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diff --git a/docs/AttributeReference.rst b/docs/AttributeReference.rst index a763ddeaeb10..d7a3f74f83fb 100644 --- a/docs/AttributeReference.rst +++ b/docs/AttributeReference.rst @@ -1,13 +1,3118 @@ .. ------------------------------------------------------------------- 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 +=================== + + +interrupt +--------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +abi_tag (gnu::abi_tag) +---------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "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. Therefor, 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", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +Marks a function as acquiring a capability. + + +alloc_size (gnu::alloc_size) +---------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "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 poiner. ``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``. + + +interrupt +--------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +Clang supports the GNU style ``__attribute__((interrupt))`` attribute on +x86/x86-64 targets.The compiler generates function entry and exit sequences +suitable for use in an interrupt handler when this attribute is present. +The 'IRET' instruction, instead of the 'RET' instruction, is used to return +from interrupt or exception handlers. All registers, except for the EFLAGS +register which is restored by the 'IRET' instruction, are preserved by the +compiler. + +Any interruptible-without-stack-switch code must be compiled with +-mno-red-zone since interrupt handlers can and will, because of the +hardware design, touch the red zone. + +1. interrupt handler must be declared with a mandatory pointer argument: + + .. code-block:: c + + struct interrupt_frame + { + uword_t ip; + uword_t cs; + uword_t flags; + uword_t sp; + uword_t ss; + }; + + __attribute__ ((interrupt)) + void f (struct interrupt_frame *frame) { + ... + } + +2. exception handler: + + The exception handler is very similar to the interrupt handler with + a different mandatory function signature: + + .. code-block:: c + + __attribute__ ((interrupt)) + void f (struct interrupt_frame *frame, uword_t error_code) { + ... + } + + and compiler pops 'ERROR_CODE' off stack before the 'IRET' instruction. + + The exception handler should only be used for exceptions which push an + error code and all other exceptions must use the interrupt handler. + The system will crash if the wrong handler is used. + + +assert_capability (assert_shared_capability, clang::assert_capability, clang::assert_shared_capability) +------------------------------------------------------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "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", "__declspec", "Keyword", "Pragma" + + "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 +------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "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 Mac OS X 10.4, +deprecated in Mac OS X 10.6, and obsoleted in Mac OS X 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 Mac OS X 10.5, a call to ``f()`` +succeeds. If Clang is instructed to compile code for Mac OS X 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 Mac OS X 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 Mac OS X 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 + + +_Noreturn +--------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","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. + + +noreturn +-------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","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. + + +carries_dependency +------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "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. + + +convergent (clang::convergent) +------------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "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]]; + + +deprecated (gnu::deprecated) +---------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "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", "__declspec", "Keyword", "Pragma" + + "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) +---------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "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", "__declspec", "Keyword", "Pragma" + + "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. + + +flatten (gnu::flatten) +---------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "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. + + +format (gnu::format) +-------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "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. + + +ifunc (gnu::ifunc) +------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +``__attribute__((ifunc("resolver")))`` is used to mark that the address of a declaration should be resolved at runtime by calling a resolver function. + +The symbol name of the resolver function is given in quotes. A function with this name (after mangling) must be defined in the current translation unit; it may be ``static``. The resolver function should take no arguments and return a pointer. + +The ``ifunc`` attribute may only be used on a function declaration. A function declaration with an ``ifunc`` attribute is considered to be a definition of the declared entity. The entity must not have weak linkage; for example, in C++, it cannot be applied to a declaration if a definition at that location would be considered inline. + +Not all targets support this attribute. ELF targets support this attribute when using binutils v2.20.1 or higher and glibc v2.11.1 or higher. Non-ELF targets currently do not support this attribute. + + +internal_linkage (clang::internal_linkage) +------------------------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +The ``internal_linkage`` attribute changes the linkage type of the declaration to internal. +This is similar to C-style ``static``, but can be used on classes and class methods. When applied to a class definition, +this attribute affects all methods and static data members of that class. +This can be used to contain the ABI of a C++ library by excluding unwanted class methods from the export tables. + + +interrupt +--------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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 behaviour 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". + + +noalias +------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","X","", "" + +The ``noalias`` attribute indicates that the only memory accesses inside +function are loads and stores from objects pointed to by its pointer-typed +arguments, with arbitrary offsets. + + +noduplicate (clang::noduplicate) +-------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +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. + + +no_sanitize (clang::no_sanitize) +-------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +Use the ``no_sanitize`` attribute on a function declaration to specify +that a particular instrumentation or set of instrumentations should not be +applied to that function. The attribute takes a list of string literals, +which have the same meaning as values accepted by the ``-fno-sanitize=`` +flag. For example, ``__attribute__((no_sanitize("address", "thread")))`` +specifies that AddressSanitizer and ThreadSanitizer should not be applied +to the function. + +See :ref:`Controlling Code Generation <controlling-code-generation>` for a +full list of supported sanitizer flags. + + +no_sanitize_address (no_address_safety_analysis, gnu::no_address_safety_analysis, gnu::no_sanitize_address) +----------------------------------------------------------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +.. _langext-address_sanitizer: + +Use ``__attribute__((no_sanitize_address))`` on a function declaration to +specify that address safety instrumentation (e.g. AddressSanitizer) should +not be applied to that function. + + +no_sanitize_thread +------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +.. _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. + + +no_sanitize_memory +------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +.. _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. + + +no_split_stack (gnu::no_split_stack) +------------------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +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. + + +not_tail_called (clang::not_tail_called) +---------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +The ``not_tail_called`` attribute prevents tail-call optimization on statically bound calls. It has no effect on indirect calls. Virtual functions, objective-c methods, and functions marked as ``always_inline`` cannot be marked as ``not_tail_called``. + +For example, it prevents tail-call optimization in the following case: + + .. code-block:: c + + int __attribute__((not_tail_called)) foo1(int); + + int foo2(int a) { + return foo1(a); // No tail-call optimization on direct calls. + } + +However, it doesn't prevent tail-call optimization in this case: + + .. code-block:: c + + int __attribute__((not_tail_called)) foo1(int); + + int foo2(int a) { + int (*fn)(int) = &foo1; + + // not_tail_called has