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[SCEV] Fix a hang introduced by collectForPHI #158153

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Merged
merged 4 commits into from
Sep 12, 2025
Merged

[SCEV] Fix a hang introduced by collectForPHI #158153

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c1c6c64
[SCEV] Fix an compiler hang introduced by collectForPHI
preames Sep 11, 2025
a7133dd
Minimal change to actually hand on old opt
preames Sep 12, 2025
7dee654
instnamer for hand reduce
preames Sep 12, 2025
074ecdc
Further hand reduction
preames Sep 12, 2025
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9 changes: 9 additions & 0 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -15445,6 +15445,12 @@ void ScalarEvolution::LoopGuards::collectFromPHI(
const BasicBlock *InBlock = Phi.getIncomingBlock(IncomingIdx);
if (!VisitedBlocks.insert(InBlock).second)
return {nullptr, scCouldNotCompute};

// Avoid analyzing unreachable blocks so that we don't get trapped
// traversing cycles with ill-formed dominance or infinite cycles
if (!SE.DT.isReachableFromEntry(InBlock))
return {nullptr, scCouldNotCompute};

auto [G, Inserted] = IncomingGuards.try_emplace(InBlock, LoopGuards(SE));
if (Inserted)
collectFromBlock(SE, G->second, Phi.getParent(), InBlock, VisitedBlocks,
Expand Down Expand Up @@ -15499,6 +15505,9 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
ScalarEvolution &SE, ScalarEvolution::LoopGuards &Guards,
const BasicBlock *Block, const BasicBlock *Pred,
SmallPtrSetImpl<const BasicBlock *> &VisitedBlocks, unsigned Depth) {

assert(SE.DT.isReachableFromEntry(Block) && SE.DT.isReachableFromEntry(Pred));

SmallVector<const SCEV *> ExprsToRewrite;
auto CollectCondition = [&](ICmpInst::Predicate Predicate, const SCEV *LHS,
const SCEV *RHS,
Expand Down
26 changes: 26 additions & 0 deletions ...st/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
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Expand Up @@ -364,3 +364,29 @@ body:
exit:
ret void
}

define void @hang_due_to_unreachable_phi_inblock() personality ptr null {
bb:
br label %bb6

self-loop: ; preds = %self-loop
%dead = invoke ptr null()
to label %self-loop unwind label %bb4
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@nikic nikic Sep 12, 2025

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Would be nice to replace this invoke + landingpad with simple branches (which I think should work?)

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@preames preames Sep 12, 2025

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I tried that, and it no longer hung on old opt. I might have gone too simple (I used a poison condition), but I suspect we might special case that idiom somewhere. I'll give it one more go before landing to be sure.

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@preames preames Sep 12, 2025

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Yeah, trying multiple variants, up to an including volatile stores, doesn't seem to trip the hang any more. Not sure why we need the invoke, but we appear to.

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@preames preames Sep 12, 2025

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Oh, I figured out the difference.

for (; Pair.first;
       Pair = SE.getPredecessorWithUniqueSuccessorForBB(Pair.first)) {
    VisitedBlocks.insert(Pair.second);
    const BranchInst *LoopEntryPredicate =
        dyn_cast<BranchInst>(Pair.first->getTerminator());
    if (!LoopEntryPredicate || LoopEntryPredicate->isUnconditional())
      continue;

    Terms.emplace_back(LoopEntryPredicate->getCondition(),
                       LoopEntryPredicate->getSuccessor(0) == Pair.second);
    NumCollectedConditions++;

    // If we are recursively collecting guards stop after 2
    // conditions to limit compile-time impact for now.
    if (Depth > 0 && NumCollectedConditions == 2)
      break;
  }

This the loop which is actually infintte. The Depth on this example is 1. As a result, if the cycle we're looping on contains a conditional branch, we hit the NumCollectedConditions guard. Per the comment, it wasn't meant to be functional, but that explains why we hit this so rarely.


bb4: ; preds = %self-loop
%i5 = landingpad { ptr, i32 }
cleanup
br label %bb6

bb6: ; preds = %bb4, %bb
%i7 = phi ptr [ null, %bb4 ], [ null, %bb ]
br label %bb8

bb8: ; preds = %bb8, %bb6
%i9 = phi ptr [ null, %bb8 ], [ null, %bb6 ]
%i11 = icmp eq ptr %i9, null
br i1 %i11, label %bb12, label %bb8

bb12: ; preds = %bb8, %bb6
ret void
}
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