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[X86] combineSelect - move vselect(cond, pshufb(x), pshufb(y)) -> or(pshufb(x), pshufb(y)) fold #145475

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Merged
merged 1 commit into from
Jun 24, 2025
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[X86] combineSelect - move vselect(cond, pshufb(x), pshufb(y)) -> or(pshufb(x), pshufb(y)) fold #145475

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78 changes: 38 additions & 40 deletions llvm/lib/Target/X86/X86ISelLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -47690,12 +47690,47 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
return V;

if (N->getOpcode() == ISD::VSELECT || N->getOpcode() == X86ISD::BLENDV) {
SmallVector<int, 64> Mask;
if (createShuffleMaskFromVSELECT(Mask, Cond,
SmallVector<int, 64> CondMask;
if (createShuffleMaskFromVSELECT(CondMask, Cond,
N->getOpcode() == X86ISD::BLENDV)) {
// Convert vselects with constant condition into shuffles.
if (DCI.isBeforeLegalizeOps())
return DAG.getVectorShuffle(VT, DL, LHS, RHS, Mask);
return DAG.getVectorShuffle(VT, DL, LHS, RHS, CondMask);

// fold vselect(cond, pshufb(x), pshufb(y)) -> or (pshufb(x), pshufb(y))
// by forcing the unselected elements to zero.
// TODO: Can we handle more shuffles with this?
if (LHS.hasOneUse() && RHS.hasOneUse()) {
SmallVector<SDValue, 1> LHSOps, RHSOps;
SmallVector<int, 64> LHSMask, RHSMask, ByteMask;
SDValue LHSShuf = peekThroughOneUseBitcasts(LHS);
SDValue RHSShuf = peekThroughOneUseBitcasts(RHS);
if (LHSShuf.getOpcode() == X86ISD::PSHUFB &&
RHSShuf.getOpcode() == X86ISD::PSHUFB &&
scaleShuffleMaskElts(VT.getSizeInBits() / 8, CondMask, ByteMask) &&
getTargetShuffleMask(LHSShuf, true, LHSOps, LHSMask) &&
getTargetShuffleMask(RHSShuf, true, RHSOps, RHSMask)) {
assert(ByteMask.size() == LHSMask.size() &&
ByteMask.size() == RHSMask.size() && "Shuffle mask mismatch");
for (auto [I, M] : enumerate(ByteMask)) {
// getConstVector sets negative shuffle mask values as undef, so
// ensure we hardcode SM_SentinelZero values to zero (0x80).
if (M < (int)ByteMask.size()) {
LHSMask[I] = isUndefOrZero(LHSMask[I]) ? 0x80 : LHSMask[I];
RHSMask[I] = 0x80;
} else {
LHSMask[I] = 0x80;
RHSMask[I] = isUndefOrZero(RHSMask[I]) ? 0x80 : RHSMask[I];
}
}
MVT ByteVT = LHSShuf.getSimpleValueType();
LHS = DAG.getNode(X86ISD::PSHUFB, DL, ByteVT, LHSOps[0],
getConstVector(LHSMask, ByteVT, DAG, DL, true));
RHS = DAG.getNode(X86ISD::PSHUFB, DL, ByteVT, RHSOps[0],
getConstVector(RHSMask, ByteVT, DAG, DL, true));
return DAG.getBitcast(VT, DAG.getNode(ISD::OR, DL, ByteVT, LHS, RHS));
}
}

// Attempt to combine as shuffle.
SDValue Op(N, 0);
Expand All @@ -47704,43 +47739,6 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
}
}

// fold vselect(cond, pshufb(x), pshufb(y)) -> or (pshufb(x), pshufb(y))
// by forcing the unselected elements to zero.
// TODO: Can we handle more shuffles with this?
if (N->getOpcode() == ISD::VSELECT && CondVT.isVector() && LHS.hasOneUse() &&
RHS.hasOneUse()) {
SmallVector<SDValue, 1> LHSOps, RHSOps;
SmallVector<int, 64> LHSMask, RHSMask, CondMask, ByteMask;
SDValue LHSShuf = peekThroughOneUseBitcasts(LHS);
SDValue RHSShuf = peekThroughOneUseBitcasts(RHS);
if (LHSShuf.getOpcode() == X86ISD::PSHUFB &&
RHSShuf.getOpcode() == X86ISD::PSHUFB &&
createShuffleMaskFromVSELECT(CondMask, Cond) &&
scaleShuffleMaskElts(VT.getSizeInBits() / 8, CondMask, ByteMask) &&
getTargetShuffleMask(LHSShuf, true, LHSOps, LHSMask) &&
getTargetShuffleMask(RHSShuf, true, RHSOps, RHSMask)) {
assert(ByteMask.size() == LHSMask.size() &&
ByteMask.size() == RHSMask.size() && "Shuffle mask mismatch");
for (auto [I, M] : enumerate(ByteMask)) {
// getConstVector sets negative shuffle mask values as undef, so ensure
// we hardcode SM_SentinelZero values to zero (0x80).
if (M < (int)ByteMask.size()) {
LHSMask[I] = isUndefOrZero(LHSMask[I]) ? 0x80 : LHSMask[I];
RHSMask[I] = 0x80;
} else {
LHSMask[I] = 0x80;
RHSMask[I] = isUndefOrZero(RHSMask[I]) ? 0x80 : RHSMask[I];
}
}
MVT ByteVT = LHSShuf.getSimpleValueType();
LHS = DAG.getNode(X86ISD::PSHUFB, DL, ByteVT, LHSOps[0],
getConstVector(LHSMask, ByteVT, DAG, DL, true));
RHS = DAG.getNode(X86ISD::PSHUFB, DL, ByteVT, RHSOps[0],
getConstVector(RHSMask, ByteVT, DAG, DL, true));
return DAG.getBitcast(VT, DAG.getNode(ISD::OR, DL, ByteVT, LHS, RHS));
}
}

// If we have SSE[12] support, try to form min/max nodes. SSE min/max
// instructions match the semantics of the common C idiom x<y?x:y but not
// x<=y?x:y, because of how they handle negative zero (which can be
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