[PATCH] D112298: [InstCombine] Generalize sadd.sat combine to compute sign bits.

[Dave Green via Phabricator via llvm-commits]

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There is a combine in instcombine to transform a saturated add/sub into a saddsat/ssubsat, currently handling inputs which are both sign extended (https://alive2.llvm.org/ce/z/68qpTn). This can generalize to, for example ashr of at least the bitwidth (https://alive2.llvm.org/ce/z/4TFyX- and https://alive2.llvm.org/ce/z/qDWzFs for example). My understanding is that, this means it generalizes further to "the number of sign bits", needing to be enough to truncate to the size of the saturate. (An example using or for instance: https://alive2.llvm.org/ce/z/EI_h_A).

So this patch makes use of ComputeNumSignBits in matchSAddSubSat to generalize the fold to any inputs with enough sign bits known, truncating the inputs to the new size of the saturate.


https://reviews.llvm.org/D112298

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
  llvm/test/Transforms/InstCombine/sadd_sat.ll

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