[PATCH] D109151: [InstCombine] Convert xor (ashr X, BW-1), C -> select(X >=s 0, C, ~C)

[Dave Green via Phabricator via llvm-commits]

dmgreen created this revision.
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The sequence of instructions `xor (ashr X, BW-1), C` (or with a truncation `xor (trunc (ashr X, BW-1)), C`) takes a value, produces all zeros or all ones and with it optionally negates a constant depending on whether the original input was positive or negative. This is the same as checking if the value is positive, and selecting between the constant and ~constant.

This is a fairly general version of the fold as part of D108049 <https://reviews.llvm.org/D108049>, which really only needs the constant to be a saturate INTMAX/INTMIN pair.


https://reviews.llvm.org/D109151

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
  llvm/test/Transforms/InstCombine/truncating-saturate.ll
  llvm/test/Transforms/InstCombine/xor-ashr.ll

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