[ObjC][ARC] Don't move a retain call living outside a loop into the loop

body

We started seeing cases where ARC optimizer would move retain calls into
loop bodies, causing imbalance in the number of retain and release
calls, after changes were made to delete inert ARC calls since the inert
calls that used to block code motion are gone.

Fix the bug by setting the CFG hazard flag when visiting a loop header.

rdar://problem/56908836

llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp

@@ -1569,6 +1569,15 @@ ObjCARCOpt::VisitTopDown(BasicBlock *BB,
     }
   }
 
+  // Check that BB and MyStates have the same number of predecessors. This
+  // prevents retain calls that live outside a loop from being moved into the
+  // loop.
+  if (!BB->hasNPredecessors(MyStates.pred_end() - MyStates.pred_begin()))
+    for (auto I = MyStates.top_down_ptr_begin(),
+              E = MyStates.top_down_ptr_end();
+         I != E; ++I)
+      I->second.SetCFGHazardAfflicted(true);
+
   LLVM_DEBUG(dbgs() << "Before:\n"
                     << BBStates[BB] << "\n"
                     << "Performing Dataflow:\n");

llvm/test/Transforms/ObjCARC/cfg-hazards.ll

@@ -430,6 +430,48 @@ exit:
   ret void
 }
 
+; The retain call in the entry block shouldn't be moved to the loop body.
+
+; CHECK: define void @test14(i8* %[[VAL0:.*]],
+; CHECK: %[[V1:.*]] = tail call i8* @llvm.objc.retain(i8* %[[VAL0]])
+; CHECK: %[[CMP:.*]] = icmp eq i8* %[[VAL0]], null
+; CHECK: br i1 %[[CMP]], label %{{.*}}, label %{{.*}}
+
+define void @test14(i8* %val0, i8 %val1) {
+entry:
+  %v1 = tail call i8* @llvm.objc.retain(i8* %val0)
+  %cmp = icmp eq i8* %val0, null
+  br i1 %cmp, label %if.end27, label %if.then
+
+if.then:
+  %tobool = icmp eq i8 %val1, 1
+  br label %for.body
+
+for.cond:
+  %cmp6 = icmp eq i8 %val1, 2
+  br i1 %cmp6, label %for.body, label %for.end.loopexit
+
+for.body:
+  call void @callee()
+  %tobool9 = icmp eq i8 %val1, 0
+  br i1 %tobool9, label %for.cond, label %if.then10
+
+if.then10:
+  br label %for.end
+
+for.end.loopexit:
+  br label %for.end
+
+for.end:
+  call void @callee()
+  call void @use_pointer(i8* %v1)
+  br label %if.end27
+
+if.end27:
+  call void @llvm.objc.release(i8* %v1) #0, !clang.imprecise_release !0
+  ret void
+}
+
 ; CHECK: attributes [[NUW]] = { nounwind }
 
 !0 = !{}