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Re: __builtin_expect vs. PGO
Scott Meyers <NeverRead@aristeia.com> writes:
> 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.
Yes, by the time it matters in the compiler, probabilities are
probabilities wherever they come from. The above paragraph is basically
saying that in the absence of profiling information, the guessed
probabilities are combined with the __builtin_expect probabilities. For
a simple two-way branch I think __builtin_expect will always beat a
guessed probabilities, just as it will generally beat a profiled
probability. For something like
switch (__builtin_expect (x, 1))
{
case 0: ...
case 1: ...
case 2: ...
}
I'm not really sure what will happen.
Ian