no effect on an indirect call even if the call can be + // resolved at compile time. + return (*fn)(a); + } + +Marking virtual functions as ``not_tail_called`` is an error: + + .. code-block:: c++ + + class Base { + public: + // not_tail_called on a virtual function is an error. + [[clang::not_tail_called]] virtual int foo1(); + + virtual int foo2(); + + // Non-virtual functions can be marked ``not_tail_called``. + [[clang::not_tail_called]] int foo3(); + }; + + class Derived1 : public Base { + public: + int foo1() override; + + // not_tail_called on a virtual function is an error. + [[clang::not_tail_called]] int foo2() override; + }; + + +#pragma omp declare simd +------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","", "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:: c + + #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:: c + + 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", "__declspec", "Keyword", "Pragma" + + "","","","", "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 + + +objc_boxable +------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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); + + +objc_method_family +------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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)``. + + +objc_requires_super +------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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{}; + ^ + + +objc_runtime_name +----------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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 + + +objc_runtime_visible +-------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +optnone (clang::optnone) +------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +The ``optnone`` attribute suppresses essentially all optimizations +on a function or method, regardless of the optimization level applied to +the compilation unit as a whole. This is particularly useful when you +need to debug a particular function, but it is infeasible to build the +entire application without optimization. Avoiding optimization on the +specified function can improve the quality of the debugging information +for that function. + +This attribute is incompatible with the ``always_inline`` and ``minsize`` +attributes. + + +overloadable +------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. Most importantly, if any function with a given +name is given the ``overloadable`` attribute, then all function declarations +and definitions with that name (and in that scope) must have the +``overloadable`` attribute. This rule even applies to redeclarations of +functions whose original declaration had 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 g(int) __attribute__((overloadable)); + int g(int) { } // error: redeclaration of "g" must also 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. + +Query for this feature with ``__has_extension(attribute_overloadable)``. + + +release_capability (release_shared_capability, clang::release_capability, clang::release_shared_capability) +----------------------------------------------------------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +Marks a function as releasing a capability. + + +kernel +------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +``__attribute__((kernel))`` is used to mark a ``kernel`` function in +RenderScript. + +In RenderScript, ``kernel`` functions are used to express data-parallel +computations. The RenderScript runtime efficiently parallelizes ``kernel`` +functions to run on computational resources such as multi-core CPUs and GPUs. +See the RenderScript_ documentation for more information. + +.. _RenderScript: https://developer.android.com/guide/topics/renderscript/compute.html + + +target (gnu::target) +-------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +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. + +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. + + +try_acquire_capability (try_acquire_shared_capability, clang::try_acquire_capability, clang::try_acquire_shared_capability) +--------------------------------------------------------------------------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +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). + + +nodiscard, warn_unused_result, clang::warn_unused_result, gnu::warn_unused_result +--------------------------------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +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`. + +.. 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. + + +xray_always_instrument (clang::xray_always_instrument), xray_never_instrument (clang::xray_never_instrument) +------------------------------------------------------------------------------------------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +``__attribute__((xray_always_instrument))`` or ``[[clang::xray_always_instrument]]`` is used to mark member functions (in C++), methods (in Objective C), and free functions (in C, C++, and Objective C) to be instrumented with XRay. This will cause the function to always have space at the beginning and exit points to allow for runtime patching. + +Conversely, ``__attribute__((xray_never_instrument))`` or ``[[clang::xray_never_instrument]]`` will inhibit the insertion of these instrumentation points. + +If a function has neither of these attributes, they become subject to the XRay heuristics used to determine whether a function should be instrumented or otherwise. + + +Variable Attributes +=================== + + +dllexport (gnu::dllexport) +-------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","X","", "" + +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 + + +dllimport (gnu::dllimport) +-------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","X","", "" + +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. + +.. _dllimport: https://msdn.microsoft.com/en-us/library/3y1sfaz2.aspx + + +init_seg +-------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","", "X" + +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 + + +nodebug (gnu::nodebug) +---------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +The ``nodebug`` attribute allows you to suppress debugging information for a +function or method, or for a variable that is not a parameter or a non-static +data member. + + +nosvm +----- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +OpenCL 2.0 supports the optional ``__attribute__((nosvm))`` qualifier for +pointer variable. It informs the compiler that the pointer does not refer +to a shared virtual memory region. See OpenCL v2.0 s6.7.2 for details. + +Since it is not widely used and has been removed from OpenCL 2.1, it is ignored +by Clang. + + +pass_object_size +---------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +.. 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. + + +require_constant_initialization (clang::require_constant_initialization) +------------------------------------------------------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +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++. + +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. + +.. 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. + + +section (gnu::section, __declspec(allocate)) +-------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","X","", "" + +The ``section`` attribute allows you to specify a specific section a +global variable or function should be in after translation. + + +swiftcall (gnu::swiftcall) +-------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +The ``swiftcall`` attribute indicates that a function should be called +using the Swift calling convention for a function or function pointer. + +The lowering for the Swift calling convention, as described by the Swift +ABI documentation, occurs in multiple phases. The first, "high-level" +phase breaks down the formal parameters and results into innately direct +and indirect components, adds implicit paraameters for the generic +signature, and assigns the context and error ABI treatments to parameters +where applicable. The second phase breaks down the direct parameters +and results from the first phase and assigns them to registers or the +stack. The ``swiftcall`` convention only handles this second phase of +lowering; the C function type must accurately reflect the results +of the first phase, as follows: + +- Results classified as indirect by high-level lowering should be + represented as parameters with the ``swift_indirect_result`` attribute. + +- Results classified as direct by high-level lowering should be represented + as follows: + + - First, remove any empty direct results. + + - If there are no direct results, the C result type should be ``void``. + + - If there is one direct result, the C result type should be a type with + the exact layout of that result type. + + - If there are a multiple direct results, the C result type should be + a struct type with the exact layout of a tuple of those results. + +- Parameters classified as indirect by high-level lowering should be + represented as parameters of pointer type. + +- Parameters classified as direct by high-level lowering should be + omitted if they are empty types; otherwise, they should be represented + as a parameter type with a layout