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JIT: Merge all RETURN/THROW blocks #128515

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JIT: Merge all RETURN/THROW blocks #128515

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f30defb
* call tailMergePreds repeatedly
BoyBaykiller May 23, 2026
a922217
Merge branch 'main' of https://github.com/dotnet/runtime into dedupli…
BoyBaykiller May 25, 2026
77b2d48
* process all sets of matchedCandidates at once in tailMerge instead …
BoyBaykiller May 25, 2026
a679d91
* partition matches to be continous in memory and skip them instead o…
BoyBaykiller May 27, 2026
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62 changes: 31 additions & 31 deletions src/coreclr/jit/fgopt.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5457,48 +5457,48 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)
}
};

ArrayStack<BasicBlock*> retOrThrowBlocks(getAllocator(CMK_ArrayStack));

// Visit each block
// Tail merge predecessors
//
for (BasicBlock* const block : Blocks())
{
iterateTailMerge(block);
if (block->isEmpty())
{
continue;
}
}

if (block->KindIs(BBJ_THROW))
{
retOrThrowBlocks.Push(block);
}
else if (block->KindIs(BBJ_RETURN) && (block != genReturnBB))
// Deduplicate RETURN blocks
//
do
{
predInfo.Reset();
for (BasicBlock* const block : Blocks())
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@AndyAyersMS AndyAyersMS May 24, 2026

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The set of eligible return and throw blocks never changes, so do we need to repeatedly walk the entire block list here?

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@BoyBaykiller BoyBaykiller May 24, 2026

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I would also think we don't need to, however when I tried to hoist that it caused asserts.
I didn't look further into it because the same approach is already done in iterateTailMerge() and there are also multiple comments arround this code about improving algorithm efficiency.
So I'd prefer properly understanding the entire code and improving efficiency in a separate PR, in the future.

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@AndyAyersMS AndyAyersMS May 25, 2026

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iterateTailMerge just walks the preds of a given block, not all blocks.

What asserts did you see?

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@BoyBaykiller BoyBaykiller May 25, 2026 •
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iterateTailMerge just walks the preds of a given block, not all blocks.

Yeah it happens to be only the preds here and so less of an issue but the fundamental thing of not needing to regenerate the set still applies I think.

What asserts did you see?

runtime/src/coreclr/jit/fgstmt.cpp

Lines 542 to 550 in 7f58900

else // The statement is in the middle.
{
assert(stmt->GetPrevStmt() != nullptr && stmt->GetNextStmt() != nullptr);
Statement* prev = stmt->GetPrevStmt();
prev->SetNextStmt(stmt->GetNextStmt());
stmt->GetNextStmt()->SetPrevStmt(prev);
}

Took a quick look, the issue might be that we don't remove entries from predInfo after we merged them.
So it will try to merge them a second time on the second iter - and is never making any progress.
Let me see if I can fix it...

{
// Avoid splitting a return away from a possible tail call
//
if (!block->hasSingleStmt())
if (block->isEmpty())
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@BoyBaykiller BoyBaykiller May 23, 2026

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This check was here before, but I dont think we actually need it. Because we only accept RETURN or THROW blocks and these should never be empty?

{
continue;
}

if (block->KindIs(BBJ_THROW))
{
predInfo.Push(PredInfo(block, block->lastStmt()));
}
else if (block->KindIs(BBJ_RETURN) && (block != genReturnBB))
{
Statement* const lastStmt = block->lastStmt();
Statement* const prevStmt = lastStmt->GetPrevStmt();
GenTree* const prevTree = prevStmt->GetRootNode();
if (prevTree->IsCall() && prevTree->AsCall()->CanTailCall())
// Avoid splitting a return away from a possible tail call
//
if (!block->hasSingleStmt())
{
continue;
Statement* const lastStmt = block->lastStmt();
Statement* const prevStmt = lastStmt->GetPrevStmt();
GenTree* const prevTree = prevStmt->GetRootNode();
if (prevTree->IsCall() && prevTree->AsCall()->CanTailCall())
{
continue;
}
}
}

retOrThrowBlocks.Push(block);
predInfo.Push(PredInfo(block, block->lastStmt()));
}
}
}

predInfo.Reset();
for (BasicBlock* const block : retOrThrowBlocks.BottomUpOrder())
{
predInfo.Push(PredInfo(block, block->lastStmt()));
}

tailMergePreds(nullptr);
} while (tailMergePreds(nullptr));

// Work through any retries
//
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