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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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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.

{
// 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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