eliminate all of the uses,
this attribute has no effect.  This attribute is typically combined with the
``used`` attribute to force the definition to be emitted and preserved into the
final linked image.

This attribute is only necessary on ELF targets; other targets prevent section
garbage collection by the linker when using the ``used`` attribute alone.
Using the attributes together should result in consistent behavior across
targets.

This attribute requires the linker to support the ``SHF_GNU_RETAIN`` extension.
This support is available in GNU ``ld`` and ``gold`` as of binutils 2.36, as
well as in ``ld.lld`` 13.
  }];
}

def InitPriorityDocs : Documentation {
  let Category = DocCatVariable;
  let Content = [{
In C++, the order in which global variables are initialized across translation
units is unspecified, unlike the ordering within a single translation unit. The
``init_priority`` attribute allows you to specify a relative ordering for the
initialization of objects declared at namespace scope in C++. The priority is
given as an integer constant expression between 101 and 65535 (inclusive).
Priorities outside of that range are reserved for use by the implementation. A
lower value indicates a higher priority of initialization. Note that only the
relative ordering of values is important. For example:

.. code-block:: c++

  struct SomeType { SomeType(); };
  __attribute__((init_priority(200))) SomeType Obj1;
  __attribute__((init_priority(101))) SomeType Obj2;

``Obj2`` will be initialized *before* ``Obj1`` despite the usual order of
initialization being the opposite.

On Windows, ``init_seg(compiler)`` is represented with a priority of 200 and
``init_seg(library)`` is represented with a priority of 400. ``init_seg(user)``
uses the default 65535 priority.

This attribute is only supported for C++ and Objective-C++ and is ignored in
other language modes. Currently, this attribute is not implemented on z/OS.
  }];
}

def InitSegDocs : Documentation {
  let Category = DocCatVariable;
  let Content = [{
The attribute applied by ``pragma init_seg()`` controls the section into
which global initialization function pointers are emitted. It is only
available with ``-fms-extensions``. Typically, this function pointer is
emitted into ``.CRT$XCU`` on Windows. The user can change the order of
initialization by using a different section name with the same
``.CRT$XC`` prefix and a suffix that sorts lexicographically before or
after the standard ``.CRT$XCU`` sections. See the init_seg_
documentation on MSDN for more information.

.. _init_seg: http://msdn.microsoft.com/en-us/library/7977wcck(v=vs.110).aspx
  }];
}

def TLSModelDocs : Documentation {
  let Category = DocCatVariable;
  let Content = [{
The ``tls_model`` attribute allows you to specify which thread-local storage
model to use. It accepts the following strings:

* global-dynamic
* local-dynamic
* initial-exec
* local-exec

TLS models are mutually exclusive.
  }];
}

def DLLExportDocs : Documentation {
  let Category = DocCatVariable;
  let Content = [{
The ``__declspec(dllexport)`` attribute declares a variable, function, or
Objective-C interface to be exported from the module. It is available under the
``-fdeclspec`` flag for compatibility with various compilers. The primary use
is for COFF object files which explicitly specify what interfaces are available
for external use. See the dllexport_ documentation on MSDN for more
information.

.. _dllexport: https://msdn.microsoft.com/en-us/library/3y1sfaz2.aspx
  }];
}

def DLLImportDocs : Documentation {
  let Category = DocCatVariable;
  let Content = [{
The ``__declspec(dllimport)`` attribute declares a variable, function, or
Objective-C interface to be imported from an external module. It is available
under the ``-fdeclspec`` flag for compatibility with various compilers. The
primary use is for COFF object files which explicitly specify what interfaces
are imported from external modules. See the dllimport_ documentation on MSDN
for more information.

Note that a dllimport function may still be inlined, if its definition is
available and it doesn't reference any non-dllimport functions or global
variables.

.. _dllimport: https://msdn.microsoft.com/en-us/library/3y1sfaz2.aspx
  }];
}

def ThreadDocs : Documentation {
  let Category = DocCatVariable;
  let Content = [{
The ``__declspec(thread)`` attribute declares a variable with thread local
storage. It is available under the ``-fms-extensions`` flag for MSVC
compatibility. See the documentation for `__declspec(thread)`_ on MSDN.

.. _`__declspec(thread)`: http://msdn.microsoft.com/en-us/library/9w1sdazb.aspx

In Clang, ``__declspec(thread)`` is generally equivalent in functionality to the
GNU ``__thread`` keyword. The variable must not have a destructor and must have
a constant initializer, if any. The attribute only applies to variables
declared with static storage duration, such as globals, class static data
members, and static locals.
  }];
}

def NoEscapeDocs : Documentation {
  let Category = DocCatVariable;
  let Content = [{
``noescape`` placed on a function parameter of a pointer type is used to inform
the compiler that the pointer cannot escape: that is, no reference to the object
the pointer points to that is derived from the parameter value will survive
after the function returns. Users are responsible for making sure parameters
annotated with ``noescape`` do not actually escape. Calling ``free()`` on such
a parameter does not constitute an escape.

For example:

.. code-block:: c

  int *gp;

  void nonescapingFunc(__attribute__((noescape)) int *p) {
    *p += 100; // OK.
  }

  void escapingFunc(__attribute__((noescape)) int *p) {
    gp = p; // Not OK.
  }

Additionally, when the parameter is a `block pointer
<https://clang.llvm.org/docs/BlockLanguageSpec.html>`, the same restriction
applies to copies of the block. For example:

.. code-block:: c

  typedef void (^BlockTy)();
  BlockTy g0, g1;

  void nonescapingFunc(__attribute__((noescape)) BlockTy block) {
    block(); // OK.
  }

  void escapingFunc(__attribute__((noescape)) BlockTy block) {
    g0 = block; // Not OK.
    g1 = Block_copy(block); // Not OK either.
  }

  }];
}

def MaybeUndefDocs : Documentation {
  let Category = DocCatVariable;
  let Content = [{
The ``maybe_undef`` attribute can be placed on a function parameter. It indicates
that the parameter is allowed to use undef values. It informs the compiler
to insert a freeze LLVM IR instruction on the function parameter.
Please note that this is an attribute that is used as an internal
implementation detail and not intended to be used by external users.

In languages HIP, CUDA etc., some functions have multi-threaded semantics and
it is enough for only one or some threads to provide defined arguments.
Depending on semantics, undef arguments in some threads don't produce
undefined results in the function call. Since, these functions accept undefined
arguments, ``maybe_undef`` attribute can be placed.

Sample usage:
.. code-block:: c

  void maybeundeffunc(int __attribute__((maybe_undef))param);
  }];
}

def CarriesDependencyDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The ``carries_dependency`` attribute specifies dependency propagation into and
out of functions.

When specified on a function or Objective-C method, the ``carries_dependency``
attribute means that the return value carries a dependency out of the function,
so that the implementation need not constrain ordering upon return from that
function. Implementations of the function and its caller may choose to preserve
dependencies instead of emitting memory ordering instructions such as fences.

Note, this attribute does not change the meaning of the program, but may result
in generation of more efficient code.
  }];
}

def CPUSpecificCPUDispatchDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The ``cpu_specific`` and ``cpu_dispatch`` attributes are used to define and
resolve multiversioned functions. This form of multiversioning provides a
mechanism for declaring versions across translation units and manually
specifying the resolved function list. A specified CPU defines a set of minimum
features that are required for the function to be called. The result of this is
that future processors execute the most restrictive version of the function the
new processor can execute.

In addition, unlike the ICC implementation of this feature, the selection of the
version does not consider the manufacturer or microarchitecture of the processor.
It tests solely the list of features that are both supported by the specified
processor and present in the compiler-rt library. This can be surprising at times,
as the runtime processor may be from a completely different manufacturer, as long
as it supports the same feature set.

This can additionally be surprising, as some processors are indistringuishable from
others based on the list of testable features. When this happens, the variant
is selected in an unspecified manner.

Function versions are defined with ``cpu_specific``, which takes one or more CPU
names as a parameter. For example:

.. code-block:: c

  // Declares and defines the ivybridge version of single_cpu.
  __attribute__((cpu_specific(ivybridge)))
  void single_cpu(void){}

  // Declares and defines the atom version of single_cpu.
  __attribute__((cpu_specific(atom)))
  void single_cpu(void){}

  // Declares and defines both the ivybridge and atom version of multi_cpu.
  __attribute__((cpu_specific(ivybridge, atom)))
  void multi_cpu(void){}

A dispatching (or resolving) function can be declared anywhere in a project's
source code with ``cpu_dispatch``. This attribute takes one or more CPU names
as a parameter (like ``cpu_specific``). Functions marked with ``cpu_dispatch``
are not expected to be defined, only declared. If such a marked function has a
definition, any side effects of the function are ignored; trivial function
bodies are permissible for ICC compatibility.

.. code-block:: c

  // Creates a resolver for single_cpu above.
  __attribute__((cpu_dispatch(ivybridge, atom)))
  void single_cpu(void){}

  // Creates a resolver for multi_cpu, but adds a 3rd version defined in another
  // translation unit.
  __attribute__((cpu_dispatch(ivybridge, atom, sandybridge)))
  void multi_cpu(void){}

Note that it is possible to have a resolving function that dispatches based on
more or fewer options than are present in the program. Specifying fewer will
result in the omitted options not being considered during resolution. Specifying
a version for resolution that isn't defined in the program will result in a
linking failure.

It is also possible to specify a CPU name of ``generic`` which will be resolved
if the executing processor doesn't satisfy the features required in the CPU
name. The behavior of a program executing on a processor that doesn't satisfy
any option of a multiversioned function is undefined.
  }];
}

def SYCLKernelDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The ``sycl_kernel`` attribute specifies that a function template will be used
to outline device code and to generate an OpenCL kernel.
Here is a code example of the SYCL program, which demonstrates the compiler's
outlining job:

.. code-block:: c++

  int foo(int x) { return ++x; }

  using namespace cl::sycl;
  queue Q;
  buffer<int, 1> a(range<1>{1024});
  Q.submit([&](handler& cgh) {
    auto A = a.get_access<access::mode::write>(cgh);
    cgh.parallel_for<init_a>(range<1>{1024}, [=](id<1> index) {
      A[index] = index[0] + foo(42);
    });
  }

A C++ function object passed to the ``parallel_for`` is called a "SYCL kernel".
A SYCL kernel defines the entry point to the "device part" of the code. The
compiler will emit all symbols accessible from a "kernel". In this code
example, the compiler will emit "foo" function. More details about the
compilation of functions for the device part can be found in the SYCL 1.2.1
specification Section 6.4.
To show to the compiler entry point to the "device part" of the code, the SYCL
runtime can use the ``sycl_kernel`` attribute in the following way:

.. code-block:: c++

  namespace cl {
  namespace sycl {
  class handler {
    template <typename KernelName, typename KernelType/*, ...*/>
    __attribute__((sycl_kernel)) void sycl_kernel_function(KernelType KernelFuncObj) {
      // ...
      KernelFuncObj();
    }

    template <typename KernelName, typename KernelType, int Dims>
    void parallel_for(range<Dims> NumWorkItems, KernelType KernelFunc) {
  #ifdef __SYCL_DEVICE_ONLY__
      sycl_kernel_function<KernelName, KernelType, Dims>(KernelFunc);
  #else
      // Host implementation
  #endif
    }
  };
  } // namespace sycl
  } // namespace cl

The compiler will also generate an OpenCL kernel using the function marked with
the ``sycl_kernel`` attribute.
Here is the list of SYCL device compiler expectations with regard to the
function marked with the ``sycl_kernel`` attribute:

- The function must be a template with at least two type template parameters.
  The compiler generates an OpenCL kernel and uses the first template parameter
  as a unique name for the generated OpenCL kernel. The host application uses
  this unique name to invoke the OpenCL kernel generated for the SYCL kernel
  specialized by this name and second template parameter ``KernelType`` (which
  might be an unnamed function object type).
- The function must have at least one parameter. The first parameter is
  required to be a function object type (named or unnamed i.e. lambda). The
  compiler uses function object type fields to generate OpenCL kernel
  parameters.
- The function must return void. The compiler reuses the body of marked functions to
  generate the OpenCL kernel body, and the OpenCL kernel must return ``void``.

The SYCL kernel in the previous code sample meets these expectations.
  }];
}

def SYCLSpecialClassDocs : Documentation {
  let Category = DocCatStmt;
  let Content = [{
SYCL defines some special classes (accessor, sampler, and stream) which require
specific handling during the generation of the SPIR entry point.
The ``__attribute__((sycl_special_class))`` attribute is used in SYCL
headers to indicate that a class or a struct needs a specific handling when
it is passed from host to device.
Special classes will have a mandatory ``__init`` method and an optional
``__finalize`` method (the ``__finalize`` method is used only with the
``stream`` type). Kernel parameters types are extract from the ``__init`` method
parameters. The kernel function arguments list is derived from the
arguments of the ``__init`` method. The arguments of the ``__init`` method are
copied into the kernel function argument list and the ``__init`` and
``__finalize`` methods are called at the beginning and the end of the kernel,
respectively.
The ``__init`` and ``__finalize`` methods must be defined inside the
special class.
Please note that this is an attribute that is used as an internal
implementation detail and not intended to be used by external users.

The syntax of the attribute is as follows:

.. code-block:: text

  class __attribute__((sycl_special_class)) accessor {};
  class [[clang::sycl_special_class]] accessor {};

This is a code example that illustrates the use of the attribute:

.. code-block:: c++

  class __attribute__((sycl_special_class)) SpecialType {
    int F1;
    int F2;
    void __init(int f1) {
      F1 = f1;
      F2 = f1;
    }
    void __finalize() {}
  public:
    SpecialType() = default;
    int getF2() const { return F2; }
  };

  int main () {
    SpecialType T;
    cgh.single_task([=] {
      T.getF2();
    });
  }

This would trigger the following kernel entry point in the AST:

.. code-block:: c++

  void __sycl_kernel(int f1) {
    SpecialType T;
    T.__init(f1);
    ...
    T.__finalize()
  }
  }];
}

def C11NoReturnDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
A function declared as ``_Noreturn`` shall not return to its caller. The
compiler will generate a diagnostic for a function declared as ``_Noreturn``
that appears to be capable of returning to its caller. Despite being a type
specifier, the ``_Noreturn`` attribute cannot be specified on a function
pointer type.
  }];
}

def CXX11NoReturnDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "noreturn, _Noreturn";
  let Content = [{
A function declared as ``[[noreturn]]`` shall not return to its caller. The
compiler will generate a diagnostic for a function declared as ``[[noreturn]]``
that appears to be capable of returning to its caller.

