Allow (Object *)kMyGlobalCFObj casts without bridging

Previously we allowed these casts only for constants declared in system
headers, which we assume are retain/release-neutral. Now also allow them
for constants in user headers, treating them as +0.  Practically, this
means that we will now allow:
id x = (id)kMyGlobalConst;

But unlike with system headers we cannot mix them with +1 values:
id y = (id)(b ? kMyGlobalConst : [Obj newValAtPlusOne]); // error
id z = (id)(b ? kSystemGlobalConst: [Obj newValAtPlusOne]); // OK

Thanks to John for suggesting this improvement.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@230534 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 9 insertions, 1 deletions

diff --git a/docs/AutomaticReferenceCounting.rst b/docs/AutomaticReferenceCounting.rst
index 1457b6082d..2faed23791 100644
--- a/docs/AutomaticReferenceCounting.rst
+++ b/docs/AutomaticReferenceCounting.rst
@@ -594,7 +594,9 @@ retainable pointer type <arc.misc.c-retainable>` and it is:
 * a message send, and the declared method either has the
   ``cf_returns_not_retained`` attribute or it has neither the
   ``cf_returns_retained`` attribute nor a :ref:`selector family
-  <arc.method-families>` that implies a retained result.
+  <arc.method-families>` that implies a retained result, or
+* :when-revised:`[beginning LLVM 3.6]` :revision:`a load from a` ``const``
+  :revision:`non-system global variable.`
 
 An expression is :arc-term:`known retained` if it is an rvalue of :ref:`C
 retainable pointer type <arc.misc.c-retainable>` and it is:
@@ -631,6 +633,12 @@ retain-agnostic, the conversion is treated as a ``__bridge`` cast.
   to an ObjC-typed local, and then calling ``CFRelease`` when done  ---  are a
   bit too likely to be accidentally accepted, leading to mysterious behavior.
 
+  For loads from ``const`` global variables of :ref:`C retainable pointer type
+  <arc.misc.c-retainable>`, it is reasonable to assume that global system
+  constants were initialitzed with true constants (e.g. string literals), but
+  user constants might have been initialized with something dynamically
+  allocated, using a global initializer.
+
 .. _arc.objects.restrictions.conversion-exception-contextual:
 
 Conversion from retainable object pointer type in certain contexts