[PATCH] D149418: [ValueTracking] Add additional cases for `isKnownNonZero(mul X, Y)`

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:2884
+
     break;
   }
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nikic wrote:
> goldstein.w.n wrote:
> > nikic wrote:
> > > Similar to the add case, we should compute mul known bits here based on the already computed XKnown/YKnown, rather than computing them again via the fallback.
> > > 
> > > I think just using KnownBits::mul should be enough, as the NSW case handled by computeKnownBitsMul is already handled better above.
> > > Similar to the add case, we should compute mul known bits here based on the already computed XKnown/YKnown, rather than computing them again via the fallback.
> > > 
> > > I think just using KnownBits::mul should be enough, as the NSW case handled by computeKnownBitsMul is already handled better above.
> > 
> > Added the actual logic b.c there is a trick for knowing the sign of the result that doesn't appear to be handled by `KnownBits::mul`. Also seems more future-proof incase someone has a new bright idea and improves it further.
> > 
> > But we don't recompute knownbits now :)
> But isn't all the sign bit logic behind `if (NSW)`, which we already handle separately?
> 
> 
I mainly find the computeKnownBitsMulDoMul() function pretty ugly and would prefer to avoid it :) KnownBits::mul should handle anything that is not based on nowrap flags, and we handle nowrap flags ourselves.




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  https://reviews.llvm.org/D149418/new/

https://reviews.llvm.org/D149418