[LoopVectorize] Fix crash with GC statepoint in epilogue vectorization

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7356,8 +7356,10 @@ static void fixReductionScalarResumeWhenVectorizingEpilog(
           VPlanPatternMatch::m_Select(VPlanPatternMatch::m_VPValue(),
                                       VPlanPatternMatch::m_VPValue(BackedgeVal),
                                       VPlanPatternMatch::m_VPValue()));
-    EpiRedHeaderPhi = cast<VPReductionPHIRecipe>(
+    EpiRedHeaderPhi = cast_if_present<VPReductionPHIRecipe>(
         vputils::findRecipe(BackedgeVal, IsaPred<VPReductionPHIRecipe>));
+    if (!EpiRedHeaderPhi)
+      return;
   }
 
   Value *MainResumeValue;
@@ -9361,12 +9363,22 @@ static void fixScalarResumeValuesFromBypass(BasicBlock *BypassBlock, Loop *L,
   // Fix induction resume values from the additional bypass block.
   IRBuilder<> BypassBuilder(BypassBlock, BypassBlock->getFirstInsertionPt());
   for (const auto &[IVPhi, II] : LVL.getInductionVars()) {
-    auto *Inc = cast<PHINode>(IVPhi->getIncomingValueForBlock(PH));
+    Value *IncomingFromPH = IVPhi->getIncomingValueForBlock(PH);
     Value *V = createInductionAdditionalBypassValues(
         IVPhi, II, BypassBuilder, ExpandedSCEVs, MainVectorTripCount,
         LVL.getPrimaryInduction());
-    // TODO: Directly add as extra operand to the VPResumePHI recipe.
-    Inc->setIncomingValueForBlock(BypassBlock, V);
+
+    if (auto *Inc = dyn_cast<PHINode>(IncomingFromPH)) {
+      // TODO: Directly add as extra operand to the VPResumePHI recipe.
+      Inc->setIncomingValueForBlock(BypassBlock, V);
+    } else {
+      // If the incoming value from preheader is not a PHI node (e.g., a
+      // constant in functions with GC statepoint), directly add an incoming
+      // value from BypassBlock to IVPhi. The incoming value from PH remains
+      // unchanged (IncomingFromPH).
+      if (IVPhi->getBasicBlockIndex(BypassBlock) == -1)
+        IVPhi->addIncoming(V, BypassBlock);
+    }
   }
 }
 

llvm/test/Transforms/LoopVectorize/pr179407-gc-statepoint.ll

@@ -0,0 +1,40 @@
+; RUN: opt -passes=loop-vectorize -enable-epilogue-vectorization -force-vector-width=2 -epilogue-vectorization-force-VF=2 -S < %s | FileCheck %s
+;
+; Test case for issue #179407: LoopVectorize should not crash when
+; vectorizing loops in functions with GC statepoint.
+;
+; The issue was that fixScalarResumeValuesFromBypass assumed
+; IVPhi->getIncomingValueForBlock(PH) always returns a PHINode, but in
+; functions with GC statepoint, it could return a constant or other value.
+;
+; This test verifies that epilogue vectorization works correctly with
+; GC statepoint by checking that the loop is vectorized and the
+; bypass blocks are properly created.
+
+; CHECK: define void @wombat
+; CHECK-SAME: gc "statepoint-example"
+; CHECK: vector.body:
+; CHECK: vec.epilog.vector.body:
+; CHECK: vec.epilog.scalar.ph:
+; CHECK: bb1:
+
+define void @wombat(i64 %arg) gc "statepoint-example" {
+bb:
+  br label %bb1
+
+bb1:                                              ; preds = %bb1, %bb
+  %phi = phi i64 [ 0, %bb ], [ %add, %bb1 ]
+  %phi2 = phi i32 [ 0, %bb ], [ %or, %bb1 ]
+  %phi3 = phi i32 [ 0, %bb ], [ %select, %bb1 ]
+  %icmp = icmp eq i32 0, 0
+  %select = select i1 %icmp, i32 0, i32 %phi3
+  %or = or i32 %phi2, 0
+  %add = add i64 %phi, 1
+  %icmp4 = icmp ult i64 %phi, %arg
+  br i1 %icmp4, label %bb1, label %bb5
+
+bb5:                                              ; preds = %bb1
+  %phi6 = phi i32 [ %select, %bb1 ]
+  %phi7 = phi i32 [ %or, %bb1 ]
+  ret void
+}