[PATCH] D67071: [CodeGen] Handle SMULFIXSAT with scale zero in TargetLowering::expandFixedPointMul

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope created this revision.
bjope added reviewers: RKSimon, bevinh, leonardchan.
Herald added subscribers: jsji, MaskRay, hiraditya, nemanjai.
Herald added a project: LLVM.

Normally TargetLowering::expandFixedPointMul would handle
SMULFIXSAT with scale zero by using an SMULO to compute the
product and determine if saturation is needed (if overflow
happened). But if SMULO isn't custom/legal it falls through
and uses the same technique, using MULHS/SMUL_LOHI, as used
for non-zero scales.

Problem was that when checking for overflow (handling saturation)
when not using MULO we did not expect to find a zero scale. So
we ended up in an assertion when doing

  APInt::getLowBitsSet(VTSize, Scale - 1)

This patch fixes the problem by adding a new special case for
how saturation is computed when scale is zero.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D67071

Files:
  llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
  llvm/test/CodeGen/PowerPC/smulfixsat.ll

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