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[AMDGPU] Make sort ordering in AMDGPURewriteAGPRCopyMFMAImpl::eliminateSpillsOfReassignedVGPRs() strict weak. #162493

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Merged
merged 5 commits into from
Oct 8, 2025
Merged

[AMDGPU] Make sort ordering in AMDGPURewriteAGPRCopyMFMAImpl::eliminateSpillsOfReassignedVGPRs() strict weak. #162493

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56f9ded
Make sort ordering strictly weak.
weiweichen Oct 8, 2025
a364ce6
Actually make it strict weak ordering.
weiweichen Oct 8, 2025
e7e2598
Remove empty line.
weiweichen Oct 8, 2025
d6fc4ef
Address review comments.
weiweichen Oct 8, 2025
b17feb7
Merge branch 'main' of github.com:llvm/llvm-project into weiweic/issu…
weiweichen Oct 8, 2025
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4 changes: 3 additions & 1 deletion llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -490,7 +490,9 @@ void AMDGPURewriteAGPRCopyMFMAImpl::eliminateSpillsOfReassignedVGPRs() const {
return true;

// Tie breaker by number to avoid need for stable sort
return A->reg().stackSlotIndex() < B->reg().stackSlotIndex();
// The ordering have to be strictly weak.
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Suggested change
// The ordering have to be strictly weak.
// The ordering has to be strictly weak.

return (A->reg().stackSlotIndex() < B->reg().stackSlotIndex()) &&
((A->weight() <= B->weight()) && (A->getSize() <= B->getSize()));
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Copilot AI Oct 8, 2025

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This comparator logic is incorrect and still violates strict weak ordering. The current implementation can return false for both comp(a,b) and comp(b,a) when elements have the same stackSlotIndex, which breaks antisymmetry. For strict weak ordering, when primary criteria are equal, you should use a single tie-breaker. Consider using: return A->reg().stackSlotIndex() < B->reg().stackSlotIndex(); as the sole comparator, or if multiple criteria are needed, use lexicographic ordering like: if (A->weight() != B->weight()) return A->weight() < B->weight(); if (A->getSize() != B->getSize()) return A->getSize() < B->getSize(); return A->reg().stackSlotIndex() < B->reg().stackSlotIndex();

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Consider using: return A->reg().stackSlotIndex() < B->reg().stackSlotIndex();

As mentioned in the issue, this is not the right solution for making strict weak ordering.

But it's also true that my change has the issue with when comp(a,b) and comp(b,a) can both be false 🤦 . Updating the logic to use the lexicographic ordering suggestion.

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// FIXME: The APIs for dealing with the LiveInterval of a frame index are
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