[PATCH] D12706: Handle non-constant shifts in computeKnownBits, and use computeKnownBits for constant folding in InstCombine/Simplify

John Regehr via llvm-commits llvm-commits at lists.llvm.org
Thu Sep 24 02:58:10 PDT 2015

regehr added a comment.

Hi Duncan,

Of course I agree in principle -- but keep in mind that Souper's 
computation of known bits is effectively optimal, and that my patch 
eliminated the vast majority of observed imprecisions.

Also I have implemented precise abstract transfer functions for things 
like this in the past, and always found them hard to get right!  No fun 
at all.

But really, I was hoping to simply take Hal's patch and add in the parts 
of mine that fail to overlap with his!  For example his patch does not 
address bswap or ctpop.




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