[PATCH] D85463: [GlobalISel] Add combine for (x & mask) -> x when (x & mask) == x

[Jessica Paquette via Phabricator via llvm-commits]

paquette added a comment.

Also yeah, it looks like this is handled in `SelectionDAG::GetDemandedBits`:

  case ISD::AND: {
    // X & -1 -> X (ignoring bits which aren't demanded).
    // Also handle the case where masked out bits in X are known to be zero.
    if (ConstantSDNode *RHSC = isConstOrConstSplat(V.getOperand(1))) {
      const APInt &AndVal = RHSC->getAPIntValue();
      if (DemandedBits.isSubsetOf(AndVal) ||
          DemandedBits.isSubsetOf(computeKnownBits(V.getOperand(0)).Zero |
                                  AndVal))
        return V.getOperand(0);
    }

So I guess that it might make sense to partially move this into GISelKnownBits.


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  https://reviews.llvm.org/D85463/new/

https://reviews.llvm.org/D85463