[NFCI][ValueTracking] getUnderlyingObject(): gracefully handle cycles

Normally, this function just doesn't bother about cycles,
and hopes that the caller supplied small-enough depth
so that at worst it will take a potentially large,
but limited amount of time. But that obviously doesn't work
if there is no depth limit.

This reapples 36f1c3db66f7268ea3183bcf0bbf05b3e1c570b4,
but without asserting, just bailout once cycle is detected.

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 79399cf..5fe0269 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4165,7 +4165,13 @@ static bool isSameUnderlyingObjectInLoop(const PHINode *PN,
 const Value *llvm::getUnderlyingObject(const Value *V, unsigned MaxLookup) {
   if (!V->getType()->isPointerTy())
     return V;
+  // Keep track of all the values we have recursed through.
+  SmallPtrSet<const Value *, 8> Visited;
   for (unsigned Count = 0; MaxLookup == 0 || Count < MaxLookup; ++Count) {
+    // Did we encounter this value already?
+    if (!Visited.insert(V).second)
+      return V; // Cycle detected, we must be in an unreachable code.
+    // Otherwise, recurse further.
     if (auto *GEP = dyn_cast<GEPOperator>(V)) {
       V = GEP->getPointerOperand();
     } else if (Operator::getOpcode(V) == Instruction::BitCast ||