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

[John Regehr via llvm-commits]

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.

John


Repository:
  rL LLVM

http://reviews.llvm.org/D12706