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[X86] Merge insertsubvector(load(p0),load_subv(p0),hi) -> subvbroadcast(p0) if either load is oneuse #128857

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Merged
merged 3 commits into from
Feb 27, 2025
Merged

[X86] Merge insertsubvector(load(p0),load_subv(p0),hi) -> subvbroadcast(p0) if either load is oneuse #128857

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1e18162
[X86] Merge insertsubvector(load(p0),load_subv(p0),hi) -> subvbroadca…
RKSimon Feb 26, 2025
5b93950
Ensure ensure all subvector uses are replaced
RKSimon Feb 26, 2025
f1798e8
Merge branch 'upstream' into x86-insertvector-subvbroadcast-oneuse
RKSimon Feb 26, 2025
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19 changes: 13 additions & 6 deletions llvm/lib/Target/X86/X86ISelLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -58567,15 +58567,22 @@ static SDValue combineINSERT_SUBVECTOR(SDNode *N, SelectionDAG &DAG,

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Does Vec have oneuse matter? Can't the loaded value be used more than once?

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Let me see if I can get this to always replace uses of subvec - then we can get rid of the oneuse checks entirely

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@RKSimon RKSimon Feb 26, 2025 •
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The oneuse checks are still preventing some regressions due to shouldReduceLoadWidth being so permissive, but I've at least ensured that any other uses of SubVec are replaced now.

// If we're splatting the lower half subvector of a full vector load into the
// upper half, attempt to create a subvector broadcast.
if (IdxVal == (OpVT.getVectorNumElements() / 2) && SubVec.hasOneUse() &&
Vec.getValueSizeInBits() == (2 * SubVec.getValueSizeInBits())) {
// TODO: Drop hasOneUse checks.
if (IdxVal == (OpVT.getVectorNumElements() / 2) &&
Vec.getValueSizeInBits() == (2 * SubVec.getValueSizeInBits()) &&
(Vec.hasOneUse() || SubVec.hasOneUse())) {
auto *VecLd = dyn_cast<LoadSDNode>(Vec);
auto *SubLd = dyn_cast<LoadSDNode>(SubVec);
if (VecLd && SubLd &&
DAG.areNonVolatileConsecutiveLoads(SubLd, VecLd,
SubVec.getValueSizeInBits() / 8, 0))
return getBROADCAST_LOAD(X86ISD::SUBV_BROADCAST_LOAD, dl, OpVT, SubVecVT,
SubLd, 0, DAG);
DAG.areNonVolatileConsecutiveLoads(
SubLd, VecLd, SubVec.getValueSizeInBits() / 8, 0)) {
SDValue BcastLd = getBROADCAST_LOAD(X86ISD::SUBV_BROADCAST_LOAD, dl, OpVT,
SubVecVT, SubLd, 0, DAG);
SDValue NewSubVec = DAG.getNode(ISD::EXTRACT_SUBVECTOR, dl, SubVecVT,
BcastLd, DAG.getVectorIdxConstant(0, dl));
DCI.CombineTo(SubLd, NewSubVec, BcastLd.getValue(1));
Comment on lines +58581 to +58583
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This only required when SubVec has multiple user. Do we need to add a condition check?

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we don't need the check, oneuse cases will be handled fine, but it might slightly improve perf

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its easier if we always call it, just in case there's weird cases like only Subvec's chain is still in use.

return BcastLd;
}
}

return SDValue();
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