Date: Sat, 4 Aug 2018 13:15:49 +0000 (UTC) From: Dimitry Andric <dim@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-vendor@freebsd.org Subject: svn commit: r337301 - in vendor/clang/dist-release_70: docs include/clang/Basic lib/AST lib/Basic lib/Sema test/CodeGen test/Driver test/SemaCXX Message-ID: <201808041315.w74DFnLM083392@repo.freebsd.org>
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Author: dim Date: Sat Aug 4 13:15:49 2018 New Revision: 337301 URL: https://svnweb.freebsd.org/changeset/base/337301 Log: Vendor import of clang release_70 branch r338892: https://llvm.org/svn/llvm-project/cfe/branches/release_70@338892 Modified: vendor/clang/dist-release_70/docs/AttributeReference.rst vendor/clang/dist-release_70/docs/ClangCommandLineReference.rst vendor/clang/dist-release_70/docs/ReleaseNotes.rst vendor/clang/dist-release_70/include/clang/Basic/Builtins.def vendor/clang/dist-release_70/include/clang/Basic/DiagnosticASTKinds.td vendor/clang/dist-release_70/lib/AST/ExprConstant.cpp vendor/clang/dist-release_70/lib/Basic/Version.cpp vendor/clang/dist-release_70/lib/Sema/SemaInit.cpp vendor/clang/dist-release_70/test/CodeGen/builtin-memfns.c vendor/clang/dist-release_70/test/Driver/darwin-infer-simulator-sdkroot.c vendor/clang/dist-release_70/test/Driver/integrated-as.c vendor/clang/dist-release_70/test/SemaCXX/constexpr-string.cpp Modified: vendor/clang/dist-release_70/docs/AttributeReference.rst ============================================================================== --- vendor/clang/dist-release_70/docs/AttributeReference.rst Sat Aug 4 13:15:11 2018 (r337300) +++ vendor/clang/dist-release_70/docs/AttributeReference.rst Sat Aug 4 13:15:49 2018 (r337301) @@ -1,13 +1,3899 @@ .. ------------------------------------------------------------------- NOTE: This file is automatically generated by running clang-tblgen - -gen-attr-docs. Do not edit this file by hand!! The contents for - this file are automatically generated by a server-side process. - - Please do not commit this file. The file exists for local testing - purposes only. + -gen-attr-docs. Do not edit this file by hand!! ------------------------------------------------------------------- =================== Attributes in Clang -=================== \ No newline at end of file +=================== +.. contents:: + :local: + +Introduction +============ + +This page lists the attributes currently supported by Clang. + +Function Attributes +=================== + + +#pragma omp declare simd +------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "","","","","", "X", "" + +The `declare simd` construct can be applied to a function to enable the creation +of one or more versions that can process multiple arguments using SIMD +instructions from a single invocation in a SIMD loop. The `declare simd` +directive is a declarative directive. There may be multiple `declare simd` +directives for a function. The use of a `declare simd` construct on a function +enables the creation of SIMD versions of the associated function that can be +used to process multiple arguments from a single invocation from a SIMD loop +concurrently. +The syntax of the `declare simd` construct is as follows: + + .. code-block:: none + + #pragma omp declare simd [clause[[,] clause] ...] new-line + [#pragma omp declare simd [clause[[,] clause] ...] new-line] + [...] + function definition or declaration + +where clause is one of the following: + + .. code-block:: none + + simdlen(length) + linear(argument-list[:constant-linear-step]) + aligned(argument-list[:alignment]) + uniform(argument-list) + inbranch + notinbranch + + +#pragma omp declare target +-------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "","","","","", "X", "" + +The `declare target` directive specifies that variables and functions are mapped +to a device for OpenMP offload mechanism. + +The syntax of the declare target directive is as follows: + + .. code-block:: c + + #pragma omp declare target new-line + declarations-definition-seq + #pragma omp end declare target new-line + + +_Noreturn +--------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "","","","","X", "", "" + +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. + + +abi_tag (gnu::abi_tag) +---------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","","","", "", "X" + +The ``abi_tag`` attribute can be applied to a function, variable, class or +inline namespace declaration to modify the mangled name of the entity. It gives +the ability to distinguish between different versions of the same entity but +with different ABI versions supported. For example, a newer version of a class +could have a different set of data members and thus have a different size. Using +the ``abi_tag`` attribute, it is possible to have different mangled names for +a global variable of the class type. Therefore, the old code could keep using +the old manged name and the new code will use the new mangled name with tags. + + +acquire_capability (acquire_shared_capability, clang::acquire_capability, clang::acquire_shared_capability) +----------------------------------------------------------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","","","", "", "" + +Marks a function as acquiring a capability. + + +alloc_align (gnu::alloc_align) +------------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","","","", "", "" + +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 +poitner is not sufficiently aligned. + + +alloc_size (gnu::alloc_size) +---------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","","","", "", "X" + +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``. + + +artificial (gnu::artificial) +---------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","","","", "", "" + +The ``artificial`` attribute can be applied to an inline function. If such a +function is inlined, the attribute indicates that debuggers should associate +the resulting instructions with the call site, rather than with the +corresponding line within the inlined callee. + + +assert_capability (assert_shared_capability, clang::assert_capability, clang::assert_shared_capability) +------------------------------------------------------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","","","", "", "" + +Marks a function that dynamically tests whether a capability is held, and halts +the program if it is not held. + + +assume_aligned (gnu::assume_aligned) +------------------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","","","", "", "X" + +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. + + +availability (clang::availability, clang::availability) +------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","X","","", "", "X" + +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. Only the availability attribute with the +platform corresponding to the target platform will be used; any others will be +ignored. 