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

llvm-svn: 251041

5 files changed, 188 insertions, 72 deletions

diff --git a/clang/lib/CodeGen/CGBlocks.cpp b/clang/lib/CodeGen/CGBlocks.cpp
index cfb09e72c7b..3f7256b9271 100644
--- a/clang/lib/CodeGen/CGBlocks.cpp
+++ b/clang/lib/CodeGen/CGBlocks.cpp
@@ -1393,31 +1393,31 @@ CodeGenFunction::GenerateCopyHelperFunction(const CGBlockInfo &blockInfo) {
         flags = BLOCK_FIELD_IS_BLOCK;
 
       // Special rules for ARC captures:
-      if (getLangOpts().ObjCAutoRefCount) {
-        Qualifiers qs = type.getQualifiers();
-
-        // We need to register __weak direct captures with the runtime.
-        if (qs.getObjCLifetime() == Qualifiers::OCL_Weak) {
-          useARCWeakCopy = true;
-
-        // We need to retain the copied value for __strong direct captures.
-        } else if (qs.getObjCLifetime() == Qualifiers::OCL_Strong) {
-          // If it's a block pointer, we have to copy the block and
-          // assign that to the destination pointer, so we might as
-          // well use _Block_object_assign.  Otherwise we can avoid that.
-          if (!isBlockPointer)
-            useARCStrongCopy = true;
-
-        // Otherwise the memcpy is fine.
-        } else {
-          continue;
-        }
+      Qualifiers qs = type.getQualifiers();
+
+      // We need to register __weak direct captures with the runtime.
+      if (qs.getObjCLifetime() == Qualifiers::OCL_Weak) {
+        useARCWeakCopy = true;
+
+      // We need to retain the copied value for __strong direct captures.
+      } else if (qs.getObjCLifetime() == Qualifiers::OCL_Strong) {
+        // If it's a block pointer, we have to copy the block and
+        // assign that to the destination pointer, so we might as
+        // well use _Block_object_assign.  Otherwise we can avoid that.
+        if (!isBlockPointer)
+          useARCStrongCopy = true;
 
       // Non-ARC captures of retainable pointers are strong and
       // therefore require a call to _Block_object_assign.
-      } else {
+      } else if (!qs.getObjCLifetime() && !getLangOpts().ObjCAutoRefCount) {
         // fall through
+
+      // Otherwise the memcpy is fine.
+      } else {
+        continue;
       }
+
+    // For all other types, the memcpy is fine.
     } else {
       continue;
     }
@@ -1564,21 +1564,24 @@ CodeGenFunction::GenerateDestroyHelperFunction(const CGBlockInfo &blockInfo) {
         flags = BLOCK_FIELD_IS_BLOCK;
 
       // Special rules for ARC captures.
-      if (getLangOpts().ObjCAutoRefCount) {
-        Qualifiers qs = type.getQualifiers();
+      Qualifiers qs = type.getQualifiers();
 
-        // Don't generate special dispose logic for a captured object
-        // unless it's __strong or __weak.
-        if (!qs.hasStrongOrWeakObjCLifetime())
-          continue;
+      // Use objc_storeStrong for __strong direct captures; the
+      // dynamic tools really like it when we do this.
+      if (qs.getObjCLifetime() == Qualifiers::OCL_Strong) {
+        useARCStrongDestroy = true;
+
+      // Support __weak direct captures.
+      } else if (qs.getObjCLifetime() == Qualifiers::OCL_Weak) {
+        useARCWeakDestroy = true;
 
-        // Support __weak direct captures.
-        if (qs.getObjCLifetime() == Qualifiers::OCL_Weak)
-          useARCWeakDestroy = true;
+      // Non-ARC captures are strong, and we need to use _Block_object_dispose.
+      } else if (!qs.hasObjCLifetime() && !getLangOpts().ObjCAutoRefCount) {
+        // fall through
 
-        // Tools really want us to use objc_storeStrong here.
-        else
-          useARCStrongDestroy = true;
+      // Otherwise, we have nothing to do.
+      } else {
+        continue;
       }
     } else {
       continue;
@@ -1958,8 +1961,6 @@ CodeGenFunction::buildByrefHelpers(llvm::StructType &byrefType,
 