The ``[[_Noreturn]]`` spelling is deprecated and only exists to ease code
migration for code using ``[[noreturn]]`` after including ``<stdnoreturn.h>``.
  }];
}

def NoMergeDocs : Documentation {
  let Category = DocCatStmt;
  let Content = [{
If a statement is marked ``nomerge`` and contains call expressions, those call
expressions inside the statement will not be merged during optimization. This 
attribute can be used to prevent the optimizer from obscuring the source
location of certain calls. For example, it will prevent tail merging otherwise
identical code sequences that raise an exception or terminate the program. Tail
merging normally reduces the precision of source location information, making
stack traces less useful for debugging. This attribute gives the user control
over the tradeoff between code size and debug information precision. 

``nomerge`` attribute can also be used as function attribute to prevent all 
calls to the specified function from merging. It has no effect on indirect 
calls.
  }];
}

def NoInlineDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
This function attribute suppresses the inlining of a function at the call sites
of the function.

``[[clang::noinline]]`` spelling can be used as a statement attribute; other
spellings of the attribute are not supported on statements. If a statement is
marked ``[[clang::noinline]]`` and contains calls, those calls inside the
statement will not be inlined by the compiler.

``__noinline__`` can be used as a keyword in CUDA/HIP languages. This is to
avoid diagnostics due to usage of ``__attribute__((__noinline__))``
with ``__noinline__`` defined as a macro as ``__attribute__((noinline))``.

.. code-block:: c

  int example(void) {
    int r;
    [[clang::noinline]] foo();
    [[clang::noinline]] r = bar();
    return r;
  }

  }];
}

def MustTailDocs : Documentation {
  let Category = DocCatStmt;
  let Content = [{
If a ``return`` statement is marked ``musttail``, this indicates that the
compiler must generate a tail call for the program to be correct, even when
optimizations are disabled. This guarantees that the call will not cause
unbounded stack growth if it is part of a recursive cycle in the call graph.

If the callee is a virtual function that is implemented by a thunk, there is
no guarantee in general that the thunk tail-calls the implementation of the
virtual function, so such a call in a recursive cycle can still result in
unbounded stack growth.

``clang::musttail`` can only be applied to a ``return`` statement whose value
is the result of a function call (even functions returning void must use
``return``, although no value is returned). The target function must have the
same number of arguments as the caller. The types of the return value and all
arguments must be similar according to C++ rules (differing only in cv
qualifiers or array size), including the implicit "this" argument, if any.
Any variables in scope, including all arguments to the function and the
return value must be trivially destructible. The calling convention of the
caller and callee must match, and they must not be variadic functions or have
old style K&R C function declarations.

``clang::musttail`` provides assurances that the tail call can be optimized on
all targets, not just one.
  }];
}

def AssertCapabilityDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "assert_capability, assert_shared_capability";
  let Content = [{
Marks a function that dynamically tests whether a capability is held, and halts
the program if it is not held.
  }];
}

def AcquireCapabilityDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "acquire_capability, acquire_shared_capability";
  let Content = [{
Marks a function as acquiring a capability.
  }];
}

def TryAcquireCapabilityDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "try_acquire_capability, try_acquire_shared_capability";
  let Content = [{
Marks a function that attempts to acquire a capability. This function may fail to
actually acquire the capability; they accept a Boolean value determining
whether acquiring the capability means success (true), or failing to acquire
the capability means success (false).
  }];
}

def ReleaseCapabilityDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "release_capability, release_shared_capability";
  let Content = [{
Marks a function as releasing a capability.
  }];
}

def AssumeAlignedDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Use ``__attribute__((assume_aligned(<alignment>[,<offset>]))`` on a function
declaration to specify that the return value of the function (which must be a
pointer type) has the specified offset, in bytes, from an address with the
specified alignment. The offset is taken to be zero if omitted.

.. code-block:: c++

  // The returned pointer value has 32-byte alignment.
  void *a() __attribute__((assume_aligned (32)));

  // The returned pointer value is 4 bytes greater than an address having
  // 32-byte alignment.
  void *b() __attribute__((assume_aligned (32, 4)));

Note that this attribute provides information to the compiler regarding a
condition that the code already ensures is true. It does not cause the compiler
to enforce the provided alignment assumption.
  }];
}

def AllocSizeDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The ``alloc_size`` attribute can be placed on functions that return pointers in
order to hint to the compiler how many bytes of memory will be available at the
returned pointer. ``alloc_size`` takes one or two arguments.

- ``alloc_size(N)`` implies that argument number N equals the number of
  available bytes at the returned pointer.
- ``alloc_size(N, M)`` implies that the product of argument number N and
  argument number M equals the number of available bytes at the returned
  pointer.

Argument numbers are 1-based.

An example of how to use ``alloc_size``

.. code-block:: c

  void *my_malloc(int a) __attribute__((alloc_size(1)));
  void *my_calloc(int a, int b) __attribute__((alloc_size(1, 2)));

  int main() {
    void *const p = my_malloc(100);
    assert(__builtin_object_size(p, 0) == 100);
    void *const a = my_calloc(20, 5);
    assert(__builtin_object_size(a, 0) == 100);
  }

.. Note:: This attribute works differently in clang than it does in GCC.
  Specifically, clang will only trace ``const`` pointers (as above); we give up
  on pointers that are not marked as ``const``. In the vast majority of cases,
  this is unimportant, because LLVM has support for the ``alloc_size``
  attribute. However, this may cause mildly unintuitive behavior when used with
  other attributes, such as ``enable_if``.
  }];
}

def CodeSegDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The ``__declspec(code_seg)`` attribute enables the placement of code into separate
named segments that can be paged or locked in memory individually. This attribute
is used to control the placement of instantiated templates and compiler-generated
code. See the documentation for `__declspec(code_seg)`_ on MSDN.

.. _`__declspec(code_seg)`: http://msdn.microsoft.com/en-us/library/dn636922.aspx
  }];
}

def AllocAlignDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Use ``__attribute__((alloc_align(<alignment>))`` on a function
declaration to specify that the return value of the function (which must be a
pointer type) is at least as aligned as the value of the indicated parameter. The
parameter is given by its index in the list of formal parameters; the first
parameter has index 1 unless the function is a C++ non-static member function,
in which case the first parameter has index 2 to account for the implicit ``this``
parameter.

.. code-block:: c++

  // The returned pointer has the alignment specified by the first parameter.
  void *a(size_t align) __attribute__((alloc_align(1)));

  // The returned pointer has the alignment specified by the second parameter.
  void *b(void *v, size_t align) __attribute__((alloc_align(2)));

  // The returned pointer has the alignment specified by the second visible
  // parameter, however it must be adjusted for the implicit 'this' parameter.
  void *Foo::b(void *v, size_t align) __attribute__((alloc_align(3)));

Note that this attribute merely informs the compiler that a function always
returns a sufficiently aligned pointer. It does not cause the compiler to
emit code to enforce that alignment. The behavior is undefined if the returned
pointer is not sufficiently aligned.
  }];
}

def EnableIfDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
.. Note:: Some features of this attribute are experimental. The meaning of
  multiple enable_if attributes on a single declaration is subject to change in
  a future version of clang. Also, the ABI is not standardized and the name
  mangling may change in future versions. To avoid that, use asm labels.

The ``enable_if`` attribute can be placed on function declarations to control
which overload is selected based on the values of the function's arguments.
When combined with the ``overloadable`` attribute, this feature is also
available in C.

.. code-block:: c++

  int isdigit(int c);
  int isdigit(int c) __attribute__((enable_if(c <= -1 || c > 255, "chosen when 'c' is out of range"))) __attribute__((unavailable("'c' must have the value of an unsigned char or EOF")));

  void foo(char c) {
    isdigit(c);
    isdigit(10);
    isdigit(-10);  // results in a compile-time error.
  }

The enable_if attribute takes two arguments, the first is an expression written
in terms of the function parameters, the second is a string explaining why this
overload candidate could not be selected to be displayed in diagnostics. The
expression is part of the function signature for the purposes of determining
whether it is a redeclaration (following the rules used when determining
whether a C++ template specialization is ODR-equivalent), but is not part of
the type.

The enable_if expression is evaluated as if it were the body of a
bool-returning constexpr function declared with the arguments of the function
it is being applied to, then called with the parameters at the call site. If the
result is false or could not be determined through constant expression
evaluation, then this overload will not be chosen and the provided string may
be used in a diagnostic if the compile fails as a result.

Because the enable_if expression is an unevaluated context, there are no global
state changes, nor the ability to pass information from the enable_if
expression to the function body. For example, suppose we want calls to
strnlen(strbuf, maxlen) to resolve to strnlen_chk(strbuf, maxlen, size of
strbuf) only if the size of strbuf can be determined:

.. code-block:: c++

  __attribute__((always_inline))
  static inline size_t strnlen(const char *s, size_t maxlen)
    __attribute__((overloadable))
    __attribute__((enable_if(__builtin_object_size(s, 0) != -1))),
                             "chosen when the buffer size is known but 'maxlen' is not")))
  {
    return strnlen_chk(s, maxlen, __builtin_object_size(s, 0));
  }

Multiple enable_if attributes may be applied to a single declaration. In this
case, the enable_if expressions are evaluated from left to right in the
following manner. First, the candidates whose enable_if expressions evaluate to
false or cannot be evaluated are discarded. If the remaining candidates do not
share ODR-equivalent enable_if expressions, the overload resolution is
ambiguous. Otherwise, enable_if overload resolution continues with the next
enable_if attribute on the candidates that have not been discarded and have
remaining enable_if attributes. In this way, we pick the most specific
overload out of a number of viable overloads using enable_if.

.. code-block:: c++

  void f() __attribute__((enable_if(true, "")));  // #1
  void f() __attribute__((enable_if(true, ""))) __attribute__((enable_if(true, "")));  // #2

  void g(int i, int j) __attribute__((enable_if(i, "")));  // #1
  void g(int i, int j) __attribute__((enable_if(j, ""))) __attribute__((enable_if(true)));  // #2

In this example, a call to f() is always resolved to #2, as the first enable_if
expression is ODR-equivalent for both declarations, but #1 does not have another
enable_if expression to continue evaluating, so the next round of evaluation has
only a single candidate. In a call to g(1, 1), the call is ambiguous even though
#2 has more enable_if attributes, because the first enable_if expressions are
not ODR-equivalent.

Query for this feature with ``__has_attribute(enable_if)``.

Note that functions with one or more ``enable_if`` attributes may not have
their address taken, unless all of the conditions specified by said
``enable_if`` are constants that evaluate to ``true``. For example:

.. code-block:: c

  const int TrueConstant = 1;
  const int FalseConstant = 0;
  int f(int a) __attribute__((enable_if(a > 0, "")));
  int g(int a) __attribute__((enable_if(a == 0 || a != 0, "")));
  int h(int a) __attribute__((enable_if(1, "")));
  int i(int a) __attribute__((enable_if(TrueConstant, "")));
  int j(int a) __attribute__((enable_if(FalseConstant, "")));

  void fn() {
    int (*ptr)(int);
    ptr = &f; // error: 'a > 0' is not always true
    ptr = &g; // error: 'a == 0 || a != 0' is not a truthy constant
    ptr = &h; // OK: 1 is a truthy constant
    ptr = &i; // OK: 'TrueConstant' is a truthy constant
    ptr = &j; // error: 'FalseConstant' is a constant, but not truthy
  }

Because ``enable_if`` evaluation happens during overload resolution,
``enable_if`` may give unintuitive results when used with templates, depending
on when overloads are resolved. In the example below, clang will emit a
diagnostic about no viable overloads for ``foo`` in ``bar``, but not in ``baz``:

.. code-block:: c++

  double foo(int i) __attribute__((enable_if(i > 0, "")));
  void *foo(int i) __attribute__((enable_if(i <= 0, "")));
  template <int I>
  auto bar() { return foo(I); }

  template <typename T>
  auto baz() { return foo(T::number); }

  struct WithNumber { constexpr static int number = 1; };
  void callThem() {
    bar<sizeof(WithNumber)>();
    baz<WithNumber>();
  }

This is because, in ``bar``, ``foo`` is resolved prior to template
instantiation, so the value for ``I`` isn't known (thus, both ``enable_if``
conditions for ``foo`` fail). However, in ``baz``, ``foo`` is resolved during
template instantiation, so the value for ``T::number`` is known.
  }];
}

def DiagnoseIfDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The ``diagnose_if`` attribute can be placed on function declarations to emit
warnings or errors at compile-time if calls to the attributed function meet
certain user-defined criteria. For example:

.. code-block:: c

  int abs(int a)
    __attribute__((diagnose_if(a >= 0, "Redundant abs call", "warning")));
  int must_abs(int a)
    __attribute__((diagnose_if(a >= 0, "Redundant abs call", "error")));

  int val = abs(1); // warning: Redundant abs call
  int val2 = must_abs(1); // error: Redundant abs call
  int val3 = abs(val);
  int val4 = must_abs(val); // Because run-time checks are not emitted for
                            // diagnose_if attributes, this executes without
                            // issue.


``diagnose_if`` is closely related to ``enable_if``, with a few key differences:

* Overload resolution is not aware of ``diagnose_if`` attributes: they're
  considered only after we select the best candidate from a given candidate set.
* Function declarations that differ only in their ``diagnose_if`` attributes are
  considered to be redeclarations of the same function (not overloads).
* If the condition provided to ``diagnose_if`` cannot be evaluated, no
  diagnostic will be emitted.

Otherwise, ``diagnose_if`` is essentially the logical negation of ``enable_if``.

As a result of bullet number two, ``diagnose_if`` attributes will stack on the
same function. For example:

.. code-block:: c

  int foo() __attribute__((diagnose_if(1, "diag1", "warning")));
  int foo() __attribute__((diagnose_if(1, "diag2", "warning")));

  int bar = foo(); // warning: diag1
                   // warning: diag2
  int (*fooptr)(void) = foo; // warning: diag1
                             // warning: diag2

  constexpr int supportsAPILevel(int N) { return N < 5; }
  int baz(int a)
    __attribute__((diagnose_if(!supportsAPILevel(10),
                               "Upgrade to API level 10 to use baz", "error")));
  int baz(int a)
    __attribute__((diagnose_if(!a, "0 is not recommended.", "warning")));

  int (*bazptr)(int) = baz; // error: Upgrade to API level 10 to use baz
  int v = baz(0); // error: Upgrade to API level 10 to use baz

Query for this feature with ``__has_attribute(diagnose_if)``.
  }];
}

def PassObjectSizeDocs : Documentation {
  let Category = DocCatVariable; // Technically it's a parameter doc, but eh.
  let Heading = "pass_object_size, pass_dynamic_object_size";
  let Content = [{
.. Note:: The mangling of functions with parameters that are annotated with
  ``pass_object_size`` is subject to change. You can get around this by
  using ``__asm__("foo")`` to explicitly name your functions, thus preserving
  your ABI; also, non-overloadable C functions with ``pass_object_size`` are
  not mangled.

