fix PR12075, a regression in a recent transform I added. In unreachable code, gep chains can be infinite. Just like "stripPointerCasts", use a set to keep track of visited instructions so we don't recurse infinitely.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151383 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/InstructionSimplify.cpp

@@ -1522,13 +1522,27 @@ static Value *ExtractEquivalentCondition(Value *V, CmpInst::Predicate Pred,
 
 /// stripPointerAdjustments - This is like Value::stripPointerCasts, but also
 /// removes inbounds gep operations, regardless of their indices.
-static Value *stripPointerAdjustments(Value *V) {
-  if (GEPOperator *GEP = dyn_cast<GEPOperator>(V))
-    if (GEP->isInBounds())
-      return stripPointerAdjustments(GEP->getOperand(0)->stripPointerCasts());
-  return V;
+static Value *stripPointerAdjustmentsImpl(Value *V,
+                                    SmallPtrSet<GEPOperator*, 8> &VisitedGEPs) {
+  GEPOperator *GEP = dyn_cast<GEPOperator>(V);
+  if (GEP == 0 || !GEP->isInBounds())
+    return V;
+
+  // If we've already seen this GEP, we will end up infinitely looping.  This
+  // can happen in unreachable code.
+  if (!VisitedGEPs.insert(GEP))
+    return V;
+  
+  return stripPointerAdjustmentsImpl(GEP->getOperand(0)->stripPointerCasts(),
+                                     VisitedGEPs);
 }
 
+static Value *stripPointerAdjustments(Value *V) {
+  SmallPtrSet<GEPOperator*, 8> VisitedGEPs;
+  return stripPointerAdjustmentsImpl(V, VisitedGEPs);
+}
+
+
 /// SimplifyICmpInst - Given operands for an ICmpInst, see if we can
 /// fold the result.  If not, this returns null.
 static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,

test/Transforms/InstSimplify/compare.ll

@@ -463,3 +463,13 @@ define i1 @alloca_compare(i64 %idx) {
   ; CHECK: alloca_compare
   ; CHECK: ret i1 false
 }
+
+; PR12075
+define i1 @infinite_gep() {
+  ret i1 1
+
+unreachableblock:
+  %X = getelementptr i32 *%X, i32 1
+  %Y = icmp eq i32* %X, null
+  ret i1 %Y
+}