   // If we have lifetime, that dominates.
   if (Qualifiers::ObjCLifetime lifetime = qs.getObjCLifetime()) {
-    assert(getLangOpts().ObjCAutoRefCount);
-
     switch (lifetime) {
     case Qualifiers::OCL_None: llvm_unreachable("impossible");
 
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index f61f63470c7..cc3516185ce 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -202,11 +202,8 @@ pushTemporaryCleanup(CodeGenFunction &CGF, const MaterializeTemporaryExpr *M,
   //   need to perform retain/release operations on the temporary.
   //
   // FIXME: This should be looking at E, not M.
-  if (CGF.getLangOpts().ObjCAutoRefCount &&
-      M->getType()->isObjCLifetimeType()) {
-    QualType ObjCARCReferenceLifetimeType = M->getType();
-    switch (Qualifiers::ObjCLifetime Lifetime =
-                ObjCARCReferenceLifetimeType.getObjCLifetime()) {
+  if (auto Lifetime = M->getType().getObjCLifetime()) {
+    switch (Lifetime) {
     case Qualifiers::OCL_None:
     case Qualifiers::OCL_ExplicitNone:
       // Carry on to normal cleanup handling.
@@ -247,11 +244,11 @@ pushTemporaryCleanup(CodeGenFunction &CGF, const MaterializeTemporaryExpr *M,
         }
         if (Duration == SD_FullExpression)
           CGF.pushDestroy(CleanupKind, ReferenceTemporary,
-                          ObjCARCReferenceLifetimeType, *Destroy,
+                          M->getType(), *Destroy,
                           CleanupKind & EHCleanup);
         else
           CGF.pushLifetimeExtendedDestroy(CleanupKind, ReferenceTemporary,
-                                          ObjCARCReferenceLifetimeType,
+                                          M->getType(),
                                           *Destroy, CleanupKind & EHCleanup);
         return;
 
@@ -355,10 +352,9 @@ EmitMaterializeTemporaryExpr(const MaterializeTemporaryExpr *M) {
 
     // FIXME: ideally this would use EmitAnyExprToMem, however, we cannot do so
     // as that will cause the lifetime adjustment to be lost for ARC
-  if (getLangOpts().ObjCAutoRefCount &&
-      M->getType()->isObjCLifetimeType() &&
-      M->getType().getObjCLifetime() != Qualifiers::OCL_None &&
-      M->getType().getObjCLifetime() != Qualifiers::OCL_ExplicitNone) {
+  auto ownership = M->getType().getObjCLifetime();
+  if (ownership != Qualifiers::OCL_None &&
+      ownership != Qualifiers::OCL_ExplicitNone) {
     Address Object = createReferenceTemporary(*this, M, E);
     if (auto *Var = dyn_cast<llvm::GlobalVariable>(Object.getPointer())) {
       Object = Address(llvm::ConstantExpr::getBitCast(Var,
@@ -1424,6 +1420,12 @@ RValue CodeGenFunction::EmitLoadOfLValue(LValue LV, SourceLocation Loc) {
                                                              AddrWeakObj));
   }
   if (LV.getQuals().getObjCLifetime() == Qualifiers::OCL_Weak) {
+    // In MRC mode, we do a load+autorelease.
+    if (!getLangOpts().ObjCAutoRefCount) {
+      return RValue::get(EmitARCLoadWeak(LV.getAddress()));
+    }
+
+    // In ARC mode, we load retained and then consume the value.
     llvm::Value *Object = EmitARCLoadWeakRetained(LV.getAddress());
     Object = EmitObjCConsumeObject(LV.getType(), Object);
     return RValue::get(Object);
@@ -3511,10 +3513,7 @@ RValue CodeGenFunction::EmitCallExpr(const CallExpr *E,
   if (const auto *PseudoDtor =
           dyn_cast<CXXPseudoDestructorExpr>(E->getCallee()->IgnoreParens())) {
     QualType DestroyedType = PseudoDtor->getDestroyedType();
-    if (getLangOpts().ObjCAutoRefCount &&
-        DestroyedType->isObjCLifetimeType() &&
-        (DestroyedType.getObjCLifetime() == Qualifiers::OCL_Strong ||
-         DestroyedType.getObjCLifetime() == Qualifiers::OCL_Weak)) {
+    if (DestroyedType.hasStrongOrWeakObjCLifetime()) {
       // Automatic Reference Counting:
       //   If the pseudo-expression names a retainable object with weak or
       //   strong lifetime, the object shall be released.
@@ -3534,7 +3533,7 @@ RValue CodeGenFunction::EmitCallExpr(const CallExpr *E,
         BaseQuals = BaseTy.getQualifiers();
       }
 
