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InsertSync: keep loop-carried head/tail sync within owning loop scope #514

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InsertSync: keep loop-carried head/tail sync within owning loop scope #514

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e1d6788
InsertSync: keep loop-carried head/tail sync scoped to owning loop
TaoTao-real Apr 18, 2026
03611c0
fix(insert-sync): clean and reallocate loop-anchored backward sync sa…
TaoTao-real Apr 19, 2026
120cd0a
test(sync): stabilize issue454 nested-loop FileCheck
TaoTao-real Apr 20, 2026
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34 changes: 18 additions & 16 deletions lib/PTO/Transforms/InsertSync/SyncEventIdAllocation.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -417,22 +417,24 @@ void SyncEventIdAllocation::UpdateBackwardMatchSync(
syncFront->eventIds.push_back(eventId);
syncEnd->eventIds.push_back(eventId);

if (reallocatedPipePair.count(ScopePair(setFlag))) {
auto *ptr = dyn_cast<LoopInstanceElement>(
syncIR_[setFlag->GetForEndIndex().value()].get());
assert(ptr != nullptr);
syncFront->SetSyncIRIndex(ptr->beginId);
syncEnd->SetSyncIRIndex(ptr->endId);
syncFront->reallocatedLoopHeadTailSync = true;
syncEnd->reallocatedLoopHeadTailSync = true;
syncIR_[ptr->beginId]->pipeBefore.push_back(syncFront.get());
syncIR_[ptr->endId]->pipeAfter.push_back(syncEnd.get());
} else {
syncFront->SetSyncIRIndex(0);
syncEnd->SetSyncIRIndex(syncIR_.size() - 1);
syncIR_[0]->pipeBefore.push_back(syncFront.get());
syncIR_[syncIR_.size() - 1]->pipeAfter.push_back(syncEnd.get());
}
auto *ptr = dyn_cast<LoopInstanceElement>(
syncIR_[setFlag->GetForEndIndex().value()].get());
assert(ptr != nullptr);

// Keep loop-carried head/tail syncs scoped to their owning loop.
// Hoisting them to function entry/exit can make them execute even when an
// enclosing branch is not taken, which is both noisy and semantically risky.
syncFront->SetSyncIRIndex(ptr->beginId);
syncEnd->SetSyncIRIndex(ptr->endId);
Comment on lines +427 to +428
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@gemini-code-assist gemini-code-assist Bot Apr 18, 2026

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high

Anchoring loop-carried syncs to the loop boundaries (ptr->beginId and ptr->endId) instead of the function boundaries (index 0 and size-1) is correct for scoping, but it breaks assumptions in other functions that specifically look at the function boundaries for these syncs:

  1. ClearReallocatedBackwardMatchSync (lines 572-588): This function only iterates over syncIR_[0] and syncIR_[last]. Since loop-carried syncs are now anchored to loop boundaries from the first allocation attempt, this function will fail to remove them during reallocation, leading to duplicate sync instructions in the final output.
  2. IgnoreBackHeadAndTailSync (lines 644-661): This optimization only checks syncIR_[0]. It will now miss loop-carried syncs anchored at loop boundaries, effectively disabling the MTE1->M optimization for these cases.

These functions should be refactored to either iterate over all syncIR_ elements or leverage the insertedBackwardSync set to identify and process all synthetic backward match syncs regardless of their anchor point.


// Preserve historical marker semantics: this flag is only used by the
// reallocation fallback path.
bool markAsReallocatedHeadTail = reallocatedPipePair.count(ScopePair(setFlag));
syncFront->reallocatedLoopHeadTailSync = markAsReallocatedHeadTail;
syncEnd->reallocatedLoopHeadTailSync = markAsReallocatedHeadTail;

syncIR_[ptr->beginId]->pipeBefore.push_back(syncFront.get());
syncIR_[ptr->endId]->pipeAfter.push_back(syncEnd.get());

insertedBackwardSync.insert(syncFront.get());
insertedBackwardSync.insert(syncEnd.get());
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