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JIT: Tail-merge robust crossJumpVictim selection #129141

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JIT: Tail-merge robust crossJumpVictim selection #129141

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48e1618
* remove candidate block (pred) if possible. This helps downstream ph…
BoyBaykiller Jun 7, 2026
dacb42f
* add tie-break
BoyBaykiller Jun 8, 2026
74d0856
* remove explicit isNoGcPoll heuristic
BoyBaykiller Jun 9, 2026
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118 changes: 73 additions & 45 deletions src/coreclr/jit/fgopt.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5212,6 +5212,23 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)

fgUnlinkStmt(predBlock, stmt);

bool canRemove =
predBlock->isEmpty() && !predBlock->HasFlag(BBF_DONT_REMOVE) && predBlock != fgFirstBB;
if (canRemove)
{
for (BasicBlock* const pred : predBlock->PredBlocksEditing())
{
fgReplaceJumpTarget(pred, predBlock, commSucc);
}

fgRemoveBlock(predBlock, true);

if (commSucc->hasProfileWeight())
{
commSucc->increaseBBProfileWeight(predBlock->bbWeight);
}
}

// Add one of the matching stmts to block, and
// update its flags.
//
Expand Down Expand Up @@ -5250,10 +5267,9 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)

JITDUMPEXEC(gtDispStmt(matchedPredInfo.TopRef(0).m_stmt));

BasicBlock* crossJumpVictim = nullptr;
Statement* crossJumpStmt = nullptr;
bool haveNoSplitVictim = false;
bool haveFallThroughVictim = false;
BasicBlock* crossJumpVictim = nullptr;
Statement* crossJumpStmt = nullptr;
unsigned bestRank = UINT32_MAX;

for (PredInfo& info : matchedPredInfo.TopDownOrder())
{
Expand All @@ -5267,46 +5283,34 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)
continue;
}

bool const isNoSplit = stmt == predBlock->firstStmt();
bool const isFallThrough = (predBlock->KindIs(BBJ_ALWAYS) && predBlock->JumpsToNext());
auto getRank = [=]() -> unsigned {
bool const isNoSplit = stmt == predBlock->firstStmt();
bool const isFallThrough = (predBlock->KindIs(BBJ_ALWAYS) && predBlock->JumpsToNext());

// Is this block possibly better than what we have?
//
bool useBlock = false;

if (crossJumpVictim == nullptr)
{
// Pick an initial candidate.
useBlock = true;
}
else if (isNoSplit && isFallThrough)
{
// This is the ideal choice.
// From most to least preferable.
//
useBlock = true;
}
else if (!haveNoSplitVictim && isNoSplit)
{
useBlock = true;
}
else if (!haveNoSplitVictim && !haveFallThroughVictim && isFallThrough)
{
useBlock = true;
}
if (isNoSplit && isFallThrough)
return 0;
if (isNoSplit)
return 1;
if (isFallThrough)
return 2;

return 3;
};

if (useBlock)
unsigned const rank = getRank();
bool pick = rank < bestRank;
if (rank == bestRank)
{
crossJumpVictim = predBlock;
crossJumpStmt = stmt;
haveNoSplitVictim = isNoSplit;
haveFallThroughVictim = isFallThrough;
pick = predBlock->bbNum < crossJumpVictim->bbNum;
}

// If we have the perfect victim, stop looking.
//
if (haveNoSplitVictim && haveFallThroughVictim)
if (pick)
{
break;
crossJumpVictim = predBlock;
crossJumpStmt = stmt;
bestRank = rank;
}
}

Expand All @@ -5315,7 +5319,7 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)
// If this block requires splitting, then split it.
// Note we know that stmt has a prev stmt.
//
if (haveNoSplitVictim)
if (crossJumpStmt == crossJumpTarget->firstStmt())
{
JITDUMP("Will cross-jump to " FMT_BB "\n", crossJumpTarget->bbNum);
}
Expand Down Expand Up @@ -5344,15 +5348,32 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)

// Fix up the flow.
//
if (commSucc != nullptr)
bool canRemove = predBlock->isEmpty() && !predBlock->HasFlag(BBF_DONT_REMOVE) && predBlock != fgFirstBB;
if (canRemove)
{
assert(predBlock->KindIs(BBJ_ALWAYS));
fgRedirectEdge(predBlock->TargetEdgeRef(), crossJumpTarget);
for (BasicBlock* const pred : predBlock->PredBlocksEditing())
{
fgReplaceJumpTarget(pred, predBlock, crossJumpTarget);
}
fgRemoveBlock(predBlock, true);

if (commSucc != nullptr && commSucc->hasProfileWeight())
{
commSucc->increaseBBProfileWeight(predBlock->bbWeight);
}
}
else
{
FlowEdge* const newEdge = fgAddRefPred(crossJumpTarget, predBlock);
predBlock->SetKindAndTargetEdge(BBJ_ALWAYS, newEdge);
if (commSucc != nullptr)
{
assert(predBlock->KindIs(BBJ_ALWAYS));
fgRedirectEdge(predBlock->TargetEdgeRef(), crossJumpTarget);
}
else
{
FlowEdge* const newEdge = fgAddRefPred(crossJumpTarget, predBlock);
predBlock->SetKindAndTargetEdge(BBJ_ALWAYS, newEdge);
}
}

// For tail merge we have a common successor of predBlock and
Expand Down Expand Up @@ -5402,6 +5423,13 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)
continue;
}

// If this block was already processed, skip it
//
if (predBlock->isEmpty())
{
continue;
}

Statement* lastStmt = predBlock->lastStmt();

// Block might be empty.
Expand Down Expand Up @@ -5498,9 +5526,9 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)
predInfo.Reset();
for (BasicBlock* const block : retOrThrowBlocks.BottomUpOrder())
{
// If this block was already merged, skip it
// If this block was already processed, skip it
//
if (!block->KindIs(BBJ_RETURN, BBJ_THROW))
if (!block->KindIs(BBJ_RETURN, BBJ_THROW) || block->isEmpty())
{
continue;
}
Expand Down
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