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JIT: Don't move blocks during return merging #113278

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JIT: Don't move blocks during return merging #113278

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ebeec7f
Don't move blocks during return merging
amanasifkhalid Mar 7, 2025
e4f6268
Remove unused member
amanasifkhalid Mar 7, 2025
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58 changes: 5 additions & 53 deletions src/coreclr/jit/flowgraph.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1906,7 +1906,7 @@ class MergedReturns
// cases, `fgReturnCount` and the merged return block's profile information
// will be updated to reflect or anticipate the rewrite of `returnBlock`.
//
void Record(BasicBlock* returnBlock)
bool Record(BasicBlock* returnBlock)
{
// Add this return to our tally
unsigned oldReturnCount = comp->fgReturnCount++;
Expand All @@ -1917,7 +1917,7 @@ class MergedReturns
{
// No need to merge just yet; simply record this return.
returnBlocks[oldReturnCount] = returnBlock;
return;
return false;
}

// We've reached our threshold
Expand All @@ -1939,6 +1939,7 @@ class MergedReturns
// Search limit is new return count minus one (to exclude this block).
unsigned searchLimit = comp->fgReturnCount - 1;
Merge(returnBlock, searchLimit);
return true;
}

//------------------------------------------------------------------------
Expand All @@ -1953,54 +1954,6 @@ class MergedReturns
return Merge(nullptr, 0);
}

//------------------------------------------------------------------------
// PlaceReturns: Move any generated const return blocks to an appropriate
// spot in the lexical block list.
//
// Returns:
// True if any returns were impacted.
//
// Notes:
// The goal is to set things up favorably for a reasonable layout without
// putting too much burden on fgReorderBlocks; in particular, since that
// method doesn't (currently) shuffle non-profile, non-rare code to create
// fall-through and reduce gotos, this method places each const return
// block immediately after its last predecessor, so that the flow from
// there to it can become fallthrough without requiring any motion to be
// performed by fgReorderBlocks.
//
bool PlaceReturns()
{
if (!mergingReturns)
{
// No returns generated => no returns to place.
return false;
}

for (unsigned index = 0; index < comp->fgReturnCount; ++index)
{
BasicBlock* returnBlock = returnBlocks[index];
BasicBlock* genReturnBlock = comp->genReturnBB;
if (returnBlock == genReturnBlock)
{
continue;
}

BasicBlock* insertionPoint = insertionPoints[index];
assert(insertionPoint != nullptr);

comp->fgUnlinkBlock(returnBlock);
comp->fgMoveBlocksAfter(returnBlock, returnBlock, insertionPoint);
// Treat the merged return block as belonging to the same EH region
// as the insertion point block, to make sure we don't break up
// EH regions; since returning a constant won't throw, this won't
// affect program behavior.
comp->fgExtendEHRegionAfter(insertionPoint);
}

return true;
}

private:
//------------------------------------------------------------------------
// CreateReturnBB: Create a basic block to serve as a merged return point, stored to
Expand Down Expand Up @@ -2425,6 +2378,7 @@ PhaseStatus Compiler::fgAddInternal()
// will expect to find it.
BasicBlock* mergedReturn = merger.EagerCreate();
assert(mergedReturn == genReturnBB);
madeChanges = true;
}
else
{
Expand Down Expand Up @@ -2454,12 +2408,10 @@ PhaseStatus Compiler::fgAddInternal()
{
if (block->KindIs(BBJ_RETURN) && !block->HasFlag(BBF_HAS_JMP))
{
merger.Record(block);
madeChanges |= merger.Record(block);
}
}

madeChanges |= merger.PlaceReturns();

if (compMethodRequiresPInvokeFrame())
{
// The P/Invoke helpers only require a frame variable, so only allocate the
Expand Down
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