The ``pass_object_size(Type)`` attribute can be placed on function parameters to
instruct clang to call ``__builtin_object_size(param, Type)`` at each callsite
of said function, and implicitly pass the result of this call in as an invisible
argument of type ``size_t`` directly after the parameter annotated with
``pass_object_size``. Clang will also replace any calls to
``__builtin_object_size(param, Type)`` in the function by said implicit
parameter.

Example usage:

.. code-block:: c

  int bzero1(char *const p __attribute__((pass_object_size(0))))
      __attribute__((noinline)) {
    int i = 0;
    for (/**/; i < (int)__builtin_object_size(p, 0); ++i) {
      p[i] = 0;
    }
    return i;
  }

  int main() {
    char chars[100];
    int n = bzero1(&chars[0]);
    assert(n == sizeof(chars));
    return 0;
  }

If successfully evaluating ``__builtin_object_size(param, Type)`` at the
callsite is not possible, then the "failed" value is passed in. So, using the
definition of ``bzero1`` from above, the following code would exit cleanly:

.. code-block:: c

  int main2(int argc, char *argv[]) {
    int n = bzero1(argv);
    assert(n == -1);
    return 0;
  }

``pass_object_size`` plays a part in overload resolution. If two overload
candidates are otherwise equally good, then the overload with one or more
parameters with ``pass_object_size`` is preferred. This implies that the choice
between two identical overloads both with ``pass_object_size`` on one or more
parameters will always be ambiguous; for this reason, having two such overloads
is illegal. For example:

.. code-block:: c++

  #define PS(N) __attribute__((pass_object_size(N)))
  // OK
  void Foo(char *a, char *b); // Overload A
  // OK -- overload A has no parameters with pass_object_size.
  void Foo(char *a PS(0), char *b PS(0)); // Overload B
  // Error -- Same signature (sans pass_object_size) as overload B, and both
  // overloads have one or more parameters with the pass_object_size attribute.
  void Foo(void *a PS(0), void *b);

  // OK
  void Bar(void *a PS(0)); // Overload C
  // OK
  void Bar(char *c PS(1)); // Overload D

  void main() {
    char known[10], *unknown;
    Foo(unknown, unknown); // Calls overload B
    Foo(known, unknown); // Calls overload B
    Foo(unknown, known); // Calls overload B
    Foo(known, known); // Calls overload B

    Bar(known); // Calls overload D
    Bar(unknown); // Calls overload D
  }

Currently, ``pass_object_size`` is a bit restricted in terms of its usage:

* Only one use of ``pass_object_size`` is allowed per parameter.

* It is an error to take the address of a function with ``pass_object_size`` on
  any of its parameters. If you wish to do this, you can create an overload
  without ``pass_object_size`` on any parameters.

* It is an error to apply the ``pass_object_size`` attribute to parameters that
  are not pointers. Additionally, any parameter that ``pass_object_size`` is
  applied to must be marked ``const`` at its function's definition.

Clang also supports the ``pass_dynamic_object_size`` attribute, which behaves
identically to ``pass_object_size``, but evaluates a call to
``__builtin_dynamic_object_size`` at the callee instead of
``__builtin_object_size``. ``__builtin_dynamic_object_size`` provides some extra
runtime checks when the object size can't be determined at compile-time. You can
read more about ``__builtin_dynamic_object_size`` `here
<https://clang.llvm.org/docs/LanguageExtensions.html#evaluating-object-size-dynamically>`_.

  }];
}

def OverloadableDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Clang provides support for C++ function overloading in C. Function overloading
in C is introduced using the ``overloadable`` attribute. For example, one
might provide several overloaded versions of a ``tgsin`` function that invokes
the appropriate standard function computing the sine of a value with ``float``,
``double``, or ``long double`` precision:

.. code-block:: c

  #include <math.h>
  float __attribute__((overloadable)) tgsin(float x) { return sinf(x); }
  double __attribute__((overloadable)) tgsin(double x) { return sin(x); }
  long double __attribute__((overloadable)) tgsin(long double x) { return sinl(x); }

Given these declarations, one can call ``tgsin`` with a ``float`` value to
receive a ``float`` result, with a ``double`` to receive a ``double`` result,
etc. Function overloading in C follows the rules of C++ function overloading
to pick the best overload given the call arguments, with a few C-specific
semantics:

* Conversion from ``float`` or ``double`` to ``long double`` is ranked as a
  floating-point promotion (per C99) rather than as a floating-point conversion
  (as in C++).

* A conversion from a pointer of type ``T*`` to a pointer of type ``U*`` is
  considered a pointer conversion (with conversion rank) if ``T`` and ``U`` are
  compatible types.

* A conversion from type ``T`` to a value of type ``U`` is permitted if ``T``
  and ``U`` are compatible types. This conversion is given "conversion" rank.

* If no viable candidates are otherwise available, we allow a conversion from a
  pointer of type ``T*`` to a pointer of type ``U*``, where ``T`` and ``U`` are
  incompatible. This conversion is ranked below all other types of conversions.
  Please note: ``U`` lacking qualifiers that are present on ``T`` is sufficient
  for ``T`` and ``U`` to be incompatible.

The declaration of ``overloadable`` functions is restricted to function
declarations and definitions. If a function is marked with the ``overloadable``
attribute, then all declarations and definitions of functions with that name,
except for at most one (see the note below about unmarked overloads), must have
the ``overloadable`` attribute. In addition, redeclarations of a function with
the ``overloadable`` attribute must have the ``overloadable`` attribute, and
redeclarations of a function without the ``overloadable`` attribute must *not*
have the ``overloadable`` attribute. e.g.,

.. code-block:: c

  int f(int) __attribute__((overloadable));
  float f(float); // error: declaration of "f" must have the "overloadable" attribute
  int f(int); // error: redeclaration of "f" must have the "overloadable" attribute

  int g(int) __attribute__((overloadable));
  int g(int) { } // error: redeclaration of "g" must also have the "overloadable" attribute

  int h(int);
  int h(int) __attribute__((overloadable)); // error: declaration of "h" must not
                                            // have the "overloadable" attribute

Functions marked ``overloadable`` must have prototypes. Therefore, the
following code is ill-formed:

.. code-block:: c

  int h() __attribute__((overloadable)); // error: h does not have a prototype

However, ``overloadable`` functions are allowed to use a ellipsis even if there
are no named parameters (as is permitted in C++). This feature is particularly
useful when combined with the ``unavailable`` attribute:

.. code-block:: c++

  void honeypot(...) __attribute__((overloadable, unavailable)); // calling me is an error

Functions declared with the ``overloadable`` attribute have their names mangled
according to the same rules as C++ function names. For example, the three
``tgsin`` functions in our motivating example get the mangled names
``_Z5tgsinf``, ``_Z5tgsind``, and ``_Z5tgsine``, respectively. There are two
caveats to this use of name mangling:

* Future versions of Clang may change the name mangling of functions overloaded
  in C, so you should not depend on an specific mangling. To be completely
  safe, we strongly urge the use of ``static inline`` with ``overloadable``
  functions.

* The ``overloadable`` attribute has almost no meaning when used in C++,
  because names will already be mangled and functions are already overloadable.
  However, when an ``overloadable`` function occurs within an ``extern "C"``
  linkage specification, it's name *will* be mangled in the same way as it
  would in C.

For the purpose of backwards compatibility, at most one function with the same
name as other ``overloadable`` functions may omit the ``overloadable``
attribute. In this case, the function without the ``overloadable`` attribute
will not have its name mangled.

For example:

.. code-block:: c

  // Notes with mangled names assume Itanium mangling.
  int f(int);
  int f(double) __attribute__((overloadable));
  void foo() {
    f(5); // Emits a call to f (not _Z1fi, as it would with an overload that
          // was marked with overloadable).
    f(1.0); // Emits a call to _Z1fd.
  }

Support for unmarked overloads is not present in some versions of clang. You may
query for it using ``__has_extension(overloadable_unmarked)``.

Query for this attribute with ``__has_attribute(overloadable)``.
  }];
}

def ObjCMethodFamilyDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Many methods in Objective-C have conventional meanings determined by their
selectors. It is sometimes useful to be able to mark a method as having a
particular conventional meaning despite not having the right selector, or as
not having the conventional meaning that its selector would suggest. For these
use cases, we provide an attribute to specifically describe the "method family"
that a method belongs to.

**Usage**: ``__attribute__((objc_method_family(X)))``, where ``X`` is one of
``none``, ``alloc``, ``copy``, ``init``, ``mutableCopy``, or ``new``. This
attribute can only be placed at the end of a method declaration:

.. code-block:: objc

  - (NSString *)initMyStringValue __attribute__((objc_method_family(none)));

Users who do not wish to change the conventional meaning of a method, and who
merely want to document its non-standard retain and release semantics, should
use the retaining behavior attributes (``ns_returns_retained``,
``ns_returns_not_retained``, etc).

Query for this feature with ``__has_attribute(objc_method_family)``.
  }];
}

def RetainBehaviorDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The behavior of a function with respect to reference counting for Foundation
(Objective-C), CoreFoundation (C) and OSObject (C++) is determined by a naming
convention (e.g. functions starting with "get" are assumed to return at
``+0``).

It can be overridden using a family of the following attributes. In
Objective-C, the annotation ``__attribute__((ns_returns_retained))`` applied to
a function communicates that the object is returned at ``+1``, and the caller
is responsible for freeing it.
Similarly, the annotation ``__attribute__((ns_returns_not_retained))``
specifies that the object is returned at ``+0`` and the ownership remains with
the callee.
The annotation ``__attribute__((ns_consumes_self))`` specifies that
the Objective-C method call consumes the reference to ``self``, e.g. by
attaching it to a supplied parameter.
Additionally, parameters can have an annotation
``__attribute__((ns_consumed))``, which specifies that passing an owned object
as that parameter effectively transfers the ownership, and the caller is no
longer responsible for it.
These attributes affect code generation when interacting with ARC code, and
they are used by the Clang Static Analyzer.

In C programs using CoreFoundation, a similar set of attributes:
``__attribute__((cf_returns_not_retained))``,
``__attribute__((cf_returns_retained))`` and ``__attribute__((cf_consumed))``
have the same respective semantics when applied to CoreFoundation objects.
These attributes affect code generation when interacting with ARC code, and
they are used by the Clang Static Analyzer.

Finally, in C++ interacting with XNU kernel (objects inheriting from OSObject),
the same attribute family is present:
``__attribute__((os_returns_not_retained))``,
``__attribute__((os_returns_retained))`` and ``__attribute__((os_consumed))``,
with the same respective semantics.
Similar to ``__attribute__((ns_consumes_self))``,
``__attribute__((os_consumes_this))`` specifies that the method call consumes
the reference to "this" (e.g., when attaching it to a different object supplied
as a parameter).
Out parameters (parameters the function is meant to write into,
either via pointers-to-pointers or references-to-pointers)
may be annotated with ``__attribute__((os_returns_retained))``
or ``__attribute__((os_returns_not_retained))`` which specifies that the object
written into the out parameter should (or respectively should not) be released
after use.
Since often out parameters may or may not be written depending on the exit
code of the function,
annotations ``__attribute__((os_returns_retained_on_zero))``
and ``__attribute__((os_returns_retained_on_non_zero))`` specify that
an out parameter at ``+1`` is written if and only if the function returns a zero
(respectively non-zero) error code.
Observe that return-code-dependent out parameter annotations are only
available for retained out parameters, as non-retained object do not have to be
released by the callee.
These attributes are only used by the Clang Static Analyzer.

The family of attributes ``X_returns_X_retained`` can be added to functions,
C++ methods, and Objective-C methods and properties.
Attributes ``X_consumed`` can be added to parameters of methods, functions,
and Objective-C methods.
  }];
}

def NoDebugDocs : Documentation {
  let Category = DocCatVariable;
  let Content = [{
The ``nodebug`` attribute allows you to suppress debugging information for a
function or method, for a variable that is not a parameter or a non-static
data member, or for a typedef or using declaration.
  }];
}

def StandaloneDebugDocs : Documentation {
  let Category = DocCatVariable;
  let Content = [{
The ``standalone_debug`` attribute causes debug info to be emitted for a record
type regardless of the debug info optimizations that are enabled with
-fno-standalone-debug. This attribute only has an effect when debug info
optimizations are enabled (e.g. with -fno-standalone-debug), and is C++-only.
  }];
}

def NoDuplicateDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The ``noduplicate`` attribute can be placed on function declarations to control
whether function calls to this function can be duplicated or not as a result of
optimizations. This is required for the implementation of functions with
certain special requirements, like the OpenCL "barrier" function, that might
need to be run concurrently by all the threads that are executing in lockstep
on the hardware. For example this attribute applied on the function
"nodupfunc" in the code below avoids that:

.. code-block:: c

  void nodupfunc() __attribute__((noduplicate));
  // Setting it as a C++11 attribute is also valid
  // void nodupfunc() [[clang::noduplicate]];
  void foo();
  void bar();

  nodupfunc();
  if (a > n) {
    foo();
  } else {
    bar();
  }

gets possibly modified by some optimizations into code similar to this:

.. code-block:: c

  if (a > n) {
    nodupfunc();
    foo();
  } else {
    nodupfunc();
    bar();
  }

where the call to "nodupfunc" is duplicated and sunk into the two branches
of the condition.
  }];
}

def ConvergentDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The ``convergent`` attribute can be placed on a function declaration. It is
translated into the LLVM ``convergent`` attribute, which indicates that the call
instructions of a function with this attribute cannot be made control-dependent
on any additional values.

In languages designed for SPMD/SIMT programming model, e.g. OpenCL or CUDA,
the call instructions of a function with this attribute must be executed by
all work items or threads in a work group or sub group.

This attribute is different from ``noduplicate`` because it allows duplicating
function calls if it can be proved that the duplicated function calls are
not made control-dependent on any additional values, e.g., unrolling a loop
executed by all work items.

