Define weak and __weak to mean ARC-style weak references, even in MRC.

Previously, __weak was silently accepted and ignored in MRC mode.
That makes this a potentially source-breaking change that we have to
roll out cautiously.  Accordingly, for the time being, actual support
for __weak references in MRC is experimental, and the compiler will
reject attempts to actually form such references.  The intent is to
eventually enable the feature by default in all non-GC modes.
(It is, of course, incompatible with ObjC GC's interpretation of
__weak.)

If you like, you can enable this feature with
  -Xclang -fobjc-weak
but like any -Xclang option, this option may be removed at any point,
e.g. if/when it is eventually enabled by default.

This patch also enables the use of the ARC __unsafe_unretained qualifier
in MRC.  Unlike __weak, this is being enabled immediately.  Since
variables are essentially __unsafe_unretained by default in MRC,
the only practical uses are (1) communication and (2) changing the
default behavior of by-value block capture.

As an implementation matter, this means that the ObjC ownership
qualifiers may appear in any ObjC language mode, and so this patch
removes a number of checks for getLangOpts().ObjCAutoRefCount
that were guarding the processing of these qualifiers.  I don't
expect this to be a significant drain on performance; it may even
be faster to just check for these qualifiers directly on a type
(since it's probably in a register anyway) than to do N dependent
loads to grab the LangOptions.

rdar://9674298

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@251041 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 31 insertions, 1 deletions

diff --git a/lib/CodeGen/CGObjC.cpp b/lib/CodeGen/CGObjC.cpp
index 5890e7fc9b..fce7409514 100644
--- a/lib/CodeGen/CGObjC.cpp
+++ b/lib/CodeGen/CGObjC.cpp
@@ -323,6 +323,21 @@ shouldExtendReceiverForInnerPointerMessage(const ObjCMessageExpr *message) {
   llvm_unreachable("invalid receiver kind");
 }
 
+/// Given an expression of ObjC pointer type, check whether it was
+/// immediately loaded from an ARC __weak l-value.
+static const Expr *findWeakLValue(const Expr *E) {
+  assert(E->getType()->isObjCRetainableType());
+  E = E->IgnoreParens();
+  if (auto CE = dyn_cast<CastExpr>(E)) {
+    if (CE->getCastKind() == CK_LValueToRValue) {
+      if (CE->getSubExpr()->getType().getObjCLifetime() == Qualifiers::OCL_Weak)
+        return CE->getSubExpr();
+    }
+  }
+
+  return nullptr;
+}
+
 RValue CodeGenFunction::EmitObjCMessageExpr(const ObjCMessageExpr *E,
                                             ReturnValueSlot Return) {
   // Only the lookup mechanism and first two arguments of the method
@@ -333,6 +348,17 @@ RValue CodeGenFunction::EmitObjCMessageExpr(const ObjCMessageExpr *E,
 
   const ObjCMethodDecl *method = E->getMethodDecl();
 
+  // If the method is -retain, and the receiver's being loaded from
+  // a __weak variable, peephole the entire operation to objc_loadWeakRetained.
+  if (method && E->getReceiverKind() == ObjCMessageExpr::Instance &&
+      method->getMethodFamily() == OMF_retain) {
+    if (auto lvalueExpr = findWeakLValue(E->getInstanceReceiver())) {
+      LValue lvalue = EmitLValue(lvalueExpr);
+      llvm::Value *result = EmitARCLoadWeakRetained(lvalue.getAddress());
+      return AdjustObjCObjectType(*this, E->getType(), RValue::get(result));
+    }
+  }
+
   // We don't retain the receiver in delegate init calls, and this is
   // safe because the receiver value is always loaded from 'self',
   // which we zero out.  We don't want to Block_copy block receivers,
@@ -976,7 +1002,11 @@ CodeGenFunction::generateObjCGetterBody(const ObjCImplementationDecl *classImpl,
       } else {
         // We want to load and autoreleaseReturnValue ARC __weak ivars.
         if (LV.getQuals().getObjCLifetime() == Qualifiers::OCL_Weak) {
-          value = emitARCRetainLoadOfScalar(*this, LV, ivarType);
+          if (getLangOpts().ObjCAutoRefCount) {
+            value = emitARCRetainLoadOfScalar(*this, LV, ivarType);
+          } else {
+            value = EmitARCLoadWeak(LV.getAddress());
+          }
 
         // Otherwise we want to do a simple load, suppressing the
         // final autorelease.