-      switch (PseudoDtor->getDestroyedType().getObjCLifetime()) {
+      switch (DestroyedType.getObjCLifetime()) {
       case Qualifiers::OCL_None:
       case Qualifiers::OCL_ExplicitNone:
       case Qualifiers::OCL_Autoreleasing:
diff --git a/clang/lib/CodeGen/CGExprCXX.cpp b/clang/lib/CodeGen/CGExprCXX.cpp
index ae4e02d53f0..71e4b98986b 100644
--- a/clang/lib/CodeGen/CGExprCXX.cpp
+++ b/clang/lib/CodeGen/CGExprCXX.cpp
@@ -1526,9 +1526,8 @@ static void EmitObjectDelete(CodeGenFunction &CGF,
                               /*ForVirtualBase=*/false,
                               /*Delegating=*/false,
                               Ptr);
-  else if (CGF.getLangOpts().ObjCAutoRefCount &&
-           ElementType->isObjCLifetimeType()) {
-    switch (ElementType.getObjCLifetime()) {
+  else if (auto Lifetime = ElementType.getObjCLifetime()) {
+    switch (Lifetime) {
     case Qualifiers::OCL_None:
     case Qualifiers::OCL_ExplicitNone:
     case Qualifiers::OCL_Autoreleasing:
diff --git a/clang/lib/CodeGen/CGObjC.cpp b/clang/lib/CodeGen/CGObjC.cpp
index 5890e7fc9b4..fce74095148 100644
--- a/clang/lib/CodeGen/CGObjC.cpp
+++ b/clang/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.
diff --git a/clang/lib/CodeGen/CGObjCMac.cpp b/clang/lib/CodeGen/CGObjCMac.cpp
index a44cdf8c345..44c9a90c76c 100644
--- a/clang/lib/CodeGen/CGObjCMac.cpp
+++ b/clang/lib/CodeGen/CGObjCMac.cpp
@@ -913,10 +913,28 @@ protected:
   /// BuildIvarLayout - Builds ivar layout bitmap for the class
   /// implementation for the __strong or __weak case.
   ///
+  /// \param hasMRCWeakIvars - Whether we are compiling in MRC and there
+  ///   are any weak ivars defined directly in the class.  Meaningless unless
+  ///   building a weak layout.  Does not guarantee that the layout will
+  ///   actually have any entries, because the ivar might be under-aligned.
   llvm::Constant *BuildIvarLayout(const ObjCImplementationDecl *OI,
                                   CharUnits beginOffset,
                                   CharUnits endOffset,
-                                  bool ForStrongLayout);
+                                  bool forStrongLayout,
+                                  bool hasMRCWeakIvars);
+
+  llvm::Constant *BuildStrongIvarLayout(const ObjCImplementationDecl *OI,
+                                        CharUnits beginOffset,
+                                        CharUnits endOffset) {
+    return BuildIvarLayout(OI, beginOffset, endOffset, true, false);
+  }
+
+  llvm::Constant *BuildWeakIvarLayout(const ObjCImplementationDecl *OI,
+                                      CharUnits beginOffset,
+                                      CharUnits endOffset,
+                                      bool hasMRCWeakIvars) {
+    return BuildIvarLayout(OI, beginOffset, endOffset, false, hasMRCWeakIvars);
+  }
   