exactly matching the layout of the + Swift parameter type. + +- The context parameter, if present, should be represented as a trailing + parameter with the ``swift_context`` attribute. + +- The error result parameter, if present, should be represented as a + trailing parameter (always following a context parameter) with the + ``swift_error_result`` attribute. + +``swiftcall`` does not support variadic arguments or unprototyped functions. + +The parameter ABI treatment attributes are aspects of the function type. +A function type which which applies an ABI treatment attribute to a +parameter is a different type from an otherwise-identical function type +that does not. A single parameter may not have multiple ABI treatment +attributes. + +Support for this feature is target-dependent, although it should be +supported on every target that Swift supports. Query for this support +with ``__has_attribute(swiftcall)``. This implies support for the +``swift_context``, ``swift_error_result``, and ``swift_indirect_result`` +attributes. + + +swift_context (gnu::swift_context) +---------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +The ``swift_context`` attribute marks a parameter of a ``swiftcall`` +function as having the special context-parameter ABI treatment. + +This treatment generally passes the context value in a special register +which is normally callee-preserved. + +A ``swift_context`` parameter must either be the last parameter or must be +followed by a ``swift_error_result`` parameter (which itself must always be +the last parameter). + +A context parameter must have pointer or reference type. + + +swift_error_result (gnu::swift_error_result) +-------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +The ``swift_error_result`` attribute marks a parameter of a ``swiftcall`` +function as having the special error-result ABI treatment. + +This treatment generally passes the underlying error value in and out of +the function through a special register which is normally callee-preserved. +This is modeled in C by pretending that the register is addressable memory: + +- The caller appears to pass the address of a variable of pointer type. + The current value of this variable is copied into the register before + the call; if the call returns normally, the value is copied back into the + variable. + +- The callee appears to receive the address of a variable. This address + is actually a hidden location in its own stack, initialized with the + value of the register upon entry. When the function returns normally, + the value in that hidden location is written back to the register. + +A ``swift_error_result`` parameter must be the last parameter, and it must be +preceded by a ``swift_context`` parameter. + +A ``swift_error_result`` parameter must have type ``T**`` or ``T*&`` for some +type T. Note that no qualifiers are permitted on the intermediate level. + +It is undefined behavior if the caller does not pass a pointer or +reference to a valid object. + +The standard convention is that the error value itself (that is, the +value stored in the apparent argument) will be null upon function entry, +but this is not enforced by the ABI. + + +swift_indirect_result (gnu::swift_indirect_result) +-------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +The ``swift_indirect_result`` attribute marks a parameter of a ``swiftcall`` +function as having the special indirect-result ABI treatment. + +This treatment gives the parameter the target's normal indirect-result +ABI treatment, which may involve passing it differently from an ordinary +parameter. However, only the first indirect result will receive this +treatment. Furthermore, low-level lowering may decide that a direct result +must be returned indirectly; if so, this will take priority over the +``swift_indirect_result`` parameters. + +A ``swift_indirect_result`` parameter must either be the first parameter or +follow another ``swift_indirect_result`` parameter. + +A ``swift_indirect_result`` parameter must have type ``T*`` or ``T&`` for +some object type ``T``. If ``T`` is a complete type at the point of +definition of a function, it is undefined behavior if the argument +value does not point to storage of adequate size and alignment for a +value of type ``T``. + +Making indirect results explicit in the signature allows C functions to +directly construct objects into them without relying on language +optimizations like C++'s named return value optimization (NRVO). + + +tls_model (gnu::tls_model) +-------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +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. + + +thread +------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","X","", "" + +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. + + +maybe_unused, unused, gnu::unused +--------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +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); + } + + +Type Attributes +=============== + + +align_value +----------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +empty_bases +----------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","X","", "" + +The empty_bases attribute permits the compiler to utilize the +empty-base-optimization more frequently. +This attribute only applies to struct, class, and union types. +It is only supported when using the Microsoft C++ ABI. + + +flag_enum +--------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +lto_visibility_public (clang::lto_visibility_public) +---------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","X","","", "" + +See :doc:`LTOVisibility`. + + +layout_version +-------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","X","", "" + +The layout_version attribute requests that the compiler utilize the class +layout rules of a particular compiler version. +This attribute only applies to struct, class, and union types. +It is only supported when using the Microsoft C++ ABI. + + +__single_inhertiance, __multiple_inheritance, __virtual_inheritance +------------------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","X", "" + +This collection of keywords is enabled under ``-fms-extensions`` and controls +the pointer-to-member representation used on ``*-*-win32`` targets. + +The ``*-*-win32`` targets utilize a pointer-to-member representation which +varies in size and alignment depending on the definition of the underlying +class. + +However, this is problematic when a forward declaration is only available and +no definition has been made yet. In such cases, Clang is forced to utilize the +most general representation that is available to it. + +These keywords make it possible to use a pointer-to-member representation other +than the most general one regardless of whether or not the definition will ever +be present in the current translation unit. + +This family of keywords belong between the ``class-key`` and ``class-name``: + +.. code-block:: c++ + + struct __single_inheritance S; + int S::*i; + struct S {}; + +This keyword can be applied to class templates but only has an effect when used +on full specializations: + +.. code-block:: c++ + + template <typename T, typename U> struct __single_inheritance A; // warning: inheritance model ignored on primary template + template <typename T> struct __multiple_inheritance A<T, T>; // warning: inheritance model ignored on partial specialization + template <> struct __single_inheritance A<int, float>; + +Note that choosing an inheritance model less general than strictly necessary is +an error: + +.. code-block:: c++ + + struct __multiple_inheritance S; // error: inheritance model does not match definition + int S::*i; + struct S {}; + + +novtable +-------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","X","", "" + +This attribute can be added to a class declaration or definition to signal to +the compiler that constructors and destructors will not reference the virtual +function table. It is only supported when using the Microsoft C++ ABI. + + +objc_subclassing_restricted +--------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +This attribute can be added to an Objective-C ``@interface`` declaration to +ensure that this class cannot be subclassed. + + +transparent_union (gnu::transparent_union) +------------------------------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +This attribute can be applied to a union to change the behaviour of calls to +functions that have an argument with a transparent union type. The compiler +behaviour is changed in the following manner: + +- A value whose type is any member of the transparent union can be passed as an + argument without the need to cast that value. + +- The argument is passed to the function using the calling convention of the + first member of the transparent union. Consequently, all the members of the + transparent union should have the same calling convention as its first member. + +Transparent unions are not supported in C++. + + +Statement Attributes +==================== + + +fallthrough, clang::fallthrough +------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","X","","", "" + +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(); + } + + +#pragma clang loop +------------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","", "X" + +The ``#pragma clang