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Add NXT_(UN)?LIKELY macros to help with branch prediction

This commit is contained in:
Corentin Wallez 2018-05-24 15:17:25 -04:00 committed by Corentin Wallez
parent b1341db700
commit 2141e960b7
1 changed files with 10 additions and 0 deletions
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10
src/common/Compiler.h
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@ -20,6 +20,8 @@
// - NXT_BREAKPOINT(): Raises an exception and breaks in the debugger // - NXT_BREAKPOINT(): Raises an exception and breaks in the debugger
// - NXT_BUILTIN_UNREACHABLE(): Hints the compiler that a code path is unreachable // - NXT_BUILTIN_UNREACHABLE(): Hints the compiler that a code path is unreachable
// - NXT_NO_DISCARD: An attribute that is C++17 [[nodiscard]] where available // - NXT_NO_DISCARD: An attribute that is C++17 [[nodiscard]] where available
// - NXT_(UN)?LIKELY(EXPR): Where available, hints the compiler that the expression will be true
// (resp. false) to help it generate code that leads to better branch prediction.
// Clang and GCC // Clang and GCC
#if defined(__GNUC__) #if defined(__GNUC__)
@ -36,6 +38,8 @@
# endif # endif
# define NXT_BUILTIN_UNREACHABLE() __builtin_unreachable() # define NXT_BUILTIN_UNREACHABLE() __builtin_unreachable()
# define NXT_LIKELY(x) __builtin_expect(!!(x), 1)
# define NXT_UNLIKELY(x) __builtin_expect(!!(x), 0)
# if !defined(__has_cpp_attribute) # if !defined(__has_cpp_attribute)
# define __has_cpp_attribute(name) 0 # define __has_cpp_attribute(name) 0
@ -69,6 +73,12 @@ extern void __cdecl __debugbreak(void);
#endif #endif
// Add noop replacements for macros for features that aren't supported by the compiler. // Add noop replacements for macros for features that aren't supported by the compiler.
#if !defined(NXT_LIKELY)
# define NXT_LIKELY(X) X
#endif
#if !defined(NXT_UNLIKELY)
# define NXT_UNLIKELY(X) X
#endif
#if !defined(NXT_NO_DISCARD) #if !defined(NXT_NO_DISCARD)
# define NXT_NO_DISCARD # define NXT_NO_DISCARD
#endif #endif