   Qualifiers::ObjCLifetime getBlockCaptureLifetime(QualType QT, bool ByrefLayout);
   
@@ -1060,7 +1078,8 @@ private:
   /// to store the weak ivar layout and properties. The return value
   /// has type ClassExtensionPtrTy.
   llvm::Constant *EmitClassExtension(const ObjCImplementationDecl *ID,
-                                     CharUnits InstanceSize);
+                                     CharUnits instanceSize,
+                                     bool hasMRCWeakIvars);
 
   /// EmitClassRef - Return a Value*, of type ObjCTypes.ClassPtrTy,
   /// for the given class.
@@ -2049,8 +2068,7 @@ llvm::Constant *CGObjCCommonMac::BuildGCBlockLayout(CodeGenModule &CGM,
                                                 const CGBlockInfo &blockInfo) {
   
   llvm::Constant *nullPtr = llvm::Constant::getNullValue(CGM.Int8PtrTy);
-  if (CGM.getLangOpts().getGC() == LangOptions::NonGC &&
-      !CGM.getLangOpts().ObjCAutoRefCount)
+  if (CGM.getLangOpts().getGC() == LangOptions::NonGC)
     return nullPtr;
 
   IvarLayoutBuilder builder(CGM, CharUnits::Zero(), blockInfo.BlockSize,
@@ -2129,10 +2147,15 @@ void IvarLayoutBuilder::visitBlock(const CGBlockInfo &blockInfo) {
 /// the type of the variable captured in the block.
 Qualifiers::ObjCLifetime CGObjCCommonMac::getBlockCaptureLifetime(QualType FQT,
                                                                   bool ByrefLayout) {
+  // If it has an ownership qualifier, we're done.
+  if (auto lifetime = FQT.getObjCLifetime())
+    return lifetime;
+
+  // If it doesn't, and this is ARC, it has no ownership.
   if (CGM.getLangOpts().ObjCAutoRefCount)
-    return FQT.getObjCLifetime();
+    return Qualifiers::OCL_None;
   
-  // MRR.
+  // In MRC, retainable pointers are owned by non-__block variables.
   if (FQT->isObjCObjectPointerType() || FQT->isBlockPointerType())
     return ByrefLayout ? Qualifiers::OCL_ExplicitNone : Qualifiers::OCL_Strong;
   
@@ -3060,11 +3083,24 @@ void CGObjCMac::GenerateCategory(const ObjCCategoryImplDecl *OCD) {
 }
 
 enum FragileClassFlags {
+  /// Apparently: is not a meta-class.
   FragileABI_Class_Factory                 = 0x00001,
+
+  /// Is a meta-class.
   FragileABI_Class_Meta                    = 0x00002,
+
+  /// Has a non-trivial constructor or destructor.
   FragileABI_Class_HasCXXStructors         = 0x02000,
+
+  /// Has hidden visibility.
   FragileABI_Class_Hidden                  = 0x20000,
-  FragileABI_Class_CompiledByARC           = 0x04000000
+
+  /// Class implementation was compiled under ARC.
+  FragileABI_Class_CompiledByARC           = 0x04000000,
+
+  /// Class implementation was compiled under MRC and has MRC weak ivars.
+  /// Exclusive with CompiledByARC.
+  FragileABI_Class_HasMRCWeakIvars         = 0x08000000,
 };
 
 enum NonFragileClassFlags {
@@ -3074,7 +3110,7 @@ enum NonFragileClassFlags {
   /// Is a root class.
   NonFragileABI_Class_Root                 = 0x00002,
 
-  /// Has a C++ constructor and destructor.
+  /// Has a non-trivial constructor or destructor.
   NonFragileABI_Class_HasCXXStructors      = 0x00004,
 
   /// Has hidden visibility.
@@ -3090,9 +3126,46 @@ enum NonFragileClassFlags {
   NonFragileABI_Class_CompiledByARC        = 0x00080,
 
   /// Class has non-trivial destructors, but zero-initialization is okay.
-  NonFragileABI_Class_HasCXXDestructorOnly = 0x00100
+  NonFragileABI_Class_HasCXXDestructorOnly = 0x00100,
+
+  /// Class implementation was compiled under MRC and has MRC weak ivars.
+  /// Exclusive with CompiledByARC.
+  NonFragileABI_Class_HasMRCWeakIvars      = 0x00200,
 };
 