loop`` directive allows loop optimization hints to be +specified for the subsequent loop. The directive allows vectorization, +interleaving, and unrolling to be enabled or disabled. Vector width as well +as interleave and unrolling count can be manually specified. See +`language extensions +<http://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations>`_ +for details. + + +#pragma unroll, #pragma nounroll +-------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","", "X" + +Loop unrolling optimization hints can be specified with ``#pragma unroll`` and +``#pragma nounroll``. The pragma is placed immediately before a for, while, +do-while, or c++11 range-based for loop. + +Specifying ``#pragma unroll`` without a parameter directs the loop unroller to +attempt to fully unroll the loop if the trip count is known at compile time and +attempt to partially unroll the loop if the trip count is not known at compile +time: + +.. code-block:: c++ + + #pragma unroll + for (...) { + ... + } + +Specifying the optional parameter, ``#pragma unroll _value_``, directs the +unroller to unroll the loop ``_value_`` times. The parameter may optionally be +enclosed in parentheses: + +.. code-block:: c++ + + #pragma unroll 16 + for (...) { + ... + } + + #pragma unroll(16) + for (...) { + ... + } + +Specifying ``#pragma nounroll`` indicates that the loop should not be unrolled: + +.. code-block:: c++ + + #pragma nounroll + for (...) { + ... + } + +``#pragma unroll`` and ``#pragma unroll _value_`` have identical semantics to +``#pragma clang loop unroll(full)`` and +``#pragma clang loop unroll_count(_value_)`` respectively. ``#pragma nounroll`` +is equivalent to ``#pragma clang loop unroll(disable)``. See +`language extensions +<http://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations>`_ +for further details including limitations of the unroll hints. + + +__read_only, __write_only, __read_write (read_only, write_only, read_write) +--------------------------------------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","X", "" + +The access qualifiers must be used with image object arguments or pipe arguments +to declare if they are being read or written by a kernel or function. + +The read_only/__read_only, write_only/__write_only and read_write/__read_write +names are reserved for use as access qualifiers and shall not be used otherwise. + +.. code-block:: c + + kernel void + foo (read_only image2d_t imageA, + write_only image2d_t imageB) { + ... + } + +In the above example imageA is a read-only 2D image object, and imageB is a +write-only 2D image object. + +The read_write (or __read_write) qualifier can not be used with pipe. + +More details can be found in the OpenCL C language Spec v2.0, Section 6.6. + + +__attribute__((opencl_unroll_hint)) +----------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +The opencl_unroll_hint attribute qualifier can be used to specify that a loop +(for, while and do loops) can be unrolled. This attribute qualifier can be +used to specify full unrolling or partial unrolling by a specified amount. +This is a compiler hint and the compiler may ignore this directive. See +`OpenCL v2.0 <https://www.khronos.org/registry/cl/specs/opencl-2.0.pdf>`_ +s6.11.5 for details. + + +Type Safety Checking +==================== +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); + +argument_with_type_tag +---------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +pointer_with_type_tag +--------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +type_tag_for_datatype +--------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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 + + +AMD GPU Attributes +================== + + +amdgpu_flat_work_group_size +--------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +amdgpu_num_sgpr +--------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +amdgpu_num_vgpr +--------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +amdgpu_waves_per_eu +------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +Calling Conventions +=================== +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. + +fastcall (gnu::fastcall, __fastcall, _fastcall) +----------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","X", "" + +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 + + +ms_abi (gnu::ms_abi) +-------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +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. + + +pcs (gnu::pcs) +-------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +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". + + +preserve_all +------------ +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +On X86-64 and AArch64 targets, this attribute changes the calling convention of +a function. The ``preserve_all`` calling convention attempts to make the code +in the caller even less intrusive than the ``preserve_most`` calling convention. +This calling convention also behaves identical to the ``C`` calling convention +on how arguments and return values are passed, but it uses a different set of +caller/callee-saved registers. This removes the burden of saving and +recovering a large register set before and after the call in the caller. If +the arguments are passed in callee-saved registers, then they will be +preserved by the callee across the call. This doesn't apply for values +returned in callee-saved registers. + +- On X86-64 the callee preserves all general purpose registers, except for + R11. R11 can be used as a scratch register. Furthermore it also preserves + all floating-point registers (XMMs/YMMs). + +The idea behind this convention is to support calls to runtime functions +that don't need to call out to any other functions. + +This calling convention, like the ``preserve_most`` calling convention, will be +used by a future version of the Objective-C runtime and should be considered +experimental at this time. + + +preserve_most +------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +On X86-64 and AArch64 targets, this attribute changes the calling convention of +a function. The ``preserve_most`` calling convention attempts to make the code +in the caller as unintrusive as possible. This convention behaves identically +to the ``C`` calling convention on how arguments and return values are passed, +but it uses a different set of caller/callee-saved registers. This alleviates +the burden of saving and recovering a large register set before and after the +call in the caller. If the arguments are passed in callee-saved registers, +then they will be preserved by the callee across the call. This doesn't +apply for values returned in callee-saved registers. + +- On X86-64 the callee preserves all general purpose registers, except for + R11. R11 can be used as a scratch register. Floating-point registers + (XMMs/YMMs) are not preserved and need to be saved by the caller. + +The idea behind this convention is to support calls to runtime functions +that have a hot path and a cold path. The hot path is usually a small piece +of code that doesn't use many registers. The cold path might need to call out to +another function and therefore only needs to preserve the caller-saved +registers, which haven't already been saved by the caller. The +`preserve_most` calling convention is very similar to the ``cold`` calling +convention in terms of caller/callee-saved registers, but they are used for +different types of function calls. ``coldcc`` is for function calls that are +rarely executed, whereas `preserve_most` function calls are intended to be +on the hot path and definitely executed a lot. Furthermore ``preserve_most`` +doesn't prevent the inliner from inlining the function call. + +This calling convention will be used by a future version of the Objective-C +runtime and should therefore still be considered experimental at this time. +Although this convention was created to optimize certain runtime calls to +the Objective-C runtime, it is not limited to this runtime and might be used +by other runtimes in the future too. The current implementation only +supports X86-64 and AArch64, but the intention is to support more architectures +in the future. + + +regcall (gnu::regcall, __regcall) +--------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","X", "" + +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 + + +regparm (gnu::regparm) +---------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +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. + + +stdcall (gnu::stdcall, __stdcall, _stdcall) +------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","X", "" + +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 + + +thiscall (gnu::thiscall, __thiscall, _thiscall) +----------------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","X", "" + +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 + + +vectorcall (__vectorcall, _vectorcall) +-------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","X", "" + +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 + + +Consumed Annotation Checking +============================ +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.