From c7e60ccfcd708a73008ed2df040162c66697bc18 Mon Sep 17 00:00:00 2001 From: Evan Brown Date: Wed, 27 Jul 2022 13:45:19 -0700 Subject: Add ABSL_IS_TRIVIALLY_RELOCATABLE and ABSL_ATTRIBUTE_TRIVIAL_ABI macros for use with clang's __is_trivially_relocatable and [[clang::trivial_abi]]. PiperOrigin-RevId: 463668740 Change-Id: I2d2d2f53d8184a7e4f7c848c2a5f5140c2481d72 --- absl/base/attributes.h | 37 +++++++++++++++++++++++++++++++++++++ 1 file changed, 37 insertions(+) (limited to 'absl/base/attributes.h') diff --git a/absl/base/attributes.h b/absl/base/attributes.h index e4e7a3d8..e11a064a 100644 --- a/absl/base/attributes.h +++ b/absl/base/attributes.h @@ -759,4 +759,41 @@ #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_ -- cgit v1.2.3