postgres.git / summary / log / commit / refs

commit    aa3ca5e3dd60bf0b992b74f955378f28e601292a
Author:   Andres Freund <andres@anarazel.de>
Date:     Fri Oct 14 23:05:30 2016 +0000

    Add likely/unlikely() branch hint macros.
    
    These are useful for very hot code paths. Because it's easy to guess
    wrongly about likelihood, and because such likelihoods change over time,
    they should be used sparingly.
    
    Past tests have shown it'd be a good idea to use them in some places,
    e.g. in error checks around ereports that ERROR out, but that's work for
    later.
    
    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 4ab3f8027a5..672e21e03ea 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
  * ----------------------------------------------------------------


[parent: 32fdf42cf546]