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[SelectionDAG] fold (not (sub Y, X)) -> (add X, ~Y) #147825

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[SelectionDAG] fold (not (sub Y, X)) -> (add X, ~Y) #147825

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16 changes: 9 additions & 7 deletions llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -9979,13 +9979,15 @@ SDValue DAGCombiner::visitXOR(SDNode *N) {
}
}

// fold (not (neg x)) -> (add X, -1)
// FIXME: This can be generalized to (not (sub Y, X)) -> (add X, ~Y) if
// Y is a constant or the subtract has a single use.
if (isAllOnesConstant(N1) && N0.getOpcode() == ISD::SUB &&
isNullConstant(N0.getOperand(0))) {
return DAG.getNode(ISD::ADD, DL, VT, N0.getOperand(1),
DAG.getAllOnesConstant(DL, VT));
// fold (not (sub Y, X)) -> (add X, ~Y) if Y is a constant.
// FIXME: We can also do this with single-use sub, but this causes an infinite
// loop
if (isAllOnesConstant(N1) && N0.getOpcode() == ISD::SUB) {
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Suggested change
if (isAllOnesConstant(N1) && N0.getOpcode() == ISD::SUB) {
if (N0.getOpcode() == ISD::SUB && isAllOnesConstant(N1)) {

SDValue N00 = N0.getOperand(0), N01 = N0.getOperand(1);
if (isa<ConstantSDNode>(N00)) {
SDValue NotY = DAG.getNOT(DL, N00, VT); // N00 = ~N00
return DAG.getNode(ISD::ADD, DL, VT, N01, NotY);
}
}

// fold (not (add X, -1)) -> (neg X)
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