** + +callable_when +------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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")))`` + + +consumable +---------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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``. + + +param_typestate +--------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +return_typestate +---------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +set_typestate +------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +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. + + +test_typestate +-------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","","","", "" + +Use ``__attribute__((test_typestate(tested_state)))`` to indicate that a method +returns true if the object is in the specified state.. + + +OpenCL Address Spaces +===================== +The address space qualifier may be used to specify the region of memory that is +used to allocate the object. OpenCL supports the following address spaces: +__generic(generic), __global(global), __local(local), __private(private), +__constant(constant). + + .. code-block:: c + + __constant int c = ...; + + __generic int* foo(global int* g) { + __local int* l; + private int p; + ... + return l; + } + +More details can be found in the OpenCL C language Spec v2.0, Section 6.5. + +constant (__constant) +--------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","X", "" + +The constant address space attribute signals that an object is located in +a constant (non-modifiable) memory region. It is available to all work items. +Any type can be annotated with the constant address space attribute. Objects +with the constant address space qualifier can be declared in any scope and must +have an initializer. + + +generic (__generic) +------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","X", "" + +The generic address space attribute is only available with OpenCL v2.0 and later. +It can be used with pointer types. Variables in global and local scope and +function parameters in non-kernel functions can have the generic address space +type attribute. It is intended to be a placeholder for any other address space +except for '__constant' in OpenCL code which can be used with multiple address +spaces. + + +global (__global) +----------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","X", "" + +The global address space attribute specifies that an object is allocated in +global memory, which is accessible by all work items. The content stored in this +memory area persists between kernel executions. Pointer types to the global +address space are allowed as function parameters or local variables. Starting +with OpenCL v2.0, the global address space can be used with global (program +scope) variables and static local variable as well. + + +local (__local) +--------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","X", "" + +The local address space specifies that an object is allocated in the local (work +group) memory area, which is accessible to all work items in the same work +group. The content stored in this memory region is not accessible after +the kernel execution ends. In a kernel function scope, any variable can be in +the local address space. In other scopes, only pointer types to the local address +space are allowed. Local address space variables cannot have an initializer. + + +private (__private) +------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","X", "" + +The private address space specifies that an object is allocated in the private +(work item) memory. Other work items cannot access the same memory area and its +content is destroyed after work item execution ends. Local variables can be +declared in the private address space. Function arguments are always in the +private address space. Kernel function arguments of a pointer or an array type +cannot point to the private address space. + + +Nullability Attributes +====================== +Whether a particular pointer may be "null" is an important concern when working with pointers in the C family of languages. The various nullability attributes indicate whether a particular pointer can be null or not, which makes APIs more expressive and can help static analysis tools identify bugs involving null pointers. Clang supports several kinds of nullability attributes: the ``nonnull`` and ``returns_nonnull`` attributes indicate which function or method parameters and result types can never be null, while nullability type qualifiers indicate which pointer types can be null (``_Nullable``) or cannot be null (``_Nonnull``). + +The nullability (type) qualifiers express whether a value of a given pointer type can be null (the ``_Nullable`` qualifier), doesn't have a defined meaning for null (the ``_Nonnull`` qualifier), or for which the purpose of null is unclear (the ``_Null_unspecified`` qualifier). Because nullability qualifiers are expressed within the type system, they are more general than the ``nonnull`` and ``returns_nonnull`` attributes, allowing one to express (for example) a nullable pointer to an array of nonnull pointers. Nullability qualifiers are written to the right of the pointer to which they apply. For example: + + .. code-block:: c + + // No meaningful result when 'ptr' is null (here, it happens to be undefined behavior). + int fetch(int * _Nonnull ptr) { return *ptr; } + + // 'ptr' may be null. + int fetch_or_zero(int * _Nullable ptr) { + return ptr ? *ptr : 0; + } + + // A nullable pointer to non-null pointers to const characters. + const char *join_strings(const char * _Nonnull * _Nullable strings, unsigned n); + +In Objective-C, there is an alternate spelling for the nullability qualifiers that can be used in Objective-C methods and properties using context-sensitive, non-underscored keywords. For example: + + .. code-block:: objective-c + + @interface NSView : NSResponder + - (nullable NSView *)ancestorSharedWithView:(nonnull NSView *)aView; + @property (assign, nullable) NSView *superview; + @property (readonly, nonnull) NSArray *subviews; + @end + +nonnull (gnu::nonnull) +---------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +The ``nonnull`` attribute indicates that some function parameters must not be null, and can be used in several different ways. It's original usage (`from GCC <https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes>`_) is as a function (or Objective-C method) attribute that specifies which parameters of the function are nonnull in a comma-separated list. For example: + + .. code-block:: c + + extern void * my_memcpy (void *dest, const void *src, size_t len) + __attribute__((nonnull (1, 2))); + +Here, the ``nonnull`` attribute indicates that parameters 1 and 2 +cannot have a null value. Omitting the parenthesized list of parameter indices means that all parameters of pointer type cannot be null: + + .. code-block:: c + + extern void * my_memcpy (void *dest, const void *src, size_t len) + __attribute__((nonnull)); + +Clang also allows the ``nonnull`` attribute to be placed directly on a function (or Objective-C method) parameter, eliminating the need to specify the parameter index ahead of type. For example: + + .. code-block:: c + + extern void * my_memcpy (void *dest __attribute__((nonnull)), + const void *src __attribute__((nonnull)), size_t len); + +Note that the ``nonnull`` attribute indicates that passing null to a non-null parameter is undefined behavior, which the optimizer may take advantage of to, e.g., remove null checks. The ``_Nonnull`` type qualifier indicates that a pointer cannot be null in a more general manner (because it is part of the type system) and does not imply undefined behavior, making it more widely applicable. + + +returns_nonnull (gnu::returns_nonnull) +-------------------------------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "X","X","","", "" + +The ``returns_nonnull`` attribute indicates that a particular function (or Objective-C method) always returns a non-null pointer. For example, a particular system ``malloc`` might be defined to terminate a process when memory is not available rather than returning a null pointer: + + .. code-block:: c + + extern void * malloc (size_t size) __attribute__((returns_nonnull)); + +The ``returns_nonnull`` attribute implies that returning a null pointer is undefined behavior, which the optimizer may take advantage of. The ``_Nonnull`` type qualifier indicates that a pointer cannot be null in a more general manner (because it is part of the type system) and does not imply undefined behavior, making it more widely applicable + + +_Nonnull +-------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","X", "" + +The ``_Nonnull`` nullability qualifier indicates that null is not a meaningful value for a value of the ``_Nonnull`` pointer type. For example, given a declaration such as: + + .. code-block:: c + + int fetch(int * _Nonnull ptr); + +a caller of ``fetch`` should not provide a null value, and the compiler will produce a warning if it sees a literal null value passed to ``fetch``. Note that, unlike the declaration attribute ``nonnull``, the presence of ``_Nonnull`` does not imply that passing null is undefined behavior: ``fetch`` is free to consider null undefined behavior or (perhaps for backward-compatibility reasons) defensively handle null. + + +_Null_unspecified +----------------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","X", "" + +The ``_Null_unspecified`` nullability qualifier indicates that neither the ``_Nonnull`` nor ``_Nullable`` qualifiers make sense for a particular pointer type. It is used primarily to indicate that the role of null with specific pointers in a nullability-annotated header is unclear, e.g., due to overly-complex implementations or historical factors with a long-lived API. + + +_Nullable +--------- +.. csv-table:: Supported Syntaxes + :header: "GNU", "C++11", "__declspec", "Keyword", "Pragma" + + "","","","X", "" + +The ``_Nullable`` nullability qualifier indicates that a value of the ``_Nullable`` pointer type can be null. For example, given: + + .. code-block:: c + + int fetch_or_zero(int * _Nullable ptr); + +a caller of ``fetch_or_zero`` can provide null. + + diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst index b15d9a5a147c..5b035cd3320f 100644 --- a/docs/ReleaseNotes.rst +++ b/docs/ReleaseNotes.rst @@ -31,11 +31,6 @@ the latest release, please check out the main please see the `Clang Web Site <http://clang.llvm.org>`_ or the `LLVM Web Site <http://llvm.org>`_. -Note that if you are reading this file from a Subversion checkout or the -main Clang web page, this document applies to the *next* release, not -the current one. To see the release notes for a specific release, please -see the `releases page <http://llvm.org/releases/>`_. - What's New in Clang 4.0.0? ========================== @@ -65,6 +60,8 @@ The option -Og has been added to optimize the debugging experience. For now, this option is exactly the same as -O1. However, in the future, some other optimizations might be enabled or disabled. +The option -MJ has been added to simplify adding JSON compilation +database output into existing build systems. The option .... diff --git a/docs/UsersManual.rst b/docs/UsersManual.rst index 96b7ac95cb5d..3adbfe059f89 100644 --- a/docs/UsersManual.rst +++ b/docs/UsersManual.rst @@ -41,6 +41,7 @@ specific section: variants depending on base language. - :ref:`C++ Language <cxx>` - :ref:`Objective C++ Language <objcxx>` +- :ref:`OpenCL C Language <opencl>`: v1.0, v1.1, v1.2, v2.0. In addition to these base languages and their dialects, Clang supports a broad variety of language extensions, which are documented in the @@ -1973,6 +1974,365 @@ Controlling implementation limits is provided or target does not support TLS, code generation for threadprivate variables relies on OpenMP runtime library. +.. _opencl: + +OpenCL Features +=============== + +Clang can be used to compile OpenCL kernels for execution on a device +(e.g. GPU). It is possible to compile the kernel into a binary (e.g. for AMD or +Nvidia targets) that can be uploaded to run directly on a device (e.g. using +`clCreateProgramWithBinary +<https://www.khronos.org/registry/OpenCL/specs/opencl-1.1.pdf#111>`_) or +into generic bitcode files loadable into other toolchains. + +Compiling to a binary using the default target from the installation can be done +as follows: + + .. code-block:: console + + $ echo "kernel void k(){}" > test.cl + $ clang test.cl + +Compiling for a specific target can be done by specifying the triple corresponding +to the target, for example: + + .. code-block:: console + + $ clang -target nvptx64-unknown-unknown test.cl + $ clang -target amdgcn-amd-amdhsa-opencl test.cl + +Compiling to bitcode can be done as follows: + + .. code-block:: console + + $ clang -c -emit-llvm test.cl + +This will produce a generic test.bc file that can be used in vendor toolchains +to perform machine code generation. + +Clang currently supports OpenCL C language standards up to v2.0. + +OpenCL Specific Options +----------------------- + +Most of the OpenCL build options from `the specification v2.0 section 5.8.4 +<https://www.khronos.org/registry/cl/specs/opencl-2.0.pdf#200>`_ are available. + +Examples: + + .. code-block:: console + + $ clang -cl-std=CL2.0 -cl-single-precision-constant test.cl + +Some extra options are available to support special OpenCL features. + +.. option:: -finclude-default-header + +Loads standard includes during compilations. By default OpenCL headers are not +loaded and therefore standard library includes are not available. To load them +automatically a flag has been added to the frontend (see also :ref:`the section +on the OpenCL Header <opencl_header>`): + + .. code-block:: console + + $ clang -Xclang -finclude-default-header test.cl + +Alternatively ``-include`` or ``-I`` followed by the path to the header location +can be given manually. + + .. code-block:: console + + $ clang -I<path to clang>/lib/Headers/opencl-c.h test.cl + +In this case the kernel code should contain ``#include <opencl-c.h>`` just as a +regular C include. + +.. option:: -cl-ext + +Disables support of OpenCL extensions. All OpenCL targets provide a list +of extensions that they support. Clang allows to amend this using the ``-cl-ext`` +flag with a comma-separated list of extensions prefixed with ``'+'`` or ``'-'``. +The syntax: ``-cl-ext=<(['-'|'+']<extension>[,])+>``, where extensions +can be either one of `the OpenCL specification extensions +<https://www.khronos.org/registry/cl/sdk/2.0/docs/man/xhtml/EXTENSION.html>`_ +or any known vendor extension. Alternatively, ``'all'`` can be used to enable +or disable all known extensions. +Example disabling double support for the 64-bit SPIR target: + + .. code-block:: console + + $ clang -cc1 -triple spir64-unknown-unknown -cl-ext=-cl_khr_fp64 test.cl + +Enabling all extensions except double support in R600 AMD GPU can be done using: + + .. code-block:: console + + $ clang -cc1 -triple r600-unknown-unknown -cl-ext=-all,+cl_khr_fp16 test.cl + +.. _opencl_fake_address_space_map: + +.. option:: -ffake-address-space-map + +Overrides the target address space map with a fake map. +This allows adding explicit address space IDs to the bitcode for non-segmented +memory architectures that don't have separate IDs for each of the OpenCL +logical address spaces by default. Passing ``-ffake-address-space-map`` will +add/override address spaces of the target compiled for with the following values: +``1-global``, ``2-constant``, ``3-local``, ``4-generic``. The private address +space is represented by the absence of an address space attribute in the IR (see +also :ref:`the section on the address space attribute <opencl_addrsp>`). + + .. code-block:: console + + $ clang -ffake-address-space-map test.cl + +Some other flags used for the compilation for C can also be passed while +compiling for OpenCL, examples: ``-c``, ``-O<1-4|s>``, ``-o``, ``-emit-llvm``, etc. + +OpenCL Targets +-------------- + +OpenCL targets are derived from the regular Clang target classes. The OpenCL +specific parts of the target representation provide address space mapping as +well as a set of supported extensions. + +Specific Targets +^^^^^^^^^^^^^^^^ + +There is a set of concrete HW architectures that OpenCL can be compiled for. + +- For AMD target: + + .. code-block:: console + + $ clang -target amdgcn-amd-amdhsa-opencl test.cl + +- For Nvidia architectures: + + .. code-block:: console + + $ clang -target nvptx64-unknown-unknown test.cl + + +Generic Targets +^^^^^^^^^^^^^^^ + +- SPIR is available as a generic target to allow portable bitcode to be produced + that can be used across GPU toolchains. The implementation follows `the SPIR + specification <https://www.khronos.org/spir>`_. There are two flavors + available for 32 and 64 bits. + + .. code-block:: console + + $ clang -target spir-unknown-unknown test.cl + $ clang -target spir64-unknown-unknown test.cl + + All known OpenCL extensions are supported in the SPIR targets. Clang will + generate SPIR v1.2 compatible IR for OpenCL versions up to 2.0 and SPIR v2.0 + for OpenCL v2.0. + +- x86 is used by some implementations that are x86 compatible and currently + remains for backwards compatibility (with older implementations prior to + SPIR target support). For "non-SPMD" targets which cannot spawn multiple + work-items on the fly using hardware, which covers practically all non-GPU + devices such as CPUs and DSPs, additional processing is needed for the kernels + to support multiple work-item execution. For this, a 3rd party toolchain, + such as for example `POCL <http://portablecl.org/>`_, can be used. + + This target does not support multiple memory segments and, therefore, the fake + address space map can be added using the :ref:`-ffake-address-space-map + <opencl_fake_address_space_map>` flag. + +.. _opencl_header: + +OpenCL Header +------------- + +By default Clang will not include standard headers and therefore OpenCL builtin +functions and some types (i.e. vectors) are unknown. The default CL header is, +however, provided in the Clang installation and can be enabled by passing the +``-finclude-default-header`` flag to the Clang frontend. + + .. code-block:: console + + $ echo "bool is_wg_uniform(int i){return get_enqueued_local_size(i)==get_local_size(i);}" > test.cl + $ clang -Xclang -finclude-default-header -cl-std=CL2.0 test.cl + +Because the header is very large and long to parse, PCH (:doc:`PCHInternals`) +and modules (:doc:`Modules`) are used internally to improve the compilation +speed. + +To enable modules for OpenCL: + + .. code-block:: console + + $ clang -target spir-unknown-unknown -c -emit-llvm -Xclang -finclude-default-header -fmodules -fimplicit-module-maps -fmodules-cache-path=<path to the generated module> test.cl + +OpenCL Metadata +--------------- + +Clang uses metadata to provide additional OpenCL semantics in IR needed for +backends and OpenCL runtime. + +Each kernel will have function metadata attached to it, specifying the arguments. +Kernel argument metadata is used to provide source level information for querying +at runtime, for example using the `clGetKernelArgInfo +<https://www.khronos.org/registry/OpenCL/specs/opencl-1.2.pdf#167>`_ +call. + +Note that ``-cl-kernel-arg-info`` enables more information about the original CL +code to be added e.g. kernel parameter names will appear in the OpenCL metadata +along with other information. + +The IDs used to encode the OpenCL's logical address spaces in the argument info +metadata follows the SPIR address space mapping as defined in the SPIR +specification `section 2.2 +<https://www.khronos.org/registry/spir/specs/spir_spec-2.0.pdf#18>`_ + +OpenCL-Specific Attributes +-------------------------- + +OpenCL support in Clang contains a set of attribute taken directly from the +specification as well as additional attributes. + +See also :doc:`AttributeReference`. + +nosvm +^^^^^ + +Clang supports this attribute to comply to OpenCL v2.0 conformance, but it +does not have any effect on the IR. For more details reffer to the specification +`section 6.7.2 +<https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#49>`_ + + +opencl_hint_unroll +^^^^^^^^^^^^^^^^^^ + +The implementation of this feature mirrors the unroll hint for C. +More details on the syntax can be found in the specification +`section 6.11.5 +<https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#61>`_ + +convergent +^^^^^^^^^^ + +To make sure no invalid optimizations occur for single program multiple data +(SPMD) / single instruction multiple thread (SIMT) Clang provides attributes that +can be used for special functions that have cross work item semantics. +An example is the subgroup operations such as `intel_sub_group_shuffle +<https://www.khronos.org/registry/cl/extensions/intel/cl_intel_subgroups.txt>`_ + + .. code-block:: c + + // Define custom my_sub_group_shuffle(data, c) + // that makes use of intel_sub_group_shuffle + r1 = … + if (r0) r1 = computeA(); + // Shuffle data from r1 into r3 + // of threads id r2. + r3 = my_sub_group_shuffle(r1, r2); + if (r0) r3 = computeB(); + +with non-SPMD semantics this is optimized to the following equivalent code: + + .. code-block:: c + + r1 = … + if (!r0) + // Incorrect functionality! The data in r1 + // have not been computed by all threads yet. + r3 = my_sub_group_shuffle(r1, r2); + else { + r1 = computeA(); + r3 = my_sub_group_shuffle(r1, r2); + r3 = computeB(); + } + +Declaring the function ``my_sub_group_shuffle`` with the convergent attribute +would prevent this: + + .. code-block:: c + + my_sub_group_shuffle() __attribute__((convergent)); + +Using ``convergent`` guarantees correct execution by keeping CFG equivalence +wrt operations marked as ``convergent``. CFG ``G´`` is equivalent to ``G`` wrt +node ``Ni`` : ``iff ∀ Nj (i≠j)`` domination and post-domination relations with +respect to ``Ni`` remain the same in both ``G`` and ``G´``. + +noduplicate +^^^^^^^^^^^ + +``noduplicate`` is more restrictive with respect to optimizations than +``convergent`` because a convergent function only preserves CFG equivalence. +This allows some optimizations to happen as long as the control flow remains +unmodified. + + .. code-block:: c + + for (int i=0; i<4; i++) + my_sub_group_shuffle() + +can be modified to: + + .. code-block:: c + + my_sub_group_shuffle(); + my_sub_group_shuffle(); + my_sub_group_shuffle(); + my_sub_group_shuffle(); + +while using ``noduplicate`` would disallow this. Also ``noduplicate`` doesn't +have the same safe semantics of CFG as ``convergent`` and can cause changes in +CFG that modify semantics of the original program. + +``noduplicate`` is kept for backwards compatibility only and it considered to be +deprecated for future uses. + +.. _opencl_addrsp: + +address_space +^^^^^^^^^^^^^ + +Clang has arbitrary address space support using the ``address_space(N)`` +attribute, where ``N`` is an integer number in the range ``0`` to ``16777215`` +(``0xffffffu``). + +An OpenCL implementation provides a list of standard address spaces using +keywords: ``private``, ``local``, ``global``, and ``generic``. In the AST and +in the IR local, global, or generic will be represented by the address space +attribute with the corresponding unique number. Note that private does not have +any corresponding attribute added and, therefore, is represented by the absence +of an address space number. The specific IDs for an address space do not have to +match between the AST and the IR. Typically in the AST address space numbers +represent logical segments while in the IR they represent physical segments. +Therefore, machines with flat memory segments can map all AST address space +numbers to the same physical segment ID or skip address space attribute +completely while generating the IR. However, if the address space information +is needed by the IR passes e.g. to improve alias analysis, it is recommended +to keep it and only lower to reflect physical memory segments in the late +machine passes. + +OpenCL builtins +--------------- + +There are some standard OpenCL functions that are implemented as Clang builtins: + +- All pipe functions from `section 6.13.16.2/6.13.16.3 + <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#160>`_ of + the OpenCL v2.0 kernel language specification. ` + +- Address space qualifier conversion functions ``to_global``/``to_local``/``to_private`` + from `section 6.13.9 + <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#101>`_. + +- All the ``enqueue_kernel`` functions from `section 6.13.17.1 + <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#164>`_ and + enqueue query functions from `section 6.13.17.5 + <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#171>`_. + .. _target_features: Target-Specific Features and Limitations @@ -2102,7 +2462,7 @@ Clang expects the GCC executable "gcc.exe" compiled for clang-cl ======== -clang-cl is an alternative command-line interface to Clang driver, designed for +clang-cl is an alternative command-line interface to Clang, designed for compatibility with the Visual C++ compiler, cl.exe. To enable clang-cl to find system headers, libraries, and the linker when run @@ -2110,7 +2470,7 @@ from the command-line, it should be executed inside a Visual Studio Native Tools Command Prompt or a regular Command Prompt where the environment has been set up using e.g. `vcvars32.bat <http://msdn.microsoft.com/en-us/library/f2ccy3wt.aspx>`_. -clang-cl can also be used from inside Visual Studio by using an LLVM Platform +clang-cl can also be used from inside Visual Studio by using an LLVM Platform Toolset. Command-Line Options @@ -2145,116 +2505,125 @@ Execute ``clang-cl /?`` to see a list of supported options: :: CL.EXE COMPATIBILITY OPTIONS: - /? Display available options - /arch:<value> Set architecture for code generation - /Brepro- Emit an object file which cannot be reproduced over time - /Brepro Emit an object file which can be reproduced over time - /C Don't discard comments when preprocessing - /c Compile only - /D <macro[=value]> Define macro - /EH<value> Exception handling model - /EP Disable linemarker output and preprocess to stdout - /E Preprocess to stdout - /fallback Fall back to cl.exe if clang-cl fails to compile - /FA Output assembly code file during compilation - /Fa<file or directory> Output assembly code to this file during compilation (with /FA) - /Fe<file or directory> Set output executable file or directory (ends in / or \) - /FI <value> Include file before parsing - /Fi<file> Set preprocess output file name (with /P) - /Fo<file or directory> Set output object file, or directory (ends in / or \) (with /c) + /? Display available options + /arch:<value> Set architecture for code generation + /Brepro- Emit an object file which cannot be reproduced over time + /Brepro Emit an object file which can be reproduced over time + /C Don't discard comments when preprocessing + /c Compile only + /D <macro[=value]> Define macro + /EH<value> Exception handling model + /EP Disable linemarker output and preprocess to stdout + /execution-charset:<value> + Runtime encoding, supports only UTF-8 + /E Preprocess to stdout + /fallback Fall back to cl.exe if clang-cl fails to compile + /FA Output assembly code file during compilation + /Fa<file or directory> Output assembly code to this file during compilation (with /FA) + /Fe<file or directory> Set output executable file or directory (ends in / or \) + /FI <value> Include file before parsing + /Fi<file> Set preprocess output file name (with /P) + /Fo<file or directory> Set output object file, or directory (ends in / or \) (with /c) /fp:except- /fp:except /fp:fast /fp:precise /fp:strict - /Fp<filename> Set pch filename (with /Yc and /Yu) - /GA Assume thread-local variables are defined in the executable - /Gd Set __cdecl as a default calling convention - /GF- Disable string pooling - /GR- Disable emission of RTTI data - /GR Enable emission of RTTI data - /Gr Set __fastcall as a default calling convention - /GS- Disable buffer security