libgfortran.h (unlikely, likely): Add usage comment.

2014-03-08  Tobias Burnus  <burnus@net-b.de>

        * libgfortran.h (unlikely, likely): Add usage comment.

From-SVN: r208434

libgfortran/ChangeLog

+2014-03-08  Tobias Burnus  <burnus@net-b.de>
+
+	* libgfortran.h (unlikely, likely): Add usage comment.
+
 2014-03-08  Dominique d'Humieres  <dominiq@lps.ens.fr>
 
 	PR libgfortran/60128

libgfortran/libgfortran.h

@@ -97,6 +97,16 @@ typedef off_t gfc_offset;
 #define NULL (void *) 0
 #endif
 
+
+/* The following macros can be used to annotate conditions which are likely or
+   unlikely to be true.  Avoid using them when a condition is only slightly
+   more likely/less unlikely than average to avoid the performance penalties of
+   branch misprediction. In addition, as __builtin_expect overrides the compiler
+   heuristic, do not use in conditions where one of the branches ends with a
+   call to a function with __attribute__((noreturn)): the compiler internal
+   heuristic will mark this branch as much less likely as unlikely() would
+   do.  */
+
 #ifndef __GNUC__
 #define __attribute__(x)
 #define likely(x)       (x)