[PATCH] D127801: [InstCombine] convert mask and shift of power-of-2 to cmp+select

[chenglin.bi via Phabricator via llvm-commits]

bcl5980 added a comment.

I agree with that the transform is more general. And my question is how to invert the transform on backend if shift + and is faster.
For now SelectionDag has the transform `select Cond, Pow2, 0 --> (zext Cond) << log2(Pow2)`

  // select Cond, Pow2, 0 --> (zext Cond) << log2(Pow2)
  if (C1Val.isPowerOf2() && C2Val.isZero()) {
    if (VT != MVT::i1)
      Cond = DAG.getNode(ISD::ZERO_EXTEND, DL, VT, Cond);
    SDValue ShAmtC =
        DAG.getShiftAmountConstant(C1Val.exactLogBase2(), VT, DL);
    return DAG.getNode(ISD::SHL, DL, VT, Cond, ShAmtC);
  }

And if we want to do this
`X == (log2(C) - log2(ShiftC)) ? C : 0 --> (ShiftC << X) & C`
`X == (log2(ShiftC) - log2(C)) ? C : 0 --> (ShiftC >> X) & C`
We need to assume `X < BitWidth` https://alive2.llvm.org/ce/z/-GjVsu


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https://reviews.llvm.org/D127801