Call objc_retainBlock before passing a block as a variadic argument

Copy the block to the heap before passing it to the callee in case the
block escapes in the callee.

rdar://problem/55683462

Differential Revision: https://reviews.llvm.org/D71431

clang/docs/AutomaticReferenceCounting.rst

@@ -1863,6 +1863,12 @@ call for retaining a value of block-pointer type, it has the effect of a
 ``Block_copy``.  The optimizer may remove such copies when it sees that the
 result is used only as an argument to a call.
 
+When a block pointer type is converted to a non-block pointer type (such as
+``id``), ``Block_copy`` is called. This is necessary because a block allocated
+on the stack won't get copied to the heap when the non-block pointer escapes.
+A block pointer is implicitly converted to ``id`` when it is passed to a
+function as a variadic argument.
+
 .. _arc.misc.exceptions:
 
 Exceptions

clang/lib/Sema/SemaExpr.cpp

@@ -5247,6 +5247,9 @@ bool Sema::GatherArgumentsForCall(SourceLocation CallLoc, FunctionDecl *FDecl,
       for (Expr *A : Args.slice(ArgIx)) {
         ExprResult Arg = DefaultVariadicArgumentPromotion(A, CallType, FDecl);
         Invalid |= Arg.isInvalid();
+        // Copy blocks to the heap.
+        if (A->getType()->isBlockPointerType())
+          maybeExtendBlockObject(Arg);
         AllArgs.push_back(Arg.get());
       }
     }

clang/test/CodeGenObjC/arc-blocks.m

@@ -730,5 +730,15 @@ void test20(const id x) {
   test20_callee(^{ (void)x; });
 }
 
+// CHECK-LABEL: define void @test21(
+// CHECK: %[[V6:.*]] = call i8* @llvm.objc.retainBlock(
+// CHECK: %[[V7:.*]] = bitcast i8* %[[V6]] to void ()*
+// CHECK: call void (i32, ...) @test21_callee(i32 1, void ()* %[[V7]]),
+
+void test21_callee(int n, ...);
+void test21(id x) {
+  test21_callee(1, ^{ (void)x; });
+}
+
 // CHECK: attributes [[NUW]] = { nounwind }
 // CHECK-UNOPT: attributes [[NUW]] = { nounwind }