diff options
Diffstat (limited to 'absl/base/attributes.h')
| -rw-r--r-- | absl/base/attributes.h | 26 |
1 files changed, 20 insertions, 6 deletions
diff --git a/absl/base/attributes.h b/absl/base/attributes.h index c71038c7..e4e7a3d8 100644 --- a/absl/base/attributes.h +++ b/absl/base/attributes.h @@ -682,9 +682,18 @@ // not compile (on supported platforms) unless the variable has a constant // initializer. This is useful for variables with static and thread storage // duration, because it guarantees that they will not suffer from the so-called -// "static init order fiasco". Prefer to put this attribute on the most visible -// declaration of the variable, if there's more than one, because code that -// accesses the variable can then use the attribute for optimization. +// "static init order fiasco". +// +// This attribute must be placed on the initializing declaration of the +// variable. Some compilers will give a -Wmissing-constinit warning when this +// attribute is placed on some other declaration but missing from the +// initializing declaration. +// +// In some cases (notably with thread_local variables), `ABSL_CONST_INIT` can +// also be used in a non-initializing declaration to tell the compiler that a +// variable is already initialized, reducing overhead that would otherwise be +// incurred by a hidden guard variable. Thus annotating all declarations with +// this attribute is recommended to potentially enhance optimization. // // Example: // @@ -693,14 +702,19 @@ // ABSL_CONST_INIT static MyType my_var; // }; // -// MyType MyClass::my_var = MakeMyType(...); +// ABSL_CONST_INIT MyType MyClass::my_var = MakeMyType(...); +// +// For code or headers that are assured to only build with C++20 and up, prefer +// just using the standard `constinit` keyword directly over this macro. // // Note that this attribute is redundant if the variable is declared constexpr. -#if ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization) +#if defined(__cpp_constinit) && __cpp_constinit >= 201907L +#define ABSL_CONST_INIT constinit +#elif ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization) #define ABSL_CONST_INIT [[clang::require_constant_initialization]] #else #define ABSL_CONST_INIT -#endif // ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization) +#endif // ABSL_ATTRIBUTE_PURE_FUNCTION // |