Sample usage:

.. code-block:: c

  void convfunc(void) __attribute__((convergent));
  // Setting it as a C++11 attribute is also valid in a C++ program.
  // void convfunc(void) [[clang::convergent]];

  }];
}

def NoSplitStackDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The ``no_split_stack`` attribute disables the emission of the split stack
preamble for a particular function. It has no effect if ``-fsplit-stack``
is not specified.
  }];
}

def NoUniqueAddressDocs : Documentation {
  let Category = DocCatField;
  let Content = [{
The ``no_unique_address`` attribute allows tail padding in a non-static data
member to overlap other members of the enclosing class (and in the special
case when the type is empty, permits it to fully overlap other members).
The field is laid out as if a base class were encountered at the corresponding
point within the class (except that it does not share a vptr with the enclosing
object).

Example usage:

.. code-block:: c++

  template<typename T, typename Alloc> struct my_vector {
    T *p;
    [[no_unique_address]] Alloc alloc;
    // ...
  };
  static_assert(sizeof(my_vector<int, std::allocator<int>>) == sizeof(int*));

``[[no_unique_address]]`` is a standard C++20 attribute. Clang supports its use
in C++11 onwards.
  }];
}

def ObjCRequiresSuperDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Some Objective-C classes allow a subclass to override a particular method in a
parent class but expect that the overriding method also calls the overridden
method in the parent class. For these cases, we provide an attribute to
designate that a method requires a "call to ``super``" in the overriding
method in the subclass.

**Usage**: ``__attribute__((objc_requires_super))``. This attribute can only
be placed at the end of a method declaration:

.. code-block:: objc

  - (void)foo __attribute__((objc_requires_super));

This attribute can only be applied the method declarations within a class, and
not a protocol. Currently this attribute does not enforce any placement of
where the call occurs in the overriding method (such as in the case of
``-dealloc`` where the call must appear at the end). It checks only that it
exists.

Note that on both OS X and iOS that the Foundation framework provides a
convenience macro ``NS_REQUIRES_SUPER`` that provides syntactic sugar for this
attribute:

.. code-block:: objc

  - (void)foo NS_REQUIRES_SUPER;

This macro is conditionally defined depending on the compiler's support for
this attribute. If the compiler does not support the attribute the macro
expands to nothing.

Operationally, when a method has this annotation the compiler will warn if the
implementation of an override in a subclass does not call super. For example:

.. code-block:: objc

   warning: method possibly missing a [super AnnotMeth] call
   - (void) AnnotMeth{};
                      ^
  }];
}

def ObjCRuntimeNameDocs : Documentation {
    let Category = DocCatDecl;
    let Content = [{
By default, the Objective-C interface or protocol identifier is used
in the metadata name for that object. The ``objc_runtime_name``
attribute allows annotated interfaces or protocols to use the
specified string argument in the object's metadata name instead of the
default name.

**Usage**: ``__attribute__((objc_runtime_name("MyLocalName")))``. This attribute
can only be placed before an @protocol or @interface declaration:

.. code-block:: objc

  __attribute__((objc_runtime_name("MyLocalName")))
  @interface Message
  @end

    }];
}

def ObjCRuntimeVisibleDocs : Documentation {
    let Category = DocCatDecl;
    let Content = [{
This attribute specifies that the Objective-C class to which it applies is
visible to the Objective-C runtime but not to the linker. Classes annotated
with this attribute cannot be subclassed and cannot have categories defined for
them.
    }];
}

def ObjCClassStubDocs : Documentation {
    let Category = DocCatType;
    let Content = [{
This attribute specifies that the Objective-C class to which it applies is
instantiated at runtime.

Unlike ``__attribute__((objc_runtime_visible))``, a class having this attribute
still has a "class stub" that is visible to the linker. This allows categories
to be defined. Static message sends with the class as a receiver use a special
access pattern to ensure the class is lazily instantiated from the class stub.

Classes annotated with this attribute cannot be subclassed and cannot have
implementations defined for them. This attribute is intended for use in
Swift-generated headers for classes defined in Swift.

Adding or removing this attribute to a class is an ABI-breaking change.
    }];
}

def ObjCBoxableDocs : Documentation {
    let Category = DocCatDecl;
    let Content = [{
Structs and unions marked with the ``objc_boxable`` attribute can be used
with the Objective-C boxed expression syntax, ``@(...)``.

**Usage**: ``__attribute__((objc_boxable))``. This attribute
can only be placed on a declaration of a trivially-copyable struct or union:

.. code-block:: objc

  struct __attribute__((objc_boxable)) some_struct {
    int i;
  };
  union __attribute__((objc_boxable)) some_union {
    int i;
    float f;
  };
  typedef struct __attribute__((objc_boxable)) _some_struct some_struct;

  // ...

  some_struct ss;
  NSValue *boxed = @(ss);

    }];
}

def AvailabilityDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The ``availability`` attribute can be placed on declarations to describe the
lifecycle of that declaration relative to operating system versions. Consider
the function declaration for a hypothetical function ``f``:

.. code-block:: c++

  void f(void) __attribute__((availability(macos,introduced=10.4,deprecated=10.6,obsoleted=10.7)));

The availability attribute states that ``f`` was introduced in macOS 10.4,
deprecated in macOS 10.6, and obsoleted in macOS 10.7. This information
is used by Clang to determine when it is safe to use ``f``: for example, if
Clang is instructed to compile code for macOS 10.5, a call to ``f()``
succeeds. If Clang is instructed to compile code for macOS 10.6, the call
succeeds but Clang emits a warning specifying that the function is deprecated.
Finally, if Clang is instructed to compile code for macOS 10.7, the call
fails because ``f()`` is no longer available.

The availability attribute is a comma-separated list starting with the
platform name and then including clauses specifying important milestones in the
declaration's lifetime (in any order) along with additional information. Those
clauses can be:

introduced=\ *version*
  The first version in which this declaration was introduced.

deprecated=\ *version*
  The first version in which this declaration was deprecated, meaning that
  users should migrate away from this API.

obsoleted=\ *version*
  The first version in which this declaration was obsoleted, meaning that it
  was removed completely and can no longer be used.

unavailable
  This declaration is never available on this platform.

message=\ *string-literal*
  Additional message text that Clang will provide when emitting a warning or
  error about use of a deprecated or obsoleted declaration. Useful to direct
  users to replacement APIs.

replacement=\ *string-literal*
  Additional message text that Clang will use to provide Fix-It when emitting
  a warning about use of a deprecated declaration. The Fix-It will replace
  the deprecated declaration with the new declaration specified.

Multiple availability attributes can be placed on a declaration, which may
correspond to different platforms. For most platforms, the availability
attribute with the platform corresponding to the target platform will be used;
any others will be ignored. However, the availability for ``watchOS`` and
``tvOS`` can be implicitly inferred from an ``iOS`` availability attribute.
Any explicit availability attributes for those platforms are still preferred over
the implicitly inferred availability attributes. If no availability attribute
specifies availability for the current target platform, the availability
attributes are ignored. Supported platforms are:

``ios``
  Apple's iOS operating system. The minimum deployment target is specified by
  the ``-mios-version-min=*version*`` or ``-miphoneos-version-min=*version*``
  command-line arguments.

``macos``
  Apple's macOS operating system. The minimum deployment target is
  specified by the ``-mmacosx-version-min=*version*`` command-line argument.
  ``macosx`` is supported for backward-compatibility reasons, but it is
  deprecated.

``tvos``
  Apple's tvOS operating system. The minimum deployment target is specified by
  the ``-mtvos-version-min=*version*`` command-line argument.

``watchos``
  Apple's watchOS operating system. The minimum deployment target is specified by
  the ``-mwatchos-version-min=*version*`` command-line argument.
  
``driverkit``
  Apple's DriverKit userspace kernel extensions. The minimum deployment target
  is specified as part of the triple.

A declaration can typically be used even when deploying back to a platform
version prior to when the declaration was introduced. When this happens, the
declaration is `weakly linked
<https://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html>`_,
as if the ``weak_import`` attribute were added to the declaration. A
weakly-linked declaration may or may not be present a run-time, and a program
can determine whether the declaration is present by checking whether the
address of that declaration is non-NULL.

The flag ``strict`` disallows using API when deploying back to a
platform version prior to when the declaration was introduced. An
attempt to use such API before its introduction causes a hard error.
Weakly-linking is almost always a better API choice, since it allows
users to query availability at runtime.

If there are multiple declarations of the same entity, the availability
attributes must either match on a per-platform basis or later
declarations must not have availability attributes for that
platform. For example:

.. code-block:: c

  void g(void) __attribute__((availability(macos,introduced=10.4)));
  void g(void) __attribute__((availability(macos,introduced=10.4))); // okay, matches
  void g(void) __attribute__((availability(ios,introduced=4.0))); // okay, adds a new platform
  void g(void); // okay, inherits both macos and ios availability from above.
  void g(void) __attribute__((availability(macos,introduced=10.5))); // error: mismatch

When one method overrides another, the overriding method can be more widely available than the overridden method, e.g.,:

.. code-block:: objc

  @interface A
  - (id)method __attribute__((availability(macos,introduced=10.4)));
  - (id)method2 __attribute__((availability(macos,introduced=10.4)));
  @end

  @interface B : A
  - (id)method __attribute__((availability(macos,introduced=10.3))); // okay: method moved into base class later
  - (id)method __attribute__((availability(macos,introduced=10.5))); // error: this method was available via the base class in 10.4
  @end

Starting with the macOS 10.12 SDK, the ``API_AVAILABLE`` macro from
``<os/availability.h>`` can simplify the spelling:

.. code-block:: objc

  @interface A
  - (id)method API_AVAILABLE(macos(10.11)));
  - (id)otherMethod API_AVAILABLE(macos(10.11), ios(11.0));
  @end

Availability attributes can also be applied using a ``#pragma clang attribute``.
Any explicit availability attribute whose platform corresponds to the target
platform is applied to a declaration regardless of the availability attributes
specified in the pragma. For example, in the code below,
``hasExplicitAvailabilityAttribute`` will use the ``macOS`` availability
attribute that is specified with the declaration, whereas
``getsThePragmaAvailabilityAttribute`` will use the ``macOS`` availability
attribute that is applied by the pragma.

.. code-block:: c

  #pragma clang attribute push (__attribute__((availability(macOS, introduced=10.12))), apply_to=function)
  void getsThePragmaAvailabilityAttribute(void);
  void hasExplicitAvailabilityAttribute(void) __attribute__((availability(macos,introduced=10.4)));
  #pragma clang attribute pop

For platforms like ``watchOS`` and ``tvOS``, whose availability attributes can
be implicitly inferred from an ``iOS`` availability attribute, the logic is
slightly more complex. The explicit and the pragma-applied availability
attributes whose platform corresponds to the target platform are applied as
described in the previous paragraph. However, the implicitly inferred attributes
are applied to a declaration only when there is no explicit or pragma-applied
availability attribute whose platform corresponds to the target platform. For
example, the function below will receive the ``tvOS`` availability from the
pragma rather than using the inferred ``iOS`` availability from the declaration:

.. code-block:: c

  #pragma clang attribute push (__attribute__((availability(tvOS, introduced=12.0))), apply_to=function)
  void getsThePragmaTVOSAvailabilityAttribute(void) __attribute__((availability(iOS,introduced=11.0)));
  #pragma clang attribute pop

The compiler is also able to apply implicitly inferred attributes from a pragma
as well. For example, when targeting ``tvOS``, the function below will receive
a ``tvOS`` availability attribute that is implicitly inferred from the ``iOS``
availability attribute applied by the pragma:

.. code-block:: c

  #pragma clang attribute push (__attribute__((availability(iOS, introduced=12.0))), apply_to=function)
  void infersTVOSAvailabilityFromPragma(void);
  #pragma clang attribute pop

The implicit attributes that are inferred from explicitly specified attributes
whose platform corresponds to the target platform are applied to the declaration
even if there is an availability attribute that can be inferred from a pragma.
For example, the function below will receive the ``tvOS, introduced=11.0``
availability that is inferred from the attribute on the declaration rather than
inferring availability from the pragma:

.. code-block:: c

  #pragma clang attribute push (__attribute__((availability(iOS, unavailable))), apply_to=function)
  void infersTVOSAvailabilityFromAttributeNextToDeclaration(void)
    __attribute__((availability(iOS,introduced=11.0)));
  #pragma clang attribute pop

Also see the documentation for `@available
<http://clang.llvm.org/docs/LanguageExtensions.html#objective-c-available>`_
  }];
}

def ExternalSourceSymbolDocs : Documentation {
  let Category = DocCatDecl;
  let Content = [{
The ``external_source_symbol`` attribute specifies that a declaration originates
from an external source and describes the nature of that source.

The fact that Clang is capable of recognizing declarations that were defined
externally can be used to provide better tooling support for mixed-language
projects or projects that rely on auto-generated code. For instance, an IDE that
uses Clang and that supports mixed-language projects can use this attribute to
provide a correct 'jump-to-definition' feature. For a concrete example,
consider a protocol that's defined in a Swift file:

.. code-block:: swift

  @objc public protocol SwiftProtocol {
    func method()
  }

This protocol can be used from Objective-C code by including a header file that
was generated by the Swift compiler. The declarations in that header can use
the ``external_source_symbol`` attribute to make Clang aware of the fact
that ``SwiftProtocol`` actually originates from a Swift module:

.. code-block:: objc

  __attribute__((external_source_symbol(language="Swift",defined_in="module")))
  @protocol SwiftProtocol
  @required
  - (void) method;
  @end

Consequently, when 'jump-to-definition' is performed at a location that
references ``SwiftProtocol``, the IDE can jump to the original definition in
the Swift source file rather than jumping to the Objective-C declaration in the
auto-generated header file.

The ``external_source_symbol`` attribute is a comma-separated list that includes
clauses that describe the origin and the nature of the particular declaration.
Those clauses can be:

language=\ *string-literal*
  The name of the source language in which this declaration was defined.

defined_in=\ *string-literal*
  The name of the source container in which the declaration was defined. The
  exact definition of source container is language-specific, e.g. Swift's
  source containers are modules, so ``defined_in`` should specify the Swift
  module name.

