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JIT: Tail-merge robust crossJumpVictim selection #129141

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Merged
EgorBo merged 4 commits into from
Jun 29, 2026
Merged

JIT: Tail-merge robust crossJumpVictim selection #129141

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d42fc3c
* add tie-break
BoyBaykiller Jun 8, 2026
a9d4d53
* remove explicit isNoGcPoll heuristic
BoyBaykiller Jun 9, 2026
b5bd36a
Merge branch 'main' into robust-crossJumpVictim
BoyBaykiller Jun 25, 2026
58079eb
* feedback
BoyBaykiller Jun 25, 2026
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69 changes: 31 additions & 38 deletions src/coreclr/jit/fgopt.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5285,10 +5285,9 @@ 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 @@ -5302,46 +5301,40 @@ PhaseStatus Compiler::fgHeadTailMerge(bool early)
continue;
}

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

@EgorBo EgorBo Jun 29, 2026

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not sure I understand why you wrapped it to a single-use lambda, not a big fan of this approach.

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)
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{
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->bbID < crossJumpVictim->bbID;
}

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

Expand All @@ -5350,7 +5343,7 @@ 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);
}
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