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JIT: Don't compact away block in cond-to-return folding #113931

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Merged
amanasifkhalid merged 1 commit into from
Mar 26, 2025
Merged

JIT: Don't compact away block in cond-to-return folding #113931

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15 changes: 8 additions & 7 deletions src/coreclr/jit/optimizebools.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1698,18 +1698,19 @@ bool Compiler::fgFoldCondToReturnBlock(BasicBlock* block)
}

// Both edges must be BBJ_RETURN
BasicBlock* retFalseBb = block->GetFalseTarget();
BasicBlock* retTrueBb = block->GetTrueTarget();
BasicBlock* const retFalseBb = block->GetFalseTarget();
BasicBlock* const retTrueBb = block->GetTrueTarget();

// Although, we might want to fold fallthrough BBJ_ALWAYS blocks first
if (fgCanCompactBlock(retTrueBb))
// We might want to compact BBJ_ALWAYS blocks first,
// but don't compact the conditional block away in the process
if (fgCanCompactBlock(retTrueBb) && !retTrueBb->TargetIs(block))
{
fgCompactBlock(retTrueBb);
modified = true;
}
// By the time we get to the retFalseBb, it might be removed by fgCompactBlock()
// so we need to check if it is still valid.
if (!retFalseBb->HasFlag(BBF_REMOVED) && fgCanCompactBlock(retFalseBb))
if (!retFalseBb->HasFlag(BBF_REMOVED) && fgCanCompactBlock(retFalseBb) && !retFalseBb->TargetIs(block))
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@EgorBo EgorBo Mar 26, 2025

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presumably you also don't need !retFalseBb->HasFlag(BBF_REMOVED) check

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@amanasifkhalid amanasifkhalid Mar 26, 2025

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I think we still need this too in the case of the diamond shape: If the true target jumps to the false target and we compact them, retFalseBb will be removed from the flowgraph. This check currently defends against this case.

{
fgCompactBlock(retFalseBb);
modified = true;
Expand All @@ -1720,8 +1721,8 @@ bool Compiler::fgFoldCondToReturnBlock(BasicBlock* block)
return modified;
}

retTrueBb = block->GetTrueTarget();
retFalseBb = block->GetFalseTarget();
assert(block->TrueTargetIs(retTrueBb));
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@EgorBo EgorBo Mar 26, 2025

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there is Same here - bail out if the block is no longer BBJ_COND after compacting. comment with a condition that you can remove I presume

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@amanasifkhalid amanasifkhalid Mar 26, 2025

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I don't think we can remove this: Suppose the true target jumps to the false target, and we compact the false target into the true target. This will make the conditional block degenerate, since it now branches to the same target in either case. fgReplaceJumpTarget will detect this, and convert the block into a BBJ_ALWAYS, so I think we need to check for this case in the caller.

assert(block->FalseTargetIs(retFalseBb));
if (!retTrueBb->KindIs(BBJ_RETURN) || !retFalseBb->KindIs(BBJ_RETURN) ||
!BasicBlock::sameEHRegion(block, retTrueBb) || !BasicBlock::sameEHRegion(block, retFalseBb) ||
(retTrueBb == genReturnBB) || (retFalseBb == genReturnBB))
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