Objective-C ARC. Blocks that strongly capture themselves

to call themselves will get the warning:
"Capturing <itself> strongly in this block is likely to
lead to a retain cycle". Cut down on the amount of noise
by noticing that user at some point sets the captured variable
to null in order to release it (and break the cycle). 
// rdar://16944538

llvm-svn: 210823

2 files changed, 36 insertions, 5 deletions

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index fc87b231d64..9f7a8bf7b38 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -7286,10 +7286,12 @@ namespace {
   struct FindCaptureVisitor : EvaluatedExprVisitor<FindCaptureVisitor> {
     FindCaptureVisitor(ASTContext &Context, VarDecl *variable)
       : EvaluatedExprVisitor<FindCaptureVisitor>(Context),
-        Variable(variable), Capturer(nullptr) {}
-
+        Context(Context), Variable(variable), Capturer(nullptr),
+        VarWillBeReased(false) {}
+    ASTContext &Context;
     VarDecl *Variable;
     Expr *Capturer;
+    bool VarWillBeReased;
 
     void VisitDeclRefExpr(DeclRefExpr *ref) {
       if (ref->getDecl() == Variable && !Capturer)
@@ -7314,6 +7316,21 @@ namespace {
       if (OVE->getSourceExpr())
         Visit(OVE->getSourceExpr());
     }
+    void VisitBinaryOperator(BinaryOperator *BinOp) {
+      if (!Variable || VarWillBeReased || BinOp->getOpcode() != BO_Assign)
+        return;
+      Expr *LHS = BinOp->getLHS();
+      if (const DeclRefExpr *DRE = dyn_cast_or_null<DeclRefExpr>(LHS)) {
+        if (DRE->getDecl() != Variable)
+          return;
+        if (Expr *RHS = BinOp->getRHS()) {
+          RHS = RHS->IgnoreParenCasts();
+          llvm::APSInt Value;
+          VarWillBeReased =
+            (RHS && RHS->isIntegerConstantExpr(Value, Context) && Value == 0);
+        }
+      }
+    }
   };
 }
 
@@ -7351,7 +7368,7 @@ static Expr *findCapturingExpr(Sema &S, Expr *e, RetainCycleOwner &owner) {
 
   FindCaptureVisitor visitor(S.Context, owner.Variable);
   visitor.Visit(block->getBlockDecl()->getBody());
-  return visitor.Capturer;
+  return visitor.VarWillBeReased ? nullptr : visitor.Capturer;
 }
 
 static void diagnoseRetainCycle(Sema &S, Expr *capturer,
diff --git a/clang/test/SemaObjC/warn-retain-cycle.m b/clang/test/SemaObjC/warn-retain-cycle.m
index eb4e966c772..4398d29e4a8 100644
--- a/clang/test/SemaObjC/warn-retain-cycle.m
+++ b/clang/test/SemaObjC/warn-retain-cycle.m
@@ -122,8 +122,8 @@ void doSomething(unsigned v);
   // Sanity check that we are really whitelisting 'addOperationWithBlock:' and not doing
   // something funny.
   [myOperationQueue addSomethingElse:^() { // expected-note {{block will be retained by an object strongly retained by the captured object}}
-    if (count > 20) { // expected-warning {{capturing 'self' strongly in this block is likely to lead to a retain cycle}}
-      doSomething(count);
+    if (count > 20) {
+      doSomething(count); // expected-warning {{capturing 'self' strongly in this block is likely to lead to a retain cycle}}
     }
   }];
 }
@@ -184,3 +184,17 @@ void testCopying(Test0 *obj) {
   })];
 }
 
+// rdar://16944538
+void func(int someCondition) {
+
+__block void(^myBlock)(void) = ^{
+        if (someCondition) {
+            doSomething(1);
+            myBlock();
+        }
+        else {
+	    myBlock = ((void*)0);
+        }
+   };
+
+}