USR=\ *string-literal*
  String that specifies a unified symbol resolution (USR) value for this
  declaration. USR string uniquely identifies this particular declaration, and
  is typically used when constructing an index of a codebase.
  The USR value in this attribute is expected to be generated by an external
  compiler that compiled the native declaration using its original source
  language. The exact format of the USR string and its other attributes
  are determined by the specification of this declaration's source language.
  When not specified, Clang's indexer will use the Clang USR for this symbol.
  User can query to see if Clang supports the use of the ``USR`` clause in
  the ``external_source_symbol`` attribute with
  ``__has_attribute(external_source_symbol) >= 20230206``.

generated_declaration
  This declaration was automatically generated by some tool.

The clauses can be specified in any order. The clauses that are listed above are
all optional, but the attribute has to have at least one clause.
  }];
}

def ConstInitDocs : Documentation {
  let Category = DocCatVariable;
  let Heading = "require_constant_initialization, constinit (C++20)";
  let Content = [{
This attribute specifies that the variable to which it is attached is intended
to have a `constant initializer <http://en.cppreference.com/w/cpp/language/constant_initialization>`_
according to the rules of [basic.start.static]. The variable is required to
have static or thread storage duration. If the initialization of the variable
is not a constant initializer an error will be produced. This attribute may
only be used in C++; the ``constinit`` spelling is only accepted in C++20
onwards.

Note that in C++03 strict constant expression checking is not done. Instead
the attribute reports if Clang can emit the variable as a constant, even if it's
not technically a 'constant initializer'. This behavior is non-portable.

Static storage duration variables with constant initializers avoid hard-to-find
bugs caused by the indeterminate order of dynamic initialization. They can also
be safely used during dynamic initialization across translation units.

This attribute acts as a compile time assertion that the requirements
for constant initialization have been met. Since these requirements change
between dialects and have subtle pitfalls it's important to fail fast instead
of silently falling back on dynamic initialization.

The first use of the attribute on a variable must be part of, or precede, the
initializing declaration of the variable. C++20 requires the ``constinit``
spelling of the attribute to be present on the initializing declaration if it
is used anywhere. The other spellings can be specified on a forward declaration
and omitted on a later initializing declaration.

.. code-block:: c++

  // -std=c++14
  #define SAFE_STATIC [[clang::require_constant_initialization]]
  struct T {
    constexpr T(int) {}
    ~T(); // non-trivial
  };
  SAFE_STATIC T x = {42}; // Initialization OK. Doesn't check destructor.
  SAFE_STATIC T y = 42; // error: variable does not have a constant initializer
  // copy initialization is not a constant expression on a non-literal type.
  }];
}

def WarnMaybeUnusedDocs : Documentation {
  let Category = DocCatVariable;
  let Heading = "maybe_unused, unused";
  let Content = [{
When passing the ``-Wunused`` flag to Clang, entities that are unused by the
program may be diagnosed. The ``[[maybe_unused]]`` (or
``__attribute__((unused))``) attribute can be used to silence such diagnostics
when the entity cannot be removed. For instance, a local variable may exist
solely for use in an ``assert()`` statement, which makes the local variable
unused when ``NDEBUG`` is defined.

The attribute may be applied to the declaration of a class, a typedef, a
variable, a function or method, a function parameter, an enumeration, an
enumerator, a non-static data member, or a label.

.. code-block: c++
  #include <cassert>

  [[maybe_unused]] void f([[maybe_unused]] bool thing1,
                          [[maybe_unused]] bool thing2) {
    [[maybe_unused]] bool b = thing1 && thing2;
    assert(b);
  }
  }];
}

def WarnUnusedResultsDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "nodiscard, warn_unused_result";
  let Content  = [{
Clang supports the ability to diagnose when the results of a function call
expression are discarded under suspicious circumstances. A diagnostic is
generated when a function or its return type is marked with ``[[nodiscard]]``
(or ``__attribute__((warn_unused_result))``) and the function call appears as a
potentially-evaluated discarded-value expression that is not explicitly cast to
``void``.

A string literal may optionally be provided to the attribute, which will be
reproduced in any resulting diagnostics. Redeclarations using different forms
of the attribute (with or without the string literal or with different string
literal contents) are allowed. If there are redeclarations of the entity with
differing string literals, it is unspecified which one will be used by Clang
in any resulting diagnostics.

.. code-block: c++
  struct [[nodiscard]] error_info { /*...*/ };
  error_info enable_missile_safety_mode();

  void launch_missiles();
  void test_missiles() {
    enable_missile_safety_mode(); // diagnoses
    launch_missiles();
  }
  error_info &foo();
  void f() { foo(); } // Does not diagnose, error_info is a reference.

Additionally, discarded temporaries resulting from a call to a constructor
marked with ``[[nodiscard]]`` or a constructor of a type marked
``[[nodiscard]]`` will also diagnose. This also applies to type conversions that
use the annotated ``[[nodiscard]]`` constructor or result in an annotated type.

.. code-block: c++
  struct [[nodiscard]] marked_type {/*..*/ };
  struct marked_ctor {
    [[nodiscard]] marked_ctor();
    marked_ctor(int);
  };

  struct S {
    operator marked_type() const;
    [[nodiscard]] operator int() const;
  };

  void usages() {
    marked_type(); // diagnoses.
    marked_ctor(); // diagnoses.
    marked_ctor(3); // Does not diagnose, int constructor isn't marked nodiscard.

    S s;
    static_cast<marked_type>(s); // diagnoses
    (int)s; // diagnoses
  }
  }];
}

def FallthroughDocs : Documentation {
  let Category = DocCatStmt;
  let Heading = "fallthrough";
  let Content = [{
The ``fallthrough`` (or ``clang::fallthrough``) attribute is used
to annotate intentional fall-through
between switch labels. It can only be applied to a null statement placed at a
point of execution between any statement and the next switch label. It is
common to mark these places with a specific comment, but this attribute is
meant to replace comments with a more strict annotation, which can be checked
by the compiler. This attribute doesn't change semantics of the code and can
be used wherever an intended fall-through occurs. It is designed to mimic
control-flow statements like ``break;``, so it can be placed in most places
where ``break;`` can, but only if there are no statements on the execution path
between it and the next switch label.

By default, Clang does not warn on unannotated fallthrough from one ``switch``
case to another. Diagnostics on fallthrough without a corresponding annotation
can be enabled with the ``-Wimplicit-fallthrough`` argument.

Here is an example:

.. code-block:: c++

  // compile with -Wimplicit-fallthrough
  switch (n) {
  case 22:
  case 33:  // no warning: no statements between case labels
    f();
  case 44:  // warning: unannotated fall-through
    g();
    [[clang::fallthrough]];
  case 55:  // no warning
    if (x) {
      h();
      break;
    }
    else {
      i();
      [[clang::fallthrough]];
    }
  case 66:  // no warning
    p();
    [[clang::fallthrough]]; // warning: fallthrough annotation does not
                            //          directly precede case label
    q();
  case 77:  // warning: unannotated fall-through
    r();
  }
  }];
}

def LikelihoodDocs : Documentation {
  let Category = DocCatStmt;
  let Heading = "likely and unlikely";
  let Content = [{
The ``likely`` and ``unlikely`` attributes are used as compiler hints.
The attributes are used to aid the compiler to determine which branch is
likely or unlikely to be taken. This is done by marking the branch substatement
with one of the two attributes.

It isn't allowed to annotate a single statement with both ``likely`` and
``unlikely``. Annotating the ``true`` and ``false`` branch of an ``if``
statement with the same likelihood attribute will result in a diagnostic and
the attributes are ignored on both branches.

In a ``switch`` statement it's allowed to annotate multiple ``case`` labels
or the ``default`` label with the same likelihood attribute. This makes
* all labels without an attribute have a neutral likelihood,
* all labels marked ``[[likely]]`` have an equally positive likelihood, and
* all labels marked ``[[unlikely]]`` have an equally negative likelihood.
The neutral likelihood is the more likely of path execution than the negative
likelihood. The positive likelihood is the more likely of path of execution
than the neutral likelihood.

These attributes have no effect on the generated code when using
PGO (Profile-Guided Optimization) or at optimization level 0.

In Clang, the attributes will be ignored if they're not placed on
* the ``case`` or ``default`` label of a ``switch`` statement,
* or on the substatement of an ``if`` or ``else`` statement,
* or on the substatement of an ``for`` or ``while`` statement.
The C++ Standard recommends to honor them on every statement in the
path of execution, but that can be confusing:

.. code-block:: c++

  if (b) {
    [[unlikely]] --b; // In the path of execution,
                      // this branch is considered unlikely.
  }

  if (b) {
    --b;
    if(b)
      return;
    [[unlikely]] --b; // Not in the path of execution,
  }                   // the branch has no likelihood information.

  if (b) {
    --b;
    foo(b);
    // Whether or not the next statement is in the path of execution depends
    // on the declaration of foo():
    // In the path of execution: void foo(int);
    // Not in the path of execution: [[noreturn]] void foo(int);
    // This means the likelihood of the branch depends on the declaration
    // of foo().
    [[unlikely]] --b;
  }


Below are some example usages of the likelihood attributes and their effects:

.. code-block:: c++

  if (b) [[likely]] { // Placement on the first statement in the branch.
    // The compiler will optimize to execute the code here.
  } else {
  }

  if (b)
    [[unlikely]] b++; // Placement on the first statement in the branch.
  else {
    // The compiler will optimize to execute the code here.
  }

  if (b) {
    [[unlikely]] b++; // Placement on the second statement in the branch.
  }                   // The attribute will be ignored.

  if (b) [[likely]] {
    [[unlikely]] b++; // No contradiction since the second attribute
  }                   // is ignored.

  if (b)
    ;
  else [[likely]] {
    // The compiler will optimize to execute the code here.
  }

  if (b)
    ;
  else
    // The compiler will optimize to execute the next statement.
    [[likely]] b = f();

  if (b) [[likely]]; // Both branches are likely. A diagnostic is issued
  else [[likely]];   // and the attributes are ignored.

  if (b)
    [[likely]] int i = 5; // Issues a diagnostic since the attribute
                          // isn't allowed on a declaration.

  switch (i) {
    [[likely]] case 1:    // This value is likely
      ...
      break;

    [[unlikely]] case 2:  // This value is unlikely
      ...
      [[fallthrough]];

    case 3:               // No likelihood attribute
      ...
      [[likely]] break;   // No effect

    case 4: [[likely]] {  // attribute on substatement has no effect
      ...
      break;
      }

    [[unlikely]] default: // All other values are unlikely
      ...
      break;
  }

  switch (i) {
    [[likely]] case 0:    // This value and code path is likely
      ...
      [[fallthrough]];

    case 1:               // No likelihood attribute, code path is neutral
      break;              // falling through has no effect on the likelihood

    case 2:               // No likelihood attribute, code path is neutral
      [[fallthrough]];

    [[unlikely]] default: // This value and code path are both unlikely
      break;
  }

  for(int i = 0; i != size; ++i) [[likely]] {
    ...               // The loop is the likely path of execution
  }

  for(const auto &E : Elements) [[likely]] {
    ...               // The loop is the likely path of execution
  }

  while(i != size) [[unlikely]] {
    ...               // The loop is the unlikely path of execution
  }                   // The generated code will optimize to skip the loop body

  while(true) [[unlikely]] {
    ...               // The attribute has no effect
  }                   // Clang elides the comparison and generates an infinite
                      // loop

  }];
}

def ARMInterruptDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "interrupt (ARM)";
  let Content = [{
Clang supports the GNU style ``__attribute__((interrupt("TYPE")))`` attribute on
ARM targets. This attribute may be attached to a function definition and
instructs the backend to generate appropriate function entry/exit code so that
it can be used directly as an interrupt service routine.

The parameter passed to the interrupt attribute is optional, but if
provided it must be a string literal with one of the following values: "IRQ",
"FIQ", "SWI", "ABORT", "UNDEF".

The semantics are as follows:

- If the function is AAPCS, Clang instructs the backend to realign the stack to
  8 bytes on entry. This is a general requirement of the AAPCS at public
  interfaces, but may not hold when an exception is taken. Doing this allows
  other AAPCS functions to be called.
- If the CPU is M-class this is all that needs to be done since the architecture
  itself is designed in such a way that functions obeying the normal AAPCS ABI
  constraints are valid exception handlers.
- If the CPU is not M-class, the prologue and epilogue are modified to save all
  non-banked registers that are used, so that upon return the user-mode state
  will not be corrupted. Note that to avoid unnecessary overhead, only
  general-purpose (integer) registers are saved in this way. If VFP operations
  are needed, that state must be saved manually.

  Specifically, interrupt kinds other than "FIQ" will save all core registers
  except "lr" and "sp". "FIQ" interrupts will save r0-r7.
- If the CPU is not M-class, the return instruction is changed to one of the
  canonical sequences permitted by the architecture for exception return. Where
  possible the function itself will make the necessary "lr" adjustments so that
  the "preferred return address" is selected.

  Unfortunately the compiler is unable to make this guarantee for an "UNDEF"
  handler, where the offset from "lr" to the preferred return address depends on
  the execution state of the code which generated the exception. In this case
  a sequence equivalent to "movs pc, lr" will be used.
  }];
}

def BPFPreserveAccessIndexDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Clang supports the ``__attribute__((preserve_access_index))``
attribute for the BPF target. This attribute may be attached to a
struct or union declaration, where if -g is specified, it enables
preserving struct or union member access debuginfo indices of this
struct or union, similar to clang ``__builtin_preserve_access_index()``.
  }];
}
def BTFDeclTagDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Clang supports the ``__attribute__((btf_decl_tag("ARGUMENT")))`` attribute for
all targets. This attribute may be attached to a struct/union, struct/union
field, function, function parameter, variable or typedef declaration. If -g is
specified, the ``ARGUMENT`` info will be preserved in IR and be emitted to
dwarf. For BPF targets, the ``ARGUMENT`` info will be emitted to .BTF ELF
section too.
  }];
}

def BTFTypeTagDocs : Documentation {
  let Category = DocCatType;
  let Content = [{
Clang supports the ``__attribute__((btf_type_tag("ARGUMENT")))`` attribute for
all targets. It only has effect when ``-g`` is specified on the command line and
is currently silently ignored when not applied to a pointer type (note: this
scenario may be diagnosed in the future).

The ``ARGUMENT`` string will be preserved in IR and emitted to DWARF for the
types used in variable declarations, function declarations, or typedef
declarations.

For BPF targets, the ``ARGUMENT`` string will also be emitted to .BTF ELF
section.
  }];
}

def MipsInterruptDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "interrupt (MIPS)";
  let Content = [{
Clang supports the GNU style ``__attribute__((interrupt("ARGUMENT")))`` attribute on
MIPS targets. This attribute may be attached to a function definition and instructs
the backend to generate appropriate function entry/exit code so that it can be used
directly as an interrupt service routine.

