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.

diff --git a/src/include/c.h b/src/include/c.h
index 4ab3f8027a56362f5c2ce3c4408f6e271edb40c1..3a77107e609a4d8c94fd48b6fafcf6baa3a9524c 100644 (file)
--- 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
  * ----------------------------------------------------------------