__builtin_expect vs. PGO

Scott Meyers NeverRead@aristeia.com
Wed Dec 21 19:40:00 GMT 2011

On 12/21/2011 8:49 AM, Ian Lance Taylor wrote:
> I believe the probabilities will be combined in that case.  The
> weighting of __builtin_expect is such that it will most likely be the
> predicted case, unless the profiling data says that the unexpected path
> is always taken.

Thanks for the comment.  The documentation for 
-fno-guess-branch-probability says:

   GCC will use heuristics to guess branch probabilities if they are
   not provided by profiling feedback (-fprofile-arcs). These
   heuristics are based on the control flow graph. If some branch
   probabilities are specified by `__builtin_expect', then the
   heuristics will be used to guess branch probabilities for the
   rest of the control flow graph, taking the `__builtin_expect'
   info into account. The interactions between the heuristics and
   `__builtin_expect' can be complex, and in some cases, it may be
   useful to disable the heuristics so that the effects of
   `__builtin_expect' are easier to understand.

Is this is any way relevant? On the one hand, it's talking about 
heuristics to be used in the absence of profiling feedback, and I'm 
assuming we have profiling information.  On the other hand, it's talking 
about the interaction of probabilities and __builtin_expect, and those 
probabilities presumably could come from PGO data.


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