By default, the compiler will produce a function prologue and epilogue suitable for
an interrupt service routine that handles an External Interrupt Controller (eic)
generated interrupt. This behavior can be explicitly requested with the "eic"
argument.

Otherwise, for use with vectored interrupt mode, the argument passed should be
of the form "vector=LEVEL" where LEVEL is one of the following values:
"sw0", "sw1", "hw0", "hw1", "hw2", "hw3", "hw4", "hw5". The compiler will
then set the interrupt mask to the corresponding level which will mask all
interrupts up to and including the argument.

The semantics are as follows:

- The prologue is modified so that the Exception Program Counter (EPC) and
  Status coprocessor registers are saved to the stack. The interrupt mask is
  set so that the function can only be interrupted by a higher priority
  interrupt. The epilogue will restore the previous values of EPC and Status.

- The prologue and epilogue are modified to save and restore all non-kernel
  registers as necessary.

- The FPU is disabled in the prologue, as the floating pointer registers are not
  spilled to the stack.

- The function return sequence is changed to use an exception return instruction.

- The parameter sets the interrupt mask for the function corresponding to the
  interrupt level specified. If no mask is specified the interrupt mask
  defaults to "eic".
  }];
}

def MicroMipsDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Clang supports the GNU style ``__attribute__((micromips))`` and
``__attribute__((nomicromips))`` attributes on MIPS targets. These attributes
may be attached to a function definition and instructs the backend to generate
or not to generate microMIPS code for that function.

These attributes override the ``-mmicromips`` and ``-mno-micromips`` options
on the command line.
  }];
}

def MipsLongCallStyleDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "long_call, far";
  let Content = [{
Clang supports the ``__attribute__((long_call))``, ``__attribute__((far))``,
and ``__attribute__((near))`` attributes on MIPS targets. These attributes may
only be added to function declarations and change the code generated
by the compiler when directly calling the function. The ``near`` attribute
allows calls to the function to be made using the ``jal`` instruction, which
requires the function to be located in the same naturally aligned 256MB
segment as the caller. The ``long_call`` and ``far`` attributes are synonyms
and require the use of a different call sequence that works regardless
of the distance between the functions.

These attributes have no effect for position-independent code.

These attributes take priority over command line switches such
as ``-mlong-calls`` and ``-mno-long-calls``.
  }];
}

def MipsShortCallStyleDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "short_call, near";
  let Content = [{
Clang supports the ``__attribute__((long_call))``, ``__attribute__((far))``,
``__attribute__((short__call))``, and ``__attribute__((near))`` attributes
on MIPS targets. These attributes may only be added to function declarations
and change the code generated by the compiler when directly calling
the function. The ``short_call`` and ``near`` attributes are synonyms and
allow calls to the function to be made using the ``jal`` instruction, which
requires the function to be located in the same naturally aligned 256MB segment
as the caller. The ``long_call`` and ``far`` attributes are synonyms and
require the use of a different call sequence that works regardless
of the distance between the functions.

These attributes have no effect for position-independent code.

These attributes take priority over command line switches such
as ``-mlong-calls`` and ``-mno-long-calls``.
  }];
}

def RISCVInterruptDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "interrupt (RISC-V)";
  let Content = [{
Clang supports the GNU style ``__attribute__((interrupt))`` attribute on RISCV
targets. This attribute may be attached to a function definition and instructs
the backend to generate appropriate function entry/exit code so that it can be
used directly as an interrupt service routine.

Permissible values for this parameter are ``user``, ``supervisor``,
and ``machine``. If there is no parameter, then it defaults to machine.

Repeated interrupt attribute on the same declaration will cause a warning
to be emitted. In case of repeated declarations, the last one prevails.

Refer to:
https://gcc.gnu.org/onlinedocs/gcc/RISC-V-Function-Attributes.html
https://riscv.org/specifications/privileged-isa/
The RISC-V Instruction Set Manual Volume II: Privileged Architecture
Version 1.10.
  }];
}

def RISCVRVVVectorBitsDocs : Documentation {
  let Category = DocCatType;
  let Content = [{
On RISC-V targets, the ``riscv_rvv_vector_bits(N)`` attribute is used to define
fixed-length variants of sizeless types.

For example:

.. code-block:: c

  #include <riscv_vector.h>

  #if defined(__riscv_v_fixed_vlen)
  typedef vint8m1_t fixed_vint8m1_t __attribute__((riscv_rvv_vector_bits(__riscv_v_fixed_vlen)));
  #endif

Creates a type ``fixed_vint8m1_t_t`` that is a fixed-length variant of
``vint8m1_t`` that contains exactly 512 bits. Unlike ``vint8m1_t``, this type
can be used in globals, structs, unions, and arrays, all of which are
unsupported for sizeless types.

The attribute can be attached to a single RVV vector (such as ``vint8m1_t``).
The attribute will be rejected unless
``N==(__riscv_v_fixed_vlen*LMUL)``, the implementation defined feature macro that
is enabled under the ``-mrvv-vector-bits`` flag. ``__riscv_v_fixed_vlen`` can
only be a power of 2 between 64 and 65536.

For types where LMUL!=1, ``__riscv_v_fixed_vlen`` needs to be scaled by the LMUL
of the type before passing to the attribute.

``vbool*_t`` types are not supported at this time.
}];
}

def AVRInterruptDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "interrupt (AVR)";
  let Content = [{
Clang supports the GNU style ``__attribute__((interrupt))`` attribute on
AVR targets. This attribute may be attached to a function definition and instructs
the backend to generate appropriate function entry/exit code so that it can be used
directly as an interrupt service routine.

On the AVR, the hardware globally disables interrupts when an interrupt is executed.
The first instruction of an interrupt handler declared with this attribute is a SEI
instruction to re-enable interrupts. See also the signal attribute that
does not insert a SEI instruction.
  }];
}

def AVRSignalDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Clang supports the GNU style ``__attribute__((signal))`` attribute on
AVR targets. This attribute may be attached to a function definition and instructs
the backend to generate appropriate function entry/exit code so that it can be used
directly as an interrupt service routine.

Interrupt handler functions defined with the signal attribute do not re-enable interrupts.
}];
}

def TargetDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Clang supports the GNU style ``__attribute__((target("OPTIONS")))`` attribute.
This attribute may be attached to a function definition and instructs
the backend to use different code generation options than were passed on the
command line.

The current set of options correspond to the existing "subtarget features" for
the target with or without a "-mno-" in front corresponding to the absence
of the feature, as well as ``arch="CPU"`` which will change the default "CPU"
for the function.

For X86, the attribute also allows ``tune="CPU"`` to optimize the generated
code for the given CPU without changing the available instructions.

For AArch64, ``arch="Arch"`` will set the architecture, similar to the -march
command line options. ``cpu="CPU"`` can be used to select a specific cpu,
as per the ``-mcpu`` option, similarly for ``tune=``. The attribute also allows the
"branch-protection=<args>" option, where the permissible arguments and their
effect on code generation are the same as for the command-line option
``-mbranch-protection``.

Example "subtarget features" from the x86 backend include: "mmx", "sse", "sse4.2",
"avx", "xop" and largely correspond to the machine specific options handled by
the front end.

Additionally, this attribute supports function multiversioning for ELF based
x86/x86-64 targets, which can be used to create multiple implementations of the
same function that will be resolved at runtime based on the priority of their
``target`` attribute strings. A function is considered a multiversioned function
if either two declarations of the function have different ``target`` attribute
strings, or if it has a ``target`` attribute string of ``default``. For
example:

  .. code-block:: c++

    __attribute__((target("arch=atom")))
    void foo() {} // will be called on 'atom' processors.
    __attribute__((target("default")))
    void foo() {} // will be called on any other processors.

All multiversioned functions must contain a ``default`` (fallback)
implementation, otherwise usages of the function are considered invalid.
Additionally, a function may not become multiversioned after its first use.
}];
}

def TargetVersionDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
For AArch64 target clang supports function multiversioning by
``__attribute__((target_version("OPTIONS")))`` attribute. When applied to a
function it instructs compiler to emit multiple function versions based on
``target_version`` attribute strings, which resolved at runtime depend on their
priority and target features availability. One of the versions is always
( implicitly or explicitly ) the ``default`` (fallback). Attribute strings can
contain dependent features names joined by the "+" sign.
}];
}

def TargetClonesDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Clang supports the ``target_clones("OPTIONS")`` attribute. This attribute may be
attached to a function declaration and causes function multiversioning, where
multiple versions of the function will be emitted with different code
generation options.  Additionally, these versions will be resolved at runtime
based on the priority of their attribute options. All ``target_clone`` functions
are considered multiversioned functions.

For AArch64 target:
The attribute contains comma-separated strings of target features joined by "+"
sign. For example:

  .. code-block:: c++

    __attribute__((target_clones("sha2+memtag2", "fcma+sve2-pmull128")))
    void foo() {}

For every multiversioned function a ``default`` (fallback) implementation
always generated if not specified directly.

For x86/x86-64 targets:
All multiversioned functions must contain a ``default`` (fallback)
implementation, otherwise usages of the function are considered invalid.
Additionally, a function may not become multiversioned after its first use.

The options to ``target_clones`` can either be a target-specific architecture
(specified as ``arch=CPU``), or one of a list of subtarget features.

Example "subtarget features" from the x86 backend include: "mmx", "sse", "sse4.2",
"avx", "xop" and largely correspond to the machine specific options handled by
the front end.

The versions can either be listed as a comma-separated sequence of string
literals or as a single string literal containing a comma-separated list of
versions.  For compatibility with GCC, the two formats can be mixed.  For
example, the following will emit 4 versions of the function:

  .. code-block:: c++

    __attribute__((target_clones("arch=atom,avx2","arch=ivybridge","default")))
    void foo() {}

}];
}

def MinVectorWidthDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Clang supports the ``__attribute__((min_vector_width(width)))`` attribute. This
attribute may be attached to a function and informs the backend that this
function desires vectors of at least this width to be generated. Target-specific
maximum vector widths still apply. This means even if you ask for something
larger than the target supports, you will only get what the target supports.
This attribute is meant to be a hint to control target heuristics that may
generate narrower vectors than what the target hardware supports.

This is currently used by the X86 target to allow some CPUs that support 512-bit
vectors to be limited to using 256-bit vectors to avoid frequency penalties.
This is currently enabled with the ``-prefer-vector-width=256`` command line
option. The ``min_vector_width`` attribute can be used to prevent the backend
from trying to split vector operations to match the ``prefer-vector-width``. All
X86 vector intrinsics from x86intrin.h already set this attribute. Additionally,
use of any of the X86-specific vector builtins will implicitly set this
attribute on the calling function. The intent is that explicitly writing vector
code using the X86 intrinsics will prevent ``prefer-vector-width`` from
affecting the code.
}];
}

def DocCatAMDGPUAttributes : DocumentationCategory<"AMD GPU Attributes">;

def AMDGPUFlatWorkGroupSizeDocs : Documentation {
  let Category = DocCatAMDGPUAttributes;
  let Content = [{
The flat work-group size is the number of work-items in the work-group size
specified when the kernel is dispatched. It is the product of the sizes of the
x, y, and z dimension of the work-group.

Clang supports the
``__attribute__((amdgpu_flat_work_group_size(<min>, <max>)))`` attribute for the
AMDGPU target. This attribute may be attached to a kernel function definition
and is an optimization hint.

``<min>`` parameter specifies the minimum flat work-group size, and ``<max>``
parameter specifies the maximum flat work-group size (must be greater than
``<min>``) to which all dispatches of the kernel will conform. Passing ``0, 0``
as ``<min>, <max>`` implies the default behavior (``128, 256``).

If specified, the AMDGPU target backend might be able to produce better machine
code for barriers and perform scratch promotion by estimating available group
segment size.

An error will be given if:
  - Specified values violate subtarget specifications;
  - Specified values are not compatible with values provided through other
    attributes.
  }];
}

def AMDGPUWavesPerEUDocs : Documentation {
  let Category = DocCatAMDGPUAttributes;
  let Content = [{
A compute unit (CU) is responsible for executing the wavefronts of a work-group.
It is composed of one or more execution units (EU), which are responsible for
executing the wavefronts. An EU can have enough resources to maintain the state
of more than one executing wavefront. This allows an EU to hide latency by
switching between wavefronts in a similar way to symmetric multithreading on a
CPU. In order to allow the state for multiple wavefronts to fit on an EU, the
resources used by a single wavefront have to be limited. For example, the number
of SGPRs and VGPRs. Limiting such resources can allow greater latency hiding,
but can result in having to spill some register state to memory.

Clang supports the ``__attribute__((amdgpu_waves_per_eu(<min>[, <max>])))``
attribute for the AMDGPU target. This attribute may be attached to a kernel
function definition and is an optimization hint.

``<min>`` parameter specifies the requested minimum number of waves per EU, and
*optional* ``<max>`` parameter specifies the requested maximum number of waves
per EU (must be greater than ``<min>`` if specified). If ``<max>`` is omitted,
then there is no restriction on the maximum number of waves per EU other than
the one dictated by the hardware for which the kernel is compiled. Passing
``0, 0`` as ``<min>, <max>`` implies the default behavior (no limits).

If specified, this attribute allows an advanced developer to tune the number of
wavefronts that are capable of fitting within the resources of an EU. The AMDGPU
target backend can use this information to limit resources, such as number of
SGPRs, number of VGPRs, size of available group and private memory segments, in
such a way that guarantees that at least ``<min>`` wavefronts and at most
``<max>`` wavefronts are able to fit within the resources of an EU. Requesting
more wavefronts can hide memory latency but limits available registers which
can result in spilling. Requesting fewer wavefronts can help reduce cache
thrashing, but can reduce memory latency hiding.

This attribute controls the machine code generated by the AMDGPU target backend
to ensure it is capable of meeting the requested values. However, when the
kernel is executed, there may be other reasons that prevent meeting the request,
for example, there may be wavefronts from other kernels executing on the EU.

An error will be given if:
  - Specified values violate subtarget specifications;
  - Specified values are not compatible with values provided through other
    attributes;
  - The AMDGPU target backend is unable to create machine code that can meet the
    request.
  }];
}

def AMDGPUNumSGPRNumVGPRDocs : Documentation {
  let Category = DocCatAMDGPUAttributes;
  let Content = [{
Clang supports the ``__attribute__((amdgpu_num_sgpr(<num_sgpr>)))`` and
``__attribute__((amdgpu_num_vgpr(<num_vgpr>)))`` attributes for the AMDGPU
target. These attributes may be attached to a kernel function definition and are
an optimization hint.

