[JumpThreading][VectorUtils] avoid infinite loop on unreachable IR

https://llvm.org/PR48362

It's possible that we could stub this out sooner somewhere
within JumpThreading, but I'm not sure how to do that, and
then we would still have potential danger in other callers.

I can't find a way to trigger this using 'instsimplify',
however, because that already has a bailout on unreachable
blocks.

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/VectorUtils.cpp b/llvm/lib/Analysis/VectorUtils.cpp
index bd69055..9072697 100644
--- a/llvm/lib/Analysis/VectorUtils.cpp
+++ b/llvm/lib/Analysis/VectorUtils.cpp
@@ -290,6 +290,10 @@ Value *llvm::findScalarElement(Value *V, unsigned EltNo) {
     if (EltNo == IIElt)
       return III->getOperand(1);
 
+    // Guard against infinite loop on malformed, unreachable IR.
+    if (III == III->getOperand(0))
+      return nullptr;
+
     // Otherwise, the insertelement doesn't modify the value, recurse on its
     // vector input.
     return findScalarElement(III->getOperand(0), EltNo);