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[DAGCombiner] improve readability 

This is not quite NFC because I changed the SDLoc to use the more
standard 'N' (the starting node for the fold).

This transform is a special-case of a more general fold that we
do in IR, but it seems like the general fold is needed here too
to avoid a potential regression seen in D58017.

https://rise4fun.com/Alive/3jZm
This commit is contained in:
Sanjay Patel 2019-12-12 13:14:02 -05:00
parent 7ffe7d5ed7
commit b39009bf1d
1 changed files with 11 additions and 11 deletions
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22
llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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@ -7922,26 +7922,26 @@ SDValue DAGCombiner::visitSRL(SDNode *N) {
}
}
// fold (srl (trunc (srl x, c1)), c2) -> 0 or (trunc (srl x, (add c1, c2)))
// TODO - support non-uniform vector shift amounts.
if (N1C && N0.getOpcode() == ISD::TRUNCATE &&
N0.getOperand(0).getOpcode() == ISD::SRL) {
if (auto N001C = isConstOrConstSplat(N0.getOperand(0).getOperand(1))) {
SDValue InnerShift = N0.getOperand(0);
// TODO - support non-uniform vector shift amounts.
if (auto *N001C = isConstOrConstSplat(InnerShift.getOperand(1))) {
uint64_t c1 = N001C->getZExtValue();
uint64_t c2 = N1C->getZExtValue();
EVT InnerShiftVT = N0.getOperand(0).getValueType();
EVT ShiftCountVT = N0.getOperand(0).getOperand(1).getValueType();
EVT InnerShiftVT = InnerShift.getValueType();
EVT ShiftAmtVT = InnerShift.getOperand(1).getValueType();
uint64_t InnerShiftSize = InnerShiftVT.getScalarSizeInBits();
// srl (trunc (srl x, c1)), c2 --> 0 or (trunc (srl x, (add c1, c2)))
// This is only valid if the OpSizeInBits + c1 = size of inner shift.
if (c1 + OpSizeInBits == InnerShiftSize) {
SDLoc DL(N0);
SDLoc DL(N);
if (c1 + c2 >= InnerShiftSize)
return DAG.getConstant(0, DL, VT);
return DAG.getNode(ISD::TRUNCATE, DL, VT,
DAG.getNode(ISD::SRL, DL, InnerShiftVT,
N0.getOperand(0).getOperand(0),
DAG.getConstant(c1 + c2, DL,
ShiftCountVT)));
SDValue NewShiftAmt = DAG.getConstant(c1 + c2, DL, ShiftAmtVT);
SDValue NewShift = DAG.getNode(ISD::SRL, DL, InnerShiftVT,
InnerShift.getOperand(0), NewShiftAmt);
return DAG.getNode(ISD::TRUNCATE, DL, VT, NewShift);
}
}
}