[PATCH] D95959: [ValueTracking] improve analysis for "C << X" and "C >> X"

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: nikic, lebedev.ri, xbolva00.
Herald added subscribers: hiraditya, mcrosier.
spatel requested review of this revision.
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This is based on the example/comments in:
https://llvm.org/PR48984

I tried just lifting the restriction in computeKnownBitsFromShiftOperator() as suggested, but that doesn't catch all of the cases shown here. I didn't step through to see exactly why that happened. But it seems like a reasonable compromise to cheaply check the special-case of shifting a constant.

There's a slight regression on a cmp transform as noted, but I think this is likely the more important/common pattern, so we can fix that icmp pattern later if needed.


https://reviews.llvm.org/D95959

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Transforms/InstCombine/and2.ll
  llvm/test/Transforms/InstCombine/lshr-and-negC-icmpeq-zero.ll
  llvm/test/Transforms/InstCombine/shift.ll
  llvm/test/Transforms/InstCombine/signbit-shl-and-icmpeq-zero.ll
  llvm/test/Transforms/InstCombine/zext-or-icmp.ll

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