diff options
Diffstat (limited to 'absl/base/attributes.h')
| -rw-r--r-- | absl/base/attributes.h | 58 |
1 files changed, 39 insertions, 19 deletions
diff --git a/absl/base/attributes.h b/absl/base/attributes.h index e4e7a3d8..b7826e77 100644 --- a/absl/base/attributes.h +++ b/absl/base/attributes.h @@ -716,26 +716,9 @@ #define ABSL_CONST_INIT #endif -// ABSL_ATTRIBUTE_PURE_FUNCTION -// -// ABSL_ATTRIBUTE_PURE_FUNCTION is used to annotate declarations of "pure" -// functions. A function is pure if its return value is only a function of its -// arguments. The pure attribute prohibits a function from modifying the state -// of the program that is observable by means other than inspecting the -// function's return value. Declaring such functions with the pure attribute -// allows the compiler to avoid emitting some calls in repeated invocations of -// the function with the same argument values. -// -// Example: -// -// ABSL_ATTRIBUTE_PURE_FUNCTION int64_t ToInt64Milliseconds(Duration d); -#if ABSL_HAVE_CPP_ATTRIBUTE(gnu::pure) -#define ABSL_ATTRIBUTE_PURE_FUNCTION [[gnu::pure]] -#elif ABSL_HAVE_ATTRIBUTE(pure) -#define ABSL_ATTRIBUTE_PURE_FUNCTION __attribute__((pure)) -#else +// These annotations are not available yet due to fear of breaking code. #define ABSL_ATTRIBUTE_PURE_FUNCTION -#endif +#define ABSL_ATTRIBUTE_CONST_FUNCTION // ABSL_ATTRIBUTE_LIFETIME_BOUND indicates that a resource owned by a function // parameter or implicit object parameter is retained by the return value of the @@ -759,4 +742,41 @@ #define ABSL_ATTRIBUTE_LIFETIME_BOUND #endif +// ABSL_ATTRIBUTE_TRIVIAL_ABI +// Indicates that a type is "trivially relocatable" -- meaning it can be +// relocated without invoking the constructor/destructor, using a form of move +// elision. +// +// From a memory safety point of view, putting aside destructor ordering, it's +// safe to apply ABSL_ATTRIBUTE_TRIVIAL_ABI if an object's location +// can change over the course of its lifetime: if a constructor can be run one +// place, and then the object magically teleports to another place where some +// methods are run, and then the object teleports to yet another place where it +// is destroyed. This is notably not true for self-referential types, where the +// move-constructor must keep the self-reference up to date. If the type changed +// location without invoking the move constructor, it would have a dangling +// self-reference. +// +// The use of this teleporting machinery means that the number of paired +// move/destroy operations can change, and so it is a bad idea to apply this to +// a type meant to count the number of moves. +// +// Warning: applying this can, rarely, break callers. Objects passed by value +// will be destroyed at the end of the call, instead of the end of the +// full-expression containing the call. In addition, it changes the ABI +// of functions accepting this type by value (e.g. to pass in registers). +// +// See also the upstream documentation: +// https://clang.llvm.org/docs/AttributeReference.html#trivial-abi +// +#if ABSL_HAVE_CPP_ATTRIBUTE(clang::trivial_abi) +#define ABSL_ATTRIBUTE_TRIVIAL_ABI [[clang::trivial_abi]] +#define ABSL_HAVE_ATTRIBUTE_TRIVIAL_ABI 1 +#elif ABSL_HAVE_ATTRIBUTE(trivial_abi) +#define ABSL_ATTRIBUTE_TRIVIAL_ABI __attribute__((trivial_abi)) +#define ABSL_HAVE_ATTRIBUTE_TRIVIAL_ABI 1 +#else +#define ABSL_ATTRIBUTE_TRIVIAL_ABI +#endif + #endif // ABSL_BASE_ATTRIBUTES_H_ |