check - /GS Enable buffer security check - /Gs<value> Set stack probe size - /Gv Set __vectorcall as a default calling convention - /Gw- Don't put each data item in its own section - /Gw Put each data item in its own section - /GX- Enable exception handling - /GX Enable exception handling - /Gy- Don't put each function in its own section - /Gy Put each function in its own section - /Gz Set __stdcall as a default calling convention - /help Display available options - /imsvc <dir> Add directory to system include search path, as if part of %INCLUDE% - /I <dir> Add directory to include search path - /J Make char type unsigned - /LDd Create debug DLL - /LD Create DLL - /link <options> Forward options to the linker - /MDd Use DLL debug run-time - /MD Use DLL run-time - /MTd Use static debug run-time - /MT Use static run-time - /Od Disable optimization - /Oi- Disable use of builtin functions - /Oi Enable use of builtin functions - /Os Optimize for size - /Ot Optimize for speed - /O<value> Optimization level - /o <file or directory> Set output file or directory (ends in / or \) - /P Preprocess to file - /Qvec- Disable the loop vectorization passes - /Qvec Enable the loop vectorization passes - /showIncludes Print info about included files to stderr - /std:<value> Language standard to compile for - /TC Treat all source files as C - /Tc <filename> Specify a C source file - /TP Treat all source files as C++ - /Tp <filename> Specify a C++ source file - /U <macro> Undefine macro - /vd<value> Control vtordisp placement - /vmb Use a best-case representation method for member pointers - /vmg Use a most-general representation for member pointers - /vmm Set the default most-general representation to multiple inheritance - /vms Set the default most-general representation to single inheritance - /vmv Set the default most-general representation to virtual inheritance - /volatile:iso Volatile loads and stores have standard semantics - /volatile:ms Volatile loads and stores have acquire and release semantics - /W0 Disable all warnings - /W1 Enable -Wall - /W2 Enable -Wall - /W3 Enable -Wall - /W4 Enable -Wall and -Wextra - /Wall Enable -Wall and -Wextra - /WX- Do not treat warnings as errors - /WX Treat warnings as errors - /w Disable all warnings - /Y- Disable precompiled headers, overrides /Yc and /Yu - /Yc<filename> Generate a pch file for all code up to and including <filename> - /Yu<filename> Load a pch file and use it instead of all code up to and including <filename> - /Z7 Enable CodeView debug information in object files - /Zc:sizedDealloc- Disable C++14 sized global deallocation functions - /Zc:sizedDealloc Enable C++14 sized global deallocation functions - /Zc:strictStrings Treat string literals as const - /Zc:threadSafeInit- Disable thread-safe initialization of static variables - /Zc:threadSafeInit Enable thread-safe initialization of static variables - /Zc:trigraphs- Disable trigraphs (default) - /Zc:trigraphs Enable trigraphs - /Zd Emit debug line number tables only - /Zi Alias for /Z7. Does not produce PDBs. - /Zl Don't mention any default libraries in the object file - /Zp Set the default maximum struct packing alignment to 1 - /Zp<value> Specify the default maximum struct packing alignment - /Zs Syntax-check only + /Fp<filename> Set pch filename (with /Yc and /Yu) + /GA Assume thread-local variables are defined in the executable + /Gd Set __cdecl as a default calling convention + /GF- Disable string pooling + /GR- Disable emission of RTTI data + /GR Enable emission of RTTI data + /Gr Set __fastcall as a default calling convention + /GS- Disable buffer security check + /GS Enable buffer security check + /Gs<value> Set stack probe size + /Gv Set __vectorcall as a default calling convention + /Gw- Don't put each data item in its own section + /Gw Put each data item in its own section + /GX- Enable exception handling + /GX Enable exception handling + /Gy- Don't put each function in its own section + /Gy Put each function in its own section + /Gz Set __stdcall as a default calling convention + /help Display available options + /imsvc <dir> Add directory to system include search path, as if part of %INCLUDE% + /I <dir> Add directory to include search path + /J Make char type unsigned + /LDd Create debug DLL + /LD Create DLL + /link <options> Forward options to the linker + /MDd Use DLL debug run-time + /MD Use DLL run-time + /MTd Use static debug run-time + /MT Use static run-time + /Od Disable optimization + /Oi- Disable use of builtin functions + /Oi Enable use of builtin functions + /Os Optimize for size + /Ot Optimize for speed + /O<value> Optimization level + /o <file or directory> Set output file or directory (ends in / or \) + /P Preprocess to file + /Qvec- Disable the loop vectorization passes + /Qvec Enable the loop vectorization passes + /showIncludes Print info about included files to stderr + /source-charset:<value> Source encoding, supports only UTF-8 + /std:<value> Language standard to compile for + /TC Treat all source files as C + /Tc <filename> Specify a C source file + /TP Treat all source files as C++ + /Tp <filename> Specify a C++ source file + /utf-8 Set source and runtime encoding to UTF-8 (default) + /U <macro> Undefine macro + /vd<value> Control vtordisp placement + /vmb Use a best-case representation method for member pointers + /vmg Use a most-general representation for member pointers + /vmm Set the default most-general representation to multiple inheritance + /vms Set the default most-general representation to single inheritance + /vmv Set the default most-general representation to virtual inheritance + /volatile:iso Volatile loads and stores have standard semantics + /volatile:ms Volatile loads and stores have acquire and release semantics + /W0 Disable all warnings + /W1 Enable -Wall + /W2 Enable -Wall + /W3 Enable -Wall + /W4 Enable -Wall and -Wextra + /Wall Enable -Wall and -Wextra + /WX- Do not treat warnings as errors + /WX Treat warnings as errors + /w Disable all warnings + /Y- Disable precompiled headers, overrides /Yc and /Yu + /Yc<filename> Generate a pch file for all code up to and including <filename> + /Yu<filename> Load a pch file and use it instead of all code up to and including <filename> + /Z7 Enable CodeView debug information in object files + /Zc:sizedDealloc- Disable C++14 sized global deallocation functions + /Zc:sizedDealloc Enable C++14 sized global deallocation functions + /Zc:strictStrings Treat string literals as const + /Zc:threadSafeInit- Disable thread-safe initialization of static variables + /Zc:threadSafeInit Enable thread-safe initialization of static variables + /Zc:trigraphs- Disable trigraphs (default) + /Zc:trigraphs Enable trigraphs + /Zd Emit debug line number tables only + /Zi Alias for /Z7. Does not produce PDBs. + /Zl Don't mention any default libraries in the object file + /Zp Set the default maximum struct packing alignment to 1 + /Zp<value> Specify the default maximum struct packing alignment + /Zs Syntax-check only OPTIONS: -### Print (but do not run) the commands to run for this compilation --analyze Run the static analyzer -fansi-escape-codes Use ANSI escape codes for diagnostics -fcolor-diagnostics Use colors in diagnostics + -fdelayed-template-parsing + Parse templated function definitions at the end of the translation unit + -fdiagnostics-absolute-paths + Print absolute paths in diagnostics -fdiagnostics-parseable-fixits Print fix-its in machine parseable form + -flto Enable LTO in 'full' mode -fms-compatibility-version=<value> Dot-separated value representing the Microsoft compiler version number to report in _MSC_VER (0 = don't define it (default)) @@ -2262,12 +2631,23 @@ Execute ``clang-cl /?`` to see a list of supported options: -fms-extensions Accept some non-standard constructs supported by the Microsoft compiler -fmsc-version=<value> Microsoft compiler version number to report in _MSC_VER (0 = don't define it (default)) + -fno-delayed-template-parsing + Disable delayed template parsing -fno-sanitize-coverage=<value> Disable specified features of coverage instrumentation for Sanitizers -fno-sanitize-recover=<value> Disable recovery for specified sanitizers -fno-sanitize-trap=<value> Disable trapping for specified sanitizers + -fno-standalone-debug Limit debug information produced to reduce size of debug binary + -fprofile-instr-generate=<file> + Generate instrumented code to collect execution counts into <file> + (overridden by LLVM_PROFILE_FILE env var) + -fprofile-instr-generate + Generate instrumented code to collect execution counts into default.profraw file + (overriden by '=' form of option or LLVM_PROFILE_FILE env var) + -fprofile-instr-use=<value> + Use instrumentation data for profile-guided optimization -fsanitize-blacklist=<value> Path to blacklist file for sanitizers -fsanitize-coverage=<value> @@ -2277,6 +2657,7 @@ Execute ``clang-cl /?`` to see a list of supported options: -fsanitize-trap=<value> Enable trapping for specified sanitizers -fsanitize=<check> Turn on runtime checks for various forms of undefined or suspicious behavior. See user manual for available checks + -fstandalone-debug Emit full debug info for all types used by the program -gcodeview Generate CodeView debug information -gline-tables-only Emit debug line number tables only -miamcu Use Intel MCU ABI |