[ObjCARC] Add an new attribute, objc_externally_retained

This attribute, called "objc_externally_retained", exposes clang's
notion of pseudo-__strong variables in ARC. Pseudo-strong variables
"borrow" their initializer, meaning that they don't retain/release
it, instead assuming that someone else is keeping their value alive.

If a function is annotated with this attribute, implicitly strong
parameters of that function aren't implicitly retained/released in
the function body, and are implicitly const. This is useful to expose
for performance reasons, most functions don't need the extra safety
of the retain/release, so programmers can opt out as needed.

This attribute can also apply to declarations of local variables,
with similar effect.

Differential revision: https://reviews.llvm.org/D55865

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@350422 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 7 insertions, 0 deletions

diff --git a/test/SemaObjC/externally-retained-no-arc.m b/test/SemaObjC/externally-retained-no-arc.m
new file mode 100644
index 0000000000..a548d6b88d
--- /dev/null
+++ b/test/SemaObjC/externally-retained-no-arc.m
@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 %s -verify
+
+@interface NSWidget @end
+
+__attribute__((objc_externally_retained)) void f(NSWidget *p) { // expected-warning{{'objc_externally_retained' attribute ignored}}
+  __attribute__((objc_externally_retained)) NSWidget *w; // expected-warning{{'objc_externally_retained' attribute ignored}}
+}