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[JumpThreading] NFC: Add comment referencing #199408. #199718

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[JumpThreading] NFC: Add comment referencing #199408. #199718

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7 changes: 7 additions & 0 deletions llvm/lib/Transforms/Scalar/JumpThreading.cpp
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Expand Up @@ -2789,6 +2789,13 @@ void JumpThreadingPass::unfoldSelectInstr(BasicBlock *Pred, BasicBlock *BB,
PredTerm->removeFromParent();
PredTerm->insertInto(NewBB, NewBB->end());
// Create a conditional branch and update PHI nodes.
//
// Note: Technically we should `freeze` the condition before using it in a
// conditional branch, unless we can prove it's not poison: select-on-on
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@boomanaiden154 boomanaiden154 May 26, 2026

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s/select-on-on/select-on-poison?

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@jlebar jlebar May 26, 2026

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See that's what I get when I don't have an LLM write my code. :D

// poison isn't UB, but branch-on-poison is. But doing this causes
// performance regressions, and we haven't been able to find a source-level
// correctness issue it fixes. See
// https://github.com/llvm/llvm-project/pull/199408#issuecomment-4545013881.
auto *BI = CondBrInst::Create(SI->getCondition(), NewBB, BB, Pred);
BI->applyMergedLocation(PredTerm->getDebugLoc(), SI->getDebugLoc());
BI->copyMetadata(*SI, {LLVMContext::MD_prof});
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