[SCEV] Fix a hang introduced by collectForPHI (llvm#158153)

preames · web-flow · commit 6885950931b8 · 2025-09-12T09:39:57.000-07:00
If we have a phi where one of it's source blocks is an unreachable
block, we don't want to traverse back into the unreachable region. Doing
so allows e.g. finding a trivial self loop when walking back the
predecessor chain.
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -15457,6 +15457,12 @@ 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,
@@ -15511,6 +15517,9 @@ 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,
diff --git a/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll b/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
@@ -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
+
+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
+}