+static bool hasWeakMember(QualType type) {
+  if (type.getObjCLifetime() == Qualifiers::OCL_Weak) {
+    return true;
+  }
+
+  if (auto recType = type->getAs<RecordType>()) {
+    for (auto field : recType->getDecl()->fields()) {
+      if (hasWeakMember(field->getType()))
+        return true;
+    }
+  }
+
+  return false;
+}
+
+/// For compatibility, we only want to set the "HasMRCWeakIvars" flag
+/// (and actually fill in a layout string) if we really do have any
+/// __weak ivars.
+static bool hasMRCWeakIvars(CodeGenModule &CGM,
+                            const ObjCImplementationDecl *ID) {
+  if (!CGM.getLangOpts().ObjCWeak) return false;
+  assert(CGM.getLangOpts().getGC() == LangOptions::NonGC);
+
+  for (const ObjCIvarDecl *ivar =
+         ID->getClassInterface()->all_declared_ivar_begin();
+       ivar; ivar = ivar->getNextIvar()) {
+    if (hasWeakMember(ivar->getType()))
+      return true;
+  }
+
+  return false;
+}
+
 /*
   struct _objc_class {
   Class isa;
@@ -3127,8 +3200,12 @@ void CGObjCMac::GenerateClass(const ObjCImplementationDecl *ID) {
   if (ID->hasNonZeroConstructors() || ID->hasDestructors())
     Flags |= FragileABI_Class_HasCXXStructors;
 
+  bool hasMRCWeak = false;
+
   if (CGM.getLangOpts().ObjCAutoRefCount)
     Flags |= FragileABI_Class_CompiledByARC;
+  else if ((hasMRCWeak = hasMRCWeakIvars(CGM, ID)))
+    Flags |= FragileABI_Class_HasMRCWeakIvars;
 
   CharUnits Size =
     CGM.getContext().getASTObjCImplementationLayout(ID).getSize();
@@ -3183,8 +3260,8 @@ void CGObjCMac::GenerateClass(const ObjCImplementationDecl *ID) {
   // cache is always NULL.
   Values[ 8] = llvm::Constant::getNullValue(ObjCTypes.CachePtrTy);
   Values[ 9] = Protocols;
-  Values[10] = BuildIvarLayout(ID, CharUnits::Zero(), Size, true);
-  Values[11] = EmitClassExtension(ID, Size);
+  Values[10] = BuildStrongIvarLayout(ID, CharUnits::Zero(), Size);
+  Values[11] = EmitClassExtension(ID, Size, hasMRCWeak);
   llvm::Constant *Init = llvm::ConstantStruct::get(ObjCTypes.ClassTy,
                                                    Values);
   std::string Name("OBJC_CLASS_");
@@ -3323,13 +3400,14 @@ llvm::Value *CGObjCMac::EmitSuperClassRef(const ObjCInterfaceDecl *ID) {
 */
 llvm::Constant *
 CGObjCMac::EmitClassExtension(const ObjCImplementationDecl *ID,
-                              CharUnits InstanceSize) {
+                              CharUnits InstanceSize, bool hasMRCWeakIvars) {
   uint64_t Size =
     CGM.getDataLayout().getTypeAllocSize(ObjCTypes.ClassExtensionTy);
 
   llvm::Constant *Values[3];
   Values[0] = llvm::ConstantInt::get(ObjCTypes.IntTy, Size);
-  Values[1] = BuildIvarLayout(ID, CharUnits::Zero(), InstanceSize, false);
+  Values[1] = BuildWeakIvarLayout(ID, CharUnits::Zero(), InstanceSize,
+                                  hasMRCWeakIvars);
   Values[2] = EmitPropertyList("\01l_OBJC_$_PROP_LIST_" + ID->getName(),
                                ID, ID->getClassInterface(), ObjCTypes);
 
