From 0cf78f283af702d2e938e249de8c09733d2f5f77 Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Sun, 3 Jul 2016 15:05:18 -0700 Subject: [PATCH 1/5] Add likely/unlikely() branch hint macros. These are useful for hot code paths. Because it's easy to guess wrongly about likelihood, and because such likelihoods change over time, they should be used sparingly. Discussion: <20160727004333.r3e2k2y6fvk2ntup@alap3.anarazel.de> --- src/include/c.h | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/src/include/c.h b/src/include/c.h index 4ab3f80..3a77107 100644 --- a/src/include/c.h +++ b/src/include/c.h @@ -939,6 +939,22 @@ typedef NameData *Name; #endif +/* + * Hints to the compiler about the likelihood of a branch. Both likely() and + * unlikely() return the boolean value of the contained expression. + * + * These should only be used sparingly, in very hot code paths. It's very easy + * to mis-estimate likelihoods. + */ +#if __GNUC__ >= 3 +#define likely(x) __builtin_expect((x) != 0, 1) +#define unlikely(x) __builtin_expect((x) != 0, 0) +#else +#define likely(x) ((x) != 0) +#define unlikely(x) ((x) != 0) +#endif + + /* ---------------------------------------------------------------- * Section 8: random stuff * ---------------------------------------------------------------- -- 2.9.3