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[AArch64] canCreateUndefOrPoisonForTargetNode - AArch64ISD::MOVI opcodes can't create undef/poison #149323

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Merged
RKSimon merged 6 commits into from
Sep 4, 2025
Merged

[AArch64] canCreateUndefOrPoisonForTargetNode - AArch64ISD::MOVI opcodes can't create undef/poison #149323

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8c0a882
[AArch64] canCreateUndefOrPoisonForTargetNode - AArch64ISD::MOVI opco…
RKSimon Jul 17, 2025
d1bb082
Merge branch 'main' into aarch64-nopoison-vmov
RKSimon Aug 28, 2025
e5e6688
Merge branch 'main' into aarch64-nopoison-vmov
RKSimon Sep 2, 2025
60b606f
Merge branch 'main' into aarch64-nopoison-vmov
RKSimon Sep 3, 2025
222da93
Add frozen MOV/MVN nodes to ComputeKnownBits_MOVI to demonstrate that…
RKSimon Sep 3, 2025
05d71a0
Merge branch 'main' into aarch64-nopoison-vmov
RKSimon Sep 4, 2025
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17 changes: 17 additions & 0 deletions llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -30398,6 +30398,23 @@ bool AArch64TargetLowering::SimplifyDemandedBitsForTargetNode(
Op, OriginalDemandedBits, OriginalDemandedElts, Known, TLO, Depth);
}

bool AArch64TargetLowering::canCreateUndefOrPoisonForTargetNode(
SDValue Op, const APInt &DemandedElts, const SelectionDAG &DAG,
bool PoisonOnly, bool ConsiderFlags, unsigned Depth) const {

// TODO: Add more target nodes.
switch (Op.getOpcode()) {
case AArch64ISD::MOVI:
case AArch64ISD::MOVIedit:
case AArch64ISD::MOVImsl:
case AArch64ISD::MOVIshift:
case AArch64ISD::MVNIshift:
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@RKSimon RKSimon Jul 17, 2025

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In fact I think these can be put into a AArch64TargetLowering::isGuaranteedNotToBeUndefOrPoisonForTargetNode implementation, but this should have the same effect

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@davemgreen davemgreen Jul 18, 2025

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That sounds OK to me, they should never be poison. Do we have a standard way of creating tests for changes like this?

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@RKSimon RKSimon Jul 18, 2025

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It might be that #148191 is the first case where we manage to show the problem (we need a case where the freeze needs removing quite late into lowering) - I haven't had time to investigate though.

return false;
}
return TargetLowering::canCreateUndefOrPoisonForTargetNode(
Op, DemandedElts, DAG, PoisonOnly, ConsiderFlags, Depth);
}

bool AArch64TargetLowering::isTargetCanonicalConstantNode(SDValue Op) const {
return Op.getOpcode() == AArch64ISD::DUP ||
Op.getOpcode() == AArch64ISD::MOVI ||
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6 changes: 6 additions & 0 deletions llvm/lib/Target/AArch64/AArch64ISelLowering.h
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Expand Up @@ -851,6 +851,12 @@ class AArch64TargetLowering : public TargetLowering {
TargetLoweringOpt &TLO,
unsigned Depth) const override;

bool canCreateUndefOrPoisonForTargetNode(SDValue Op,
const APInt &DemandedElts,
const SelectionDAG &DAG,
bool PoisonOnly, bool ConsiderFlags,
unsigned Depth) const override;

bool isTargetCanonicalConstantNode(SDValue Op) const override;

// With the exception of data-predicate transitions, no instructions are
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