[PATCH] D150670: [WebAssembly] Disable generation of fshl/fshr for rotates

[Paulo Matos via Phabricator via cfe-commits]

pmatos added a comment.

In D150670#4352163 <https://reviews.llvm.org/D150670#4352163>, @nikic wrote:

> 1. Say that we prefer preserving rotates over "simplifying" funnel shifts (ending up with the rot2 pattern). Basically by skipping the optimization at https://github.com/llvm/llvm-project/blob/7f54b38e28b3b66195de672848f2b5366d0d51e3/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp#L927-L931 if both fsh operands are the same. Assuming this doesn't cause test regressions, I think this would be acceptable to do. From a backend perspective, even for targets that have a native funnel shift (aarch64, x86), the difference between the rot1/rot2 patterns looks pretty neutral.

I am surprised this option is viable for example. This was my initial thought to avoid the rotate, but I assumed adding something like :

  if (!getTarget().getTriple().isWasm()) {
    APInt DemandedMaskLHS(DemandedMask.lshr(ShiftAmt));
    APInt DemandedMaskRHS(DemandedMask.shl(BitWidth - ShiftAmt));
    if (SimplifyDemandedBits(I, 0, DemandedMaskLHS, LHSKnown, Depth + 1) ||
        SimplifyDemandedBits(I, 1, DemandedMaskRHS, RHSKnown, Depth + 1))
      return I;
  }

would not be well received. Also, I cannot find precedent for doing this.


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