[PATCH] D86017: [BPI] Improve static heuristics for integer comparisons with CST

[Dávid Bolvanský via Phabricator via llvm-commits]

xbolva00 added inline comments.


================
Comment at: llvm/lib/Analysis/BranchProbabilityInfo.cpp:885
     // Otherwise -> Likely
     if (CI->isTrueWhenEqual())
       std::swap(TakenProb, UntakenProb);
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ebrevnov wrote:
> xbolva00 wrote:
> > ebrevnov wrote:
> > > Looks like I misread this code originally. I thought the intent was to set "likely" if equal and "unlikely" if not. But as I understand now isTrueWhenEqual does completely different thing. I would suggest reverting this ASAP and rework.
> > All perf inprovement is gone when only == is unlikely. So atleast the comment should be fixed.
> It should be not only == is unlikely, but != is likely as well.  Doesn't this help?
> What exact condition you have for which you need to set probabilities?
> 
 if (CI->isTrueWhenEqual())

so in this case
X == Y / X >= Y  / X <= Y -> Unlikely

Otherwise likely.

When I used
 if (Pred == EQ) then unlikely else likely, I got perf problems. So I think that LT/GT are key predicates which helps us here. 


Repository:
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  https://reviews.llvm.org/D86017/new/

https://reviews.llvm.org/D86017