If these attributes are specified, then the AMDGPU target backend will attempt
to limit the number of SGPRs and/or VGPRs used to the specified value(s). The
number of used SGPRs and/or VGPRs may further be rounded up to satisfy the
allocation requirements or constraints of the subtarget. Passing ``0`` as
``num_sgpr`` and/or ``num_vgpr`` implies the default behavior (no limits).

These attributes can be used to test the AMDGPU target backend. It is
recommended that the ``amdgpu_waves_per_eu`` attribute be used to control
resources such as SGPRs and VGPRs since it is aware of the limits for different
subtargets.

An error will be given if:
  - Specified values violate subtarget specifications;
  - Specified values are not compatible with values provided through other
    attributes;
  - The AMDGPU target backend is unable to create machine code that can meet the
    request.
  }];
}

def DocCatCallingConvs : DocumentationCategory<"Calling Conventions"> {
  let Content = [{
Clang supports several different calling conventions, depending on the target
platform and architecture. The calling convention used for a function determines
how parameters are passed, how results are returned to the caller, and other
low-level details of calling a function.
  }];
}

def PcsDocs : Documentation {
  let Category = DocCatCallingConvs;
  let Content = [{
On ARM targets, this attribute can be used to select calling conventions
similar to ``stdcall`` on x86. Valid parameter values are "aapcs" and
"aapcs-vfp".
  }];
}

def AArch64VectorPcsDocs : Documentation {
  let Category = DocCatCallingConvs;
  let Content = [{
On AArch64 targets, this attribute changes the calling convention of a
function to preserve additional floating-point and Advanced SIMD registers
relative to the default calling convention used for AArch64.

This means it is more efficient to call such functions from code that performs
extensive floating-point and vector calculations, because fewer live SIMD and FP
registers need to be saved. This property makes it well-suited for e.g.
floating-point or vector math library functions, which are typically leaf
functions that require a small number of registers.

However, using this attribute also means that it is more expensive to call
a function that adheres to the default calling convention from within such
a function. Therefore, it is recommended that this attribute is only used
for leaf functions.

For more information, see the documentation for `aarch64_vector_pcs`_ on
the Arm Developer website.

.. _`aarch64_vector_pcs`: https://developer.arm.com/products/software-development-tools/hpc/arm-compiler-for-hpc/vector-function-abi
  }];
}

def AArch64SVEPcsDocs : Documentation {
  let Category = DocCatCallingConvs;
  let Content = [{
On AArch64 targets, this attribute changes the calling convention of a
function to preserve additional Scalable Vector registers and Scalable
Predicate registers relative to the default calling convention used for
AArch64.

This means it is more efficient to call such functions from code that performs
extensive scalable vector and scalable predicate calculations, because fewer
live SVE registers need to be saved. This property makes it well-suited for SVE
math library functions, which are typically leaf functions that require a small
number of registers.

However, using this attribute also means that it is more expensive to call
a function that adheres to the default calling convention from within such
a function. Therefore, it is recommended that this attribute is only used
for leaf functions.

For more information, see the documentation for `aarch64_sve_pcs` in the
ARM C Language Extension (ACLE) documentation.

.. _`aarch64_sve_pcs`: https://github.com/ARM-software/acle/blob/main/main/acle.md#scalable-vector-extension-procedure-call-standard-attribute
  }];
}

def RegparmDocs : Documentation {
  let Category = DocCatCallingConvs;
  let Content = [{
On 32-bit x86 targets, the regparm attribute causes the compiler to pass
the first three integer parameters in EAX, EDX, and ECX instead of on the
stack. This attribute has no effect on variadic functions, and all parameters
are passed via the stack as normal.
  }];
}

def SysVABIDocs : Documentation {
  let Category = DocCatCallingConvs;
  let Content = [{
On Windows x86_64 targets, this attribute changes the calling convention of a
function to match the default convention used on Sys V targets such as Linux,
Mac, and BSD. This attribute has no effect on other targets.
  }];
}

def MSABIDocs : Documentation {
  let Category = DocCatCallingConvs;
  let Content = [{
On non-Windows x86_64 targets, this attribute changes the calling convention of
a function to match the default convention used on Windows x86_64. This
attribute has no effect on Windows targets or non-x86_64 targets.
  }];
}

def StdCallDocs : Documentation {
  let Category = DocCatCallingConvs;
  let Content = [{
On 32-bit x86 targets, this attribute changes the calling convention of a
function to clear parameters off of the stack on return. This convention does
not support variadic calls or unprototyped functions in C, and has no effect on
x86_64 targets. This calling convention is used widely by the Windows API and
COM applications. See the documentation for `__stdcall`_ on MSDN.

.. _`__stdcall`: http://msdn.microsoft.com/en-us/library/zxk0tw93.aspx
  }];
}

def FastCallDocs : Documentation {
  let Category = DocCatCallingConvs;
  let Content = [{
On 32-bit x86 targets, this attribute changes the calling convention of a
function to use ECX and EDX as register parameters and clear parameters off of
the stack on return. This convention does not support variadic calls or
unprototyped functions in C, and has no effect on x86_64 targets. This calling
convention is supported primarily for compatibility with existing code. Users
seeking register parameters should use the ``regparm`` attribute, which does
not require callee-cleanup. See the documentation for `__fastcall`_ on MSDN.

.. _`__fastcall`: http://msdn.microsoft.com/en-us/library/6xa169sk.aspx
  }];
}

def RegCallDocs : Documentation {
  let Category = DocCatCallingConvs;
  let Content = [{
On x86 targets, this attribute changes the calling convention to
`__regcall`_ convention. This convention aims to pass as many arguments
as possible in registers. It also tries to utilize registers for the
return value whenever it is possible.

.. _`__regcall`: https://software.intel.com/en-us/node/693069
  }];
}

def ThisCallDocs : Documentation {
  let Category = DocCatCallingConvs;
  let Content = [{
On 32-bit x86 targets, this attribute changes the calling convention of a
function to use ECX for the first parameter (typically the implicit ``this``
parameter of C++ methods) and clear parameters off of the stack on return. This
convention does not support variadic calls or unprototyped functions in C, and
has no effect on x86_64 targets. See the documentation for `__thiscall`_ on
MSDN.

.. _`__thiscall`: http://msdn.microsoft.com/en-us/library/ek8tkfbw.aspx
  }];
}

def VectorCallDocs : Documentation {
  let Category = DocCatCallingConvs;
  let Content = [{
On 32-bit x86 *and* x86_64 targets, this attribute changes the calling
convention of a function to pass vector parameters in SSE registers.

On 32-bit x86 targets, this calling convention is similar to ``__fastcall``.
The first two integer parameters are passed in ECX and EDX. Subsequent integer
parameters are passed in memory, and callee clears the stack. On x86_64
targets, the callee does *not* clear the stack, and integer parameters are
passed in RCX, RDX, R8, and R9 as is done for the default Windows x64 calling
convention.

On both 32-bit x86 and x86_64 targets, vector and floating point arguments are
passed in XMM0-XMM5. Homogeneous vector aggregates of up to four elements are
passed in sequential SSE registers if enough are available. If AVX is enabled,
256 bit vectors are passed in YMM0-YMM5. Any vector or aggregate type that
cannot be passed in registers for any reason is passed by reference, which
allows the caller to align the parameter memory.

See the documentation for `__vectorcall`_ on MSDN for more details.

.. _`__vectorcall`: http://msdn.microsoft.com/en-us/library/dn375768.aspx
  }];
}

def DocCatConsumed : DocumentationCategory<"Consumed Annotation Checking"> {
  let Content = [{
Clang supports additional attributes for checking basic resource management
properties, specifically for unique objects that have a single owning reference.
The following attributes are currently supported, although **the implementation
for these annotations is currently in development and are subject to change.**
  }];
}

def SetTypestateDocs : Documentation {
  let Category = DocCatConsumed;
  let Content = [{
Annotate methods that transition an object into a new state with
``__attribute__((set_typestate(new_state)))``. The new state must be
unconsumed, consumed, or unknown.
  }];
}

def CallableWhenDocs : Documentation {
  let Category = DocCatConsumed;
  let Content = [{
Use ``__attribute__((callable_when(...)))`` to indicate what states a method
may be called in. Valid states are unconsumed, consumed, or unknown. Each
argument to this attribute must be a quoted string. E.g.:

``__attribute__((callable_when("unconsumed", "unknown")))``
  }];
}

def TestTypestateDocs : Documentation {
  let Category = DocCatConsumed;
  let Content = [{
Use ``__attribute__((test_typestate(tested_state)))`` to indicate that a method
returns true if the object is in the specified state..
  }];
}

def ParamTypestateDocs : Documentation {
  let Category = DocCatConsumed;
  let Content = [{
This attribute specifies expectations about function parameters. Calls to an
function with annotated parameters will issue a warning if the corresponding
argument isn't in the expected state. The attribute is also used to set the
initial state of the parameter when analyzing the function's body.
  }];
}

def ReturnTypestateDocs : Documentation {
  let Category = DocCatConsumed;
  let Content = [{
The ``return_typestate`` attribute can be applied to functions or parameters.
When applied to a function the attribute specifies the state of the returned
value. The function's body is checked to ensure that it always returns a value
in the specified state. On the caller side, values returned by the annotated
function are initialized to the given state.

When applied to a function parameter it modifies the state of an argument after
a call to the function returns. The function's body is checked to ensure that
the parameter is in the expected state before returning.
  }];
}

def ConsumableDocs : Documentation {
  let Category = DocCatConsumed;
  let Content = [{
Each ``class`` that uses any of the typestate annotations must first be marked
using the ``consumable`` attribute. Failure to do so will result in a warning.

This attribute accepts a single parameter that must be one of the following:
``unknown``, ``consumed``, or ``unconsumed``.
  }];
}

def NoProfileInstrumentFunctionDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Use the ``no_profile_instrument_function`` attribute on a function declaration
to denote that the compiler should not instrument the function with
profile-related instrumentation, such as via the
``-fprofile-generate`` / ``-fprofile-instr-generate`` /
``-fcs-profile-generate`` / ``-fprofile-arcs`` flags.
}];
}

def NoSanitizeDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Use the ``no_sanitize`` attribute on a function or a global variable
declaration to specify that a particular instrumentation or set of
instrumentations should not be applied.

The attribute takes a list of string literals with the following accepted
values:
* all values accepted by ``-fno-sanitize=``;
* ``coverage``, to disable SanitizerCoverage instrumentation.

For example, ``__attribute__((no_sanitize("address", "thread")))`` specifies
that AddressSanitizer and ThreadSanitizer should not be applied to the function
or variable. Using ``__attribute__((no_sanitize("coverage")))`` specifies that
SanitizerCoverage should not be applied to the function.

See :ref:`Controlling Code Generation <controlling-code-generation>` for a
full list of supported sanitizer flags.
  }];
}

def DisableSanitizerInstrumentationDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
Use the ``disable_sanitizer_instrumentation`` attribute on a function,
Objective-C method, or global variable, to specify that no sanitizer
instrumentation should be applied.

This is not the same as ``__attribute__((no_sanitize(...)))``, which depending
on the tool may still insert instrumentation to prevent false positive reports.
  }];
}

def NoSanitizeAddressDocs : Documentation {
  let Category = DocCatFunction;
  // This function has multiple distinct spellings, and so it requires a custom
  // heading to be specified. The most common spelling is sufficient.
  let Heading = "no_sanitize_address, no_address_safety_analysis";
  let Content = [{
.. _langext-address_sanitizer:

Use ``__attribute__((no_sanitize_address))`` on a function or a global
variable declaration to specify that address safety instrumentation
(e.g. AddressSanitizer) should not be applied.
  }];
}

def NoSanitizeThreadDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "no_sanitize_thread";
  let Content = [{
.. _langext-thread_sanitizer:

Use ``__attribute__((no_sanitize_thread))`` on a function declaration to
specify that checks for data races on plain (non-atomic) memory accesses should
not be inserted by ThreadSanitizer. The function is still instrumented by the
tool to avoid false positives and provide meaningful stack traces.
  }];
}

def NoSanitizeMemoryDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "no_sanitize_memory";
  let Content = [{
.. _langext-memory_sanitizer:

Use ``__attribute__((no_sanitize_memory))`` on a function declaration to
specify that checks for uninitialized memory should not be inserted
(e.g. by MemorySanitizer). The function may still be instrumented by the tool
to avoid false positives in other places.
  }];
}

def CFICanonicalJumpTableDocs : Documentation {
  let Category = DocCatFunction;
  let Heading = "cfi_canonical_jump_table";
  let Content = [{
.. _langext-cfi_canonical_jump_table:

Use ``__attribute__((cfi_canonical_jump_table))`` on a function declaration to
make the function's CFI jump table canonical. See :ref:`the CFI documentation
<cfi-canonical-jump-tables>` for more details.
  }];
}

def DocCatTypeSafety : DocumentationCategory<"Type Safety Checking"> {
  let Content = [{
Clang supports additional attributes to enable checking type safety properties
that can't be enforced by the C type system. To see warnings produced by these
checks, ensure that -Wtype-safety is enabled. Use cases include:

* MPI library implementations, where these attributes enable checking that
  the buffer type matches the passed ``MPI_Datatype``;
* for HDF5 library there is a similar use case to MPI;
* checking types of variadic functions' arguments for functions like
  ``fcntl()`` and ``ioctl()``.

You can detect support for these attributes with ``__has_attribute()``. For
example:

.. code-block:: c++

  #if defined(__has_attribute)
  #  if __has_attribute(argument_with_type_tag) && \
        __has_attribute(pointer_with_type_tag) && \
        __has_attribute(type_tag_for_datatype)
  #    define ATTR_MPI_PWT(buffer_idx, type_idx) __attribute__((pointer_with_type_tag(mpi,buffer_idx,type_idx)))
  /* ... other macros ... */
  #  endif
  #endif

  #if !defined(ATTR_MPI_PWT)
  # define ATTR_MPI_PWT(buffer_idx, type_idx)
  #endif

  int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */)
      ATTR_MPI_PWT(1,3);
  }];
}

def ArgumentWithTypeTagDocs : Documentation {
  let Category = DocCatTypeSafety;
  let Heading = "argument_with_type_tag";
  let Content = [{
Use ``__attribute__((argument_with_type_tag(arg_kind, arg_idx,
type_tag_idx)))`` on a function declaration to specify that the function
accepts a type tag that determines the type of some other argument.

