[NFCI][ValueTracking] getUnderlyingObject(): assert that no cycles are encountered

Jeroen Dobbelaere in
https://lists.llvm.org/pipermail/llvm-dev/2021-March/149206.html
is reporting that this function can end up in an endless loop
when called from SROA w/ full restrict patches.

For now, simply ensure that such problems are caught earlier/easier.

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 79399cf..8c44d73 100644 (file)
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4165,7 +4165,11 @@ static bool isSameUnderlyingObjectInLoop(const PHINode *PN,
 const Value *llvm::getUnderlyingObject(const Value *V, unsigned MaxLookup) {
   if (!V->getType()->isPointerTy())
     return V;
+#ifndef NDEBUG
+  SmallPtrSet<const Value *, 8> Visited;
+#endif
   for (unsigned Count = 0; MaxLookup == 0 || Count < MaxLookup; ++Count) {
+    assert(Visited.insert(V).second && "Cycle detected. Unreachable code?");
     if (auto *GEP = dyn_cast<GEPOperator>(V)) {
       V = GEP->getPointerOperand();
     } else if (Operator::getOpcode(V) == Instruction::BitCast ||