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. + +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 + +Also see the documentation for `@available +<http://clang.llvm.org/docs/LanguageExtensions.html#objective-c-available>`_ + + +carries_dependency +------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","","","", "", "X" + +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. + + +code_seg +-------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "","","","X","", "", "" + +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 + + +convergent (clang::convergent, clang::convergent) +------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","X","","", "", "X" + +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]]; + + +cpu_dispatch (clang::cpu_dispatch, clang::cpu_dispatch) +------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","X","","", "", "X" + +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. + +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. + + +cpu_specific (clang::cpu_specific, clang::cpu_specific) +------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","X","","", "", "X" + +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. + +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. + + +deprecated (gnu::deprecated) +---------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","X","X","", "", "" + +The ``deprecated`` attribute can be applied to a function, a variable, or a +type. This is useful when identifying functions, variables, or types that are +expected to be removed in a future version of a program. + +Consider the function declaration for a hypothetical function ``f``: + +.. code-block:: c++ + + void f(void) __attribute__((deprecated("message", "replacement"))); + +When spelled as `__attribute__((deprecated))`, the deprecated attribute can have +two optional string arguments. The first one is the message to display when +emitting the warning; the second one enables the compiler to provide a Fix-It +to replace the deprecated name with a new name. Otherwise, when spelled as +`[[gnu::deprecated]] or [[deprecated]]`, the attribute can have one optional +string argument which is the message to display when emitting the warning. + + +diagnose_if +----------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","","","","", "", "" + +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 + + void abs(int a) + __attribute__((diagnose_if(a >= 0, "Redundant abs call", "warning"))); + void 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)``. + + +disable_tail_calls (clang::disable_tail_calls, clang::disable_tail_calls) +------------------------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","X","","", "", "X" + +The ``disable_tail_calls`` attribute instructs the backend to not perform tail call optimization inside the marked function. + +For example: + + .. code-block:: c + + int callee(int); + + int foo(int a) __attribute__((disable_tail_calls)) { + return callee(a); // This call is not tail-call optimized. + } + +Marking virtual functions as ``disable_tail_calls`` is legal. + + .. code-block:: c++ + + int callee(int); + + class Base { + public: + [[clang::disable_tail_calls]] virtual int foo1() { + return callee(); // This call is not tail-call optimized. + } + }; + + class Derived1 : public Base { + public: + int foo1() override { + return callee(); // This call is tail-call optimized. + } + }; + + +enable_if +--------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","","","","", "", "X" + +.. 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. + + +external_source_symbol (clang::external_source_symbol, clang::external_source_symbol) +------------------------------------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","X","","", "", "X" + +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. + +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. + + +flatten (gnu::flatten) +---------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","","","", "", "X" + +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. + + +force_align_arg_pointer (gnu::force_align_arg_pointer) +------------------------------------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","","","", "", "" + +Use this attribute to force stack alignment. + +Legacy x86 code uses 4-byte stack alignment. Newer aligned SSE instructions +(like 'movaps') that work with the stack require operands to be 16-byte aligned. +This attribute realigns the stack in the function prologue to make sure the +stack can be used with SSE instructions. + +Note that the x86_64 ABI forces 16-byte stack alignment at the call site. +Because of this, 'force_align_arg_pointer' is not needed on x86_64, except in +rare cases where the caller does not align the stack properly (e.g. flow +jumps from i386 arch code). + + .. code-block:: c + + __attribute__ ((force_align_arg_pointer)) + void f () { + ... + } + + +format (gnu::format) +-------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "C2x", "__declspec", "Keyword", "Pragma", "Pragma clang attribute" + + "X","X","","","", "", "" + +Clang supports the ``format`` attribute, which indicates that the function +accepts 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. *** DIFF OUTPUT TRUNCATED AT 1000 LINES ***
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