This attribute is primarily useful for checking arguments of variadic functions
(``pointer_with_type_tag`` can be used in most non-variadic cases).

In the attribute prototype above:
  * ``arg_kind`` is an identifier that should be used when annotating all
    applicable type tags.
  * ``arg_idx`` provides the position of a function argument. The expected type of
    this function argument will be determined by the function argument specified
    by ``type_tag_idx``. In the code example below, "3" means that the type of the
    function's third argument will be determined by ``type_tag_idx``.
  * ``type_tag_idx`` provides the position of a function argument. This function
    argument will be a type tag. The type tag will determine the expected type of
    the argument specified by ``arg_idx``. In the code example below, "2" means
    that the type tag associated with the function's second argument should agree
    with the type of the argument specified by ``arg_idx``.

For example:

.. code-block:: c++

  int fcntl(int fd, int cmd, ...)
      __attribute__(( argument_with_type_tag(fcntl,3,2) ));
  // The function's second argument will be a type tag; this type tag will
  // determine the expected type of the function's third argument.
  }];
}

def PointerWithTypeTagDocs : Documentation {
  let Category = DocCatTypeSafety;
  let Heading = "pointer_with_type_tag";
  let Content = [{
Use ``__attribute__((pointer_with_type_tag(ptr_kind, ptr_idx, type_tag_idx)))``
on a function declaration to specify that the function accepts a type tag that
determines the pointee type of some other pointer argument.

In the attribute prototype above:
  * ``ptr_kind`` is an identifier that should be used when annotating all
    applicable type tags.
  * ``ptr_idx`` provides the position of a function argument; this function
    argument will have a pointer type. The expected pointee type of this pointer
    type will be determined by the function argument specified by
    ``type_tag_idx``. In the code example below, "1" means that the pointee type
    of the function's first argument will be determined by ``type_tag_idx``.
  * ``type_tag_idx`` provides the position of a function argument; this function
    argument will be a type tag. The type tag will determine the expected pointee
    type of the pointer argument specified by ``ptr_idx``. In the code example
    below, "3" means that the type tag associated with the function's third
    argument should agree with the pointee type of the pointer argument specified
    by ``ptr_idx``.

For example:

.. code-block:: c++

  typedef int MPI_Datatype;
  int MPI_Send(void *buf, int count, MPI_Datatype datatype /*, other args omitted */)
      __attribute__(( pointer_with_type_tag(mpi,1,3) ));
  // The function's 3rd argument will be a type tag; this type tag will
  // determine the expected pointee type of the function's 1st argument.
  }];
}

def TypeTagForDatatypeDocs : Documentation {
  let Category = DocCatTypeSafety;
  let Content = [{
When declaring a variable, use
``__attribute__((type_tag_for_datatype(kind, type)))`` to create a type tag that
is tied to the ``type`` argument given to the attribute.

In the attribute prototype above:
  * ``kind`` is an identifier that should be used when annotating all applicable
    type tags.
  * ``type`` indicates the name of the type.

Clang supports annotating type tags of two forms.

  * **Type tag that is a reference to a declared identifier.**
    Use ``__attribute__((type_tag_for_datatype(kind, type)))`` when declaring that
    identifier:

    .. code-block:: c++

      typedef int MPI_Datatype;
      extern struct mpi_datatype mpi_datatype_int
          __attribute__(( type_tag_for_datatype(mpi,int) ));
      #define MPI_INT ((MPI_Datatype) &mpi_datatype_int)
      // &mpi_datatype_int is a type tag. It is tied to type "int".

  * **Type tag that is an integral literal.**
    Declare a ``static const`` variable with an initializer value and attach
    ``__attribute__((type_tag_for_datatype(kind, type)))`` on that declaration:

    .. code-block:: c++

      typedef int MPI_Datatype;
      static const MPI_Datatype mpi_datatype_int
          __attribute__(( type_tag_for_datatype(mpi,int) )) = 42;
      #define MPI_INT ((MPI_Datatype) 42)
      // The number 42 is a type tag. It is tied to type "int".


The ``type_tag_for_datatype`` attribute also accepts an optional third argument
that determines how the type of the function argument specified by either
``arg_idx`` or ``ptr_idx`` is compared against the type associated with the type
tag. (Recall that for the ``argument_with_type_tag`` attribute, the type of the
function argument specified by ``arg_idx`` is compared against the type
associated with the type tag. Also recall that for the ``pointer_with_type_tag``
attribute, the pointee type of the function argument specified by ``ptr_idx`` is
compared against the type associated with the type tag.) There are two supported
values for this optional third argument:

  * ``layout_compatible`` will cause types to be compared according to
    layout-compatibility rules (In C++11 [class.mem] p 17, 18, see the
    layout-compatibility rules for two standard-layout struct types and for two
    standard-layout union types). This is useful when creating a type tag
    associated with a struct or union type. For example:

    .. code-block:: c++

      /* In mpi.h */
      typedef int MPI_Datatype;
      struct internal_mpi_double_int { double d; int i; };
      extern struct mpi_datatype mpi_datatype_double_int
          __attribute__(( type_tag_for_datatype(mpi,
                          struct internal_mpi_double_int, layout_compatible) ));

      #define MPI_DOUBLE_INT ((MPI_Datatype) &mpi_datatype_double_int)

      int MPI_Send(void *buf, int count, MPI_Datatype datatype, ...)
          __attribute__(( pointer_with_type_tag(mpi,1,3) ));

      /* In user code */
      struct my_pair { double a; int b; };
      struct my_pair *buffer;
      MPI_Send(buffer, 1, MPI_DOUBLE_INT /*, ... */); // no warning because the
                                                       // layout of my_pair is
                                                       // compatible with that of
                                                       // internal_mpi_double_int

      struct my_int_pair { int a; int b; }
      struct my_int_pair *buffer2;
      MPI_Send(buffer2, 1, MPI_DOUBLE_INT /*, ... */); // warning because the
                                                        // layout of my_int_pair
                                                        // does not match that of
                                                        // internal_mpi_double_int

  * ``must_be_null`` specifies that the function argument specified by either
    ``arg_idx`` (for the ``argument_with_type_tag`` attribute) or ``ptr_idx`` (for
    the ``pointer_with_type_tag`` attribute) should be a null pointer constant.
    The second argument to the ``type_tag_for_datatype`` attribute is ignored. For
    example:

    .. code-block:: c++

      /* In mpi.h */
      typedef int MPI_Datatype;
      extern struct mpi_datatype mpi_datatype_null
          __attribute__(( type_tag_for_datatype(mpi, void, must_be_null) ));

      #define MPI_DATATYPE_NULL ((MPI_Datatype) &mpi_datatype_null)
      int MPI_Send(void *buf, int count, MPI_Datatype datatype, ...)
          __attribute__(( pointer_with_type_tag(mpi,1,3) ));

      /* In user code */
      struct my_pair { double a; int b; };
      struct my_pair *buffer;
      MPI_Send(buffer, 1, MPI_DATATYPE_NULL /*, ... */); // warning: MPI_DATATYPE_NULL
                                                          // was specified but buffer
                                                          // is not a null pointer
  }];
}

def FlattenDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{
The ``flatten`` attribute causes calls within the attributed function to
be inlined unless it is impossible to do so, for example if the body of the
callee is unavailable or if the callee has the ``noinline`` attribute.
  }];
}

def FormatDocs : Documentation {
  let Category = DocCatFunction;
  let Content = [{

Clang supports the ``format`` attribute, which indicates that the function
accepts (among other possibilities) a ``printf`` or ``scanf``-like format string
and corresponding arguments or a ``va_list`` that contains these arguments.

Please see `GCC documentation about format attribute
<http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_ to find details
about attribute syntax.

Clang implements two kinds of checks with this attribute.

#. Clang checks that the function with the ``format`` attribute is called with
   a format string that uses format specifiers that are allowed, and that
   arguments match the format string. This is the ``-Wformat`` warning, it is
   on by default.

#. Clang checks that the format string argument is a literal string. This is
   the ``-Wformat-nonliteral`` warning, it is off by default.

   Clang implements this mostly the same way as GCC, but there is a difference
   for functions that accept a ``va_list`` argument (for example, ``vprintf``).
   GCC does not emit ``-Wformat-nonliteral`` warning for calls to such
   functions. Clang does not warn if the format string comes from a function
   parameter, where the function is annotated with a compatible attribute,
   otherwise it warns. For example:

   .. code-block:: c

     __attribute__((__format__ (__scanf__, 1, 3)))
     void foo(const char* s, char *buf, ...) {
       va_list ap;
       va_start(ap, buf);

       vprintf(s, ap); // warning: format string is not a string literal
     }

   In this case we warn because ``s`` contains a format string for a
   ``scanf``-like function, but it is passed to a ``printf``-like function.

   If the attribute is removed, clang still warns, because the format string is
   not a string literal.

   Another example:

   .. code-block:: c

     __attribute__((__format__ (__printf__, 1, 3)))
     void foo(const char* s, char *buf, ...) {
       va_list ap;
       va_start(ap, buf);

       vprintf(s, ap); // warning
     }

   In this case Clang does not warn because the format string ``s`` and
   the corresponding arguments are annotated. If the arguments are
   incorrect, the caller of ``foo`` will receive a warning.

As an extension to GCC's behavior, Clang accepts the ``format`` attribute on
non-variadic functions. Clang checks non-variadic format functions for the same
classes of issues that can be found on variadic functions, as controlled by the
same warning flags, except that the types of formatted arguments is forced by
the function signature. For example:

.. code-block:: c

  __attribute__((__format__(__printf__, 1, 2)))
  void fmt(const char *s, const char *a, int b);

  void bar(void) {
    fmt("%s %i", "hello", 123); // OK
    fmt("%i %g", "hello", 123); // warning: arguments don't match format
    extern const char *fmt;
    fmt(fmt, "hello", 123); // warning: format string is not a string literal
  }

When using the format attribute on a variadic function, the first data parameter
_must_ be the index of the ellipsis in the parameter list. Clang will generate
a diagnostic otherwise, as it wouldn't be possible to forward that argument list
to `printf`-family functions. For instance, this is an error:

.. code-block:: c

  __attribute__((__format__(__printf__, 1, 2)))
  void fmt(const char *s, int b, ...);
  // ^ error: format attribute parameter 3 is out of bounds
  // (must be __printf__, 1, 3)

Using the ``format`` attribute on a non-variadic function emits a GCC
compatibility diagnostic.
  }];
}

def AlignValueDocs : Documentation {
  let Category = DocCatType;
  let Content = [{
The align_value attribute can be added to the typedef of a pointer type or the
declaration of a variable of pointer or reference type. It specifies that the
pointer will point to, or the reference will bind to, only objects with at
least the provided alignment. This alignment value must be some positive power
of 2.

   .. code-block:: c

     typedef double * aligned_double_ptr __attribute__((align_value(64)));
     void foo(double & x  __attribute__((align_value(128)),
              aligned_double_ptr y) { ... }

If the pointer value does not have the specified alignment at runtime, the
behavior of the program is undefined.
  }];
}

def FlagEnumDocs : Documentation {
  let Category = DocCatDecl;
  let Content = [{
This attribute can be added to an enumerator to signal to the compiler that it
is intended to be used as a flag type. This will cause the compiler to assume
that the range of the type includes all of the values that you can get by
manipulating bits of the enumerator when issuing warnings.
  }];
}

def AsmLabelDocs : Documentation {
  let Category = DocCatDecl;
  let Content = [{
This attribute can be used on a function or variable to specify its symbol name.

On some targets, all C symbols are prefixed by default with a single character,
typically ``_``. This was done historically to distinguish them from symbols
used by other languages. (This prefix is also added to the standard Itanium
C++ ABI prefix on "mangled" symbol names, so that e.g. on such targets the true
symbol name for a C++ variable declared as ``int cppvar;`` would be
``__Z6cppvar``; note the two underscores.)  This prefix is *not* added to the
symbol names specified by the ``asm`` attribute; programmers wishing to match a
C symbol name must compensate for this.

For example, consider the following C code:

.. code-block:: c

  int var1 asm("altvar") = 1;  // "altvar" in symbol table.
  int var2 = 1; // "_var2" in symbol table.

  void func1(void) asm("altfunc");
  void func1(void) {} // "altfunc" in symbol table.
  void func2(void) {} // "_func2" in symbol table.

Clang's implementation of this attribute is compatible with GCC's, `documented here <https://gcc.gnu.org/onlinedocs/gcc/Asm-Labels.html>`_.

While it is possible to use this attribute to name a special symbol used
internally by the compiler, such as an LLVM intrinsic, this is neither
recommended nor supported and may cause the compiler to crash or miscompile.
Users who wish to gain access to intrinsic behavior are strongly encouraged to
request new builtin functions.
  }];
}

def EnumExtensibilityDocs : Documentation {
  let Category = DocCatDecl;
  let Content = [{
Attribute ``enum_extensibility`` is used to distinguish between enum definitions
that are extensible and those that are not. The attribute can take either
``closed`` or ``open`` as an argument. ``closed`` indicates a variable of the
enum type takes a value that corresponds to one of the enumerators listed in the
enum definition or, when the enum is annotated with ``flag_enum``, a value that
can be constructed using values corresponding to the enumerators. ``open``
indicates a variable of the enum type can take any values allowed by the
standard and instructs clang to be more lenient when issuing warnings.

.. code-block:: c

  enum __attribute__((enum_extensibility(closed))) ClosedEnum {
    A0, A1
  };

  enum __attribute__((enum_extensibility(open))) OpenEnum {
    B0, B1
  };

  enum __attribute__((enum_extensibility(closed),flag_enum)) ClosedFlagEnum {
    C0 = 1 << 0, C1 = 1 << 1
  };

  enum __attribute__((enum_extensibility(open),flag_enum)) OpenFlagEnum {
    D0 = 1 << 0, D1 = 1 << 1
  };

  void foo1() {
    enum ClosedEnum ce;
    enum OpenEnum oe;
    enum ClosedFlagEnum cfe;
    enum OpenFlagEnum ofe;

    ce = A1;           // no warnings
    ce = 100;          // warning issued
    oe = B1;           // no warnings
    oe = 100;          // no warnings
    cfe = C0 | C1;     // no warnings
    cfe = C0 | C1 | 4; // warning issued
    ofe = D0 | D1;     // no warnings
    ofe = D0 | D1 | 4; // no warnings
  }