@@ -4827,10 +4905,13 @@ llvm::Constant *IvarLayoutBuilder::buildBitmap(CGObjCCommonMac &CGObjC,
 llvm::Constant *
 CGObjCCommonMac::BuildIvarLayout(const ObjCImplementationDecl *OMD,
                                  CharUnits beginOffset, CharUnits endOffset,
-                                 bool ForStrongLayout) {
+                                 bool ForStrongLayout, bool HasMRCWeakIvars) {
+  // If this is MRC, and we're either building a strong layout or there
+  // are no weak ivars, bail out early.
   llvm::Type *PtrTy = CGM.Int8PtrTy;
   if (CGM.getLangOpts().getGC() == LangOptions::NonGC &&
-      !CGM.getLangOpts().ObjCAutoRefCount)
+      !CGM.getLangOpts().ObjCAutoRefCount &&
+      (ForStrongLayout || !HasMRCWeakIvars))
     return llvm::Constant::getNullValue(PtrTy);
 
   const ObjCInterfaceDecl *OI = OMD->getClassInterface();
@@ -4844,8 +4925,10 @@ CGObjCCommonMac::BuildIvarLayout(const ObjCImplementationDecl *OMD,
   // runtimes, that means starting at InstanceStart.  In fragile runtimes,
   // there's no InstanceStart, so it means starting at the end of the
   // superclass, rounded up to word alignment.
+  //
+  // MRC weak layout strings follow the ARC style.
   CharUnits baseOffset;
-  if (CGM.getLangOpts().ObjCAutoRefCount) {
+  if (CGM.getLangOpts().getGC() == LangOptions::NonGC) {
     for (const ObjCIvarDecl *IVD = OI->all_declared_ivar_begin(); 
          IVD; IVD = IVD->getNextIvar())
       ivars.push_back(IVD);
@@ -5662,8 +5745,11 @@ llvm::GlobalVariable * CGObjCNonFragileABIMac::BuildClassRoTInitializer(
   CharUnits beginInstance = CharUnits::fromQuantity(InstanceStart);
   CharUnits endInstance = CharUnits::fromQuantity(InstanceSize);
 
+  bool hasMRCWeak = false;
   if (CGM.getLangOpts().ObjCAutoRefCount)
     flags |= NonFragileABI_Class_CompiledByARC;
+  else if ((hasMRCWeak = hasMRCWeakIvars(CGM, ID)))
+    flags |= NonFragileABI_Class_HasMRCWeakIvars;
 
   Values[ 0] = llvm::ConstantInt::get(ObjCTypes.IntTy, flags);
   Values[ 1] = llvm::ConstantInt::get(ObjCTypes.IntTy, InstanceStart);
@@ -5671,7 +5757,7 @@ llvm::GlobalVariable * CGObjCNonFragileABIMac::BuildClassRoTInitializer(
   // FIXME. For 64bit targets add 0 here.
   Values[ 3] = (flags & NonFragileABI_Class_Meta)
     ? GetIvarLayoutName(nullptr, ObjCTypes)
-    : BuildIvarLayout(ID, beginInstance, endInstance, true);
+    : BuildStrongIvarLayout(ID, beginInstance, endInstance);
   Values[ 4] = GetClassName(ID->getObjCRuntimeNameAsString());
   // const struct _method_list_t * const baseMethods;
   std::vector<llvm::Constant*> Methods;
@@ -5718,7 +5804,8 @@ llvm::GlobalVariable * CGObjCNonFragileABIMac::BuildClassRoTInitializer(
     Values[ 9] = llvm::Constant::getNullValue(ObjCTypes.PropertyListPtrTy);
   } else {
     Values[ 7] = EmitIvarList(ID);
-    Values[ 8] = BuildIvarLayout(ID, beginInstance, endInstance, false);
+    Values[ 8] = BuildWeakIvarLayout(ID, beginInstance, endInstance,
+                                     hasMRCWeak);
     Values[ 9] = EmitPropertyList("\01l_OBJC_$_PROP_LIST_" + ID->getObjCRuntimeNameAsString(),
                                   ID, ID->getClassInterface(), ObjCTypes);
   }