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[5.1] SimplifyCFG: fix a compile time bug from exponential jump threading. #25452

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Merged
merged 1 commit into from
Jun 14, 2019
Merged

[5.1] SimplifyCFG: fix a compile time bug from exponential jump threading. #25452

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30 changes: 18 additions & 12 deletions lib/SILOptimizer/Transforms/SimplifyCFG.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -72,9 +72,9 @@ namespace {
llvm::SmallDenseMap<SILBasicBlock*, unsigned, 32> WorklistMap;
// Keep track of loop headers - we don't want to jump-thread through them.
SmallPtrSet<SILBasicBlock *, 32> LoopHeaders;
// The number of times jump threading was done through a block.
// Used to prevent infinite jump threading loops.
llvm::SmallDenseMap<SILBasicBlock*, unsigned, 8> JumpThreadedBlocks;
// The cost (~ number of copied instructions) of jump threading per basic
// block. Used to prevent infinite jump threading loops.
llvm::SmallDenseMap<SILBasicBlock *, int, 8> JumpThreadingCost;

// Dominance and post-dominance info for the current function
DominanceInfo *DT = nullptr;
Expand Down Expand Up @@ -1014,15 +1014,19 @@ bool SimplifyCFG::tryJumpThreading(BranchInst *BI) {
}
}

ThreadingBudget -= JumpThreadingCost[SrcBB];
ThreadingBudget -= JumpThreadingCost[DestBB];

// If we don't have anything that we can simplify, don't do it.
if (ThreadingBudget == 0)
if (ThreadingBudget <= 0)
return false;

// If it looks potentially interesting, decide whether we *can* do the
// operation and whether the block is small enough to be worth duplicating.
int copyCosts = 0;
for (auto &Inst : *DestBB) {
ThreadingBudget -= getThreadingCost(&Inst);
if (ThreadingBudget <= 0)
copyCosts += getThreadingCost(&Inst);
if (ThreadingBudget <= copyCosts)
return false;

// We need to update ssa if a value is used outside the duplicated block.
Expand All @@ -1044,15 +1048,11 @@ bool SimplifyCFG::tryJumpThreading(BranchInst *BI) {
return false;
}

// Limit the number we jump-thread through a block.
// Otherwise we may end up with jump-threading indefinitely.
unsigned &NumThreaded = JumpThreadedBlocks[DestBB];
if (++NumThreaded > 16)
return false;

LLVM_DEBUG(llvm::dbgs() << "jump thread from bb" << SrcBB->getDebugID()
<< " to bb" << DestBB->getDebugID() << '\n');

JumpThreadingCost[DestBB] += copyCosts;

// Okay, it looks like we want to do this and we can. Duplicate the
// destination block into this one, rewriting uses of the BBArgs to use the
// branch arguments as we go.
Expand Down Expand Up @@ -1267,6 +1267,12 @@ bool SimplifyCFG::simplifyBranchBlock(BranchInst *BI) {
if (LoopHeaders.count(DestBB))
LoopHeaders.insert(BB);

auto Iter = JumpThreadingCost.find(DestBB);
if (Iter != JumpThreadingCost.end()) {
int costs = Iter->second;
JumpThreadingCost[BB] += costs;
}

removeFromWorklist(DestBB);
DestBB->eraseFromParent();
++NumBlocksMerged;
Expand Down
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