[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

llvm-svn: 350422

1 files changed, 9 insertions, 3 deletions

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index aa4b23b15c5..eddf90178bc 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -11214,17 +11214,23 @@ static bool CheckForModifiableLvalue(Expr *E, SourceLocation Loc, Sema &S) {
         if (var->isARCPseudoStrong() &&
             (!var->getTypeSourceInfo() ||
              !var->getTypeSourceInfo()->getType().isConstQualified())) {
-          // There are two pseudo-strong cases:
+          // There are three pseudo-strong cases:
           //  - self
           ObjCMethodDecl *method = S.getCurMethodDecl();
-          if (method && var == method->getSelfDecl())
+          if (method && var == method->getSelfDecl()) {
             DiagID = method->isClassMethod()
               ? diag::err_typecheck_arc_assign_self_class_method
               : diag::err_typecheck_arc_assign_self;
 
+          //  - Objective-C externally_retained attribute.
+          } else if (var->hasAttr<ObjCExternallyRetainedAttr>() ||
+                     isa<ParmVarDecl>(var)) {
+            DiagID = diag::err_typecheck_arc_assign_externally_retained;
+
           //  - fast enumeration variables
-          else
+          } else {
             DiagID = diag::err_typecheck_arr_assign_enumeration;
+          }
 
           SourceRange Assign;
           if (Loc != OrigLoc)