[ValueTracking] Provide getUnderlyingObjectAggressive fallback

llvm/lib/Analysis/ValueTracking.cpp

@@ -6886,7 +6886,7 @@ const Value *llvm::getUnderlyingObjectAggressive(const Value *V) {
       return FirstObject;
   } while (!Worklist.empty());
 
-  return Object;
+  return Object ? Object : FirstObject;
 }
 
 /// This is the function that does the work of looking through basic

llvm/test/Transforms/FunctionAttrs/phi_cycle.ll

@@ -0,0 +1,52 @@
+; RUN: opt -passes=function-attrs -S < %s
+
+; Regression test for a null-returning bug of getUnderlyingObjectAggressive().
+; This should not crash.
+define void @phi_cycle() {
+bb:
+  unreachable
+
+bb1:                                              ; preds = %bb17
+  br label %bb2
+
+bb2:                                              ; preds = %bb5, %bb1
+  %phi = phi ptr [ %phi6, %bb1 ], [ %phi6, %bb5 ]
+  br i1 poison, label %bb4, label %bb3
+
+bb3:                                              ; preds = %bb2
+  %getelementptr = getelementptr inbounds i8, ptr %phi, i32 poison
+  br label %bb5
+
+bb4:                                              ; preds = %bb2
+  br label %bb7
+
+bb5:                                              ; preds = %bb15, %bb3
+  %phi6 = phi ptr [ %getelementptr, %bb3 ], [ %phi16, %bb15 ]
+  br i1 poison, label %bb17, label %bb2
+
+bb7:                                              ; preds = %bb15, %bb4
+  %phi8 = phi ptr [ %phi, %bb4 ], [ %phi16, %bb15 ]
+  br i1 poison, label %bb11, label %bb9
+
+bb9:                                              ; preds = %bb7
+  %getelementptr10 = getelementptr inbounds i8, ptr %phi8, i32 1
+  store i8 poison, ptr %phi8, align 1
+  br label %bb15
+
+bb11:                                             ; preds = %bb7
+  br i1 poison, label %bb13, label %bb12
+
+bb12:                                             ; preds = %bb11
+  br label %bb13
+
+bb13:                                             ; preds = %bb12, %bb11
+  %getelementptr14 = getelementptr inbounds i8, ptr %phi8, i32 poison
+  br label %bb15
+
+bb15:                                             ; preds = %bb13, %bb9
+  %phi16 = phi ptr [ %getelementptr14, %bb13 ], [ %getelementptr10, %bb9 ]
+  br i1 poison, label %bb5, label %bb7
+
+bb17:                                             ; preds = %bb5
+  br label %bb1
+}