[IndVars] Don't assume backedge value is instruction (PR64891)

In degenerate cases, the backedge value can be folded to poison.

Fixes https://github.com/llvm/llvm-project/issues/64891.

1 files changed, 9 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index a28916b..14582c7 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -1944,13 +1944,15 @@ PHINode *WidenIV::createWideIV(SCEVExpander &Rewriter) {
   // SCEVExpander. Henceforth, we produce 1-to-1 narrow to wide uses.
   if (BasicBlock *LatchBlock = L->getLoopLatch()) {
     WideInc =
-      cast<Instruction>(WidePhi->getIncomingValueForBlock(LatchBlock));
-    WideIncExpr = SE->getSCEV(WideInc);
-    // Propagate the debug location associated with the original loop increment
-    // to the new (widened) increment.
-    auto *OrigInc =
-      cast<Instruction>(OrigPhi->getIncomingValueForBlock(LatchBlock));
-    WideInc->setDebugLoc(OrigInc->getDebugLoc());
+        dyn_cast<Instruction>(WidePhi->getIncomingValueForBlock(LatchBlock));
+    if (WideInc) {
+      WideIncExpr = SE->getSCEV(WideInc);
+      // Propagate the debug location associated with the original loop
+      // increment to the new (widened) increment.
+      auto *OrigInc =
+          cast<Instruction>(OrigPhi->getIncomingValueForBlock(LatchBlock));
+      WideInc->setDebugLoc(OrigInc->getDebugLoc());
+    }
   }
 
   LLVM_DEBUG(dbgs() << "Wide IV: " << *WidePhi << "\n");