[llvm] [DAGCombiner] Fold smax(X, -1) to or(X, ashr(X, BW-1)) for code size (PR #206242)

[via llvm-commits]

AZero13 wrote:

WAIT!

  if (CC == ISD::SETGT && isAllOnesOrAllOnesSplat(CondC) &&
      isAllOnesOrAllOnesSplat(C2)) {
    // i32 X > -1 ? C1 : -1 --> (X >>s 31) | C1
    SDValue ShAmtC = DAG.getConstant(X.getScalarValueSizeInBits() - 1, DL, VT);
    SDValue Sra = DAG.getNode(ISD::SRA, DL, VT, X, ShAmtC);
    return DAG.getNode(ISD::OR, DL, VT, Sra, C1);
  }
  
  exists though in dagcombiner.

https://github.com/llvm/llvm-project/pull/206242