diff --git a/include/clang/AST/ASTContext.h b/include/clang/AST/ASTContext.h
index 1503090..b7d3f1f 100644
--- a/include/clang/AST/ASTContext.h
+++ b/include/clang/AST/ASTContext.h
@@ -860,11 +860,8 @@
     return cudaConfigureCallDecl;
   }
 
-  /// Builds the struct used for __block variables.
-  QualType BuildByRefType(StringRef DeclName, QualType Ty) const;
-
   /// Returns true iff we need copy/dispose helpers for the given type.
-  bool BlockRequiresCopying(QualType Ty) const;
+  bool BlockRequiresCopying(QualType Ty, const VarDecl *D);
   
   
   /// Returns true, if given type has a known lifetime. HasByrefExtendedLayout is set
diff --git a/lib/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index c4dbf5d..7fee560 100644
--- a/lib/AST/ASTContext.cpp
+++ b/lib/AST/ASTContext.cpp
@@ -4361,17 +4361,45 @@
   return getTagDeclType(BlockDescriptorExtendedType);
 }
 
-bool ASTContext::BlockRequiresCopying(QualType Ty) const {
-  if (Ty->isObjCRetainableType())
+/// BlockRequiresCopying - Returns true if byref variable "D" of type "Ty"
+/// requires copy/dispose. Note that this must match the logic
+/// in buildByrefHelpers.
+bool ASTContext::BlockRequiresCopying(QualType Ty,
+                                      const VarDecl *D) {
+  if (const CXXRecordDecl *record = Ty->getAsCXXRecordDecl()) {
+    const Expr *copyExpr = getBlockVarCopyInits(D);
+    if (!copyExpr && record->hasTrivialDestructor()) return false;
+    
     return true;
-  if (getLangOpts().CPlusPlus) {
-    if (const RecordType *RT = Ty->getAs<RecordType>()) {
-      CXXRecordDecl *RD = cast<CXXRecordDecl>(RT->getDecl());
-      return RD->hasConstCopyConstructor();
-      
-    }
   }
-  return false;
+  
+  if (!Ty->isObjCRetainableType()) return false;
+  
+  Qualifiers qs = Ty.getQualifiers();
+  
+  // If we have lifetime, that dominates.
+  if (Qualifiers::ObjCLifetime lifetime = qs.getObjCLifetime()) {
+    assert(getLangOpts().ObjCAutoRefCount);
+    
+    switch (lifetime) {
+      case Qualifiers::OCL_None: llvm_unreachable("impossible");
+        
+      // These are just bits as far as the runtime is concerned.
+      case Qualifiers::OCL_ExplicitNone:
+      case Qualifiers::OCL_Autoreleasing:
+        return false;
+        
+      // Tell the runtime that this is ARC __weak, called by the
+      // byref routines.
+      case Qualifiers::OCL_Weak:
+      // ARC __strong __block variables need to be retained.
+      case Qualifiers::OCL_Strong:
+        return true;
+    }
+    llvm_unreachable("fell out of lifetime switch!");
+  }
+  return (Ty->isBlockPointerType() || isObjCNSObjectType(Ty) ||
+          Ty->isObjCObjectPointerType());
 }
 
 bool ASTContext::getByrefLifetime(QualType Ty,
@@ -4397,74 +4425,6 @@
   return true;
 }
 
-QualType
-ASTContext::BuildByRefType(StringRef DeclName, QualType Ty) const {
-  //  type = struct __Block_byref_1_X {
-  //    void *__isa;
-  //    struct __Block_byref_1_X *__forwarding;
-  //    unsigned int __flags;
-  //    unsigned int __size;
-  //    void *__copy_helper;            // as needed
-  //    void *__destroy_help            // as needed
-  //    void *__byref_variable_layout;    // Extended layout info. for byref variable as needed
-  //    int X;
-  //  } *
-
-  bool HasCopyAndDispose = BlockRequiresCopying(Ty);
-  bool HasByrefExtendedLayout;
-  Qualifiers::ObjCLifetime Lifetime;
-
-  // FIXME: Move up
-  SmallString<36> Name;
-  llvm::raw_svector_ostream(Name) << "__Block_byref_" <<
-                                  ++UniqueBlockByRefTypeID << '_' << DeclName;
-  RecordDecl *T;
-  T = CreateRecordDecl(*this, TTK_Struct, TUDecl, &Idents.get(Name.str()));
-  T->startDefinition();
-  QualType Int32Ty = IntTy;
-  assert(getIntWidth(IntTy) == 32 && "non-32bit int not supported");
-  QualType FieldTypes[] = {
-    getPointerType(VoidPtrTy),
-    getPointerType(getTagDeclType(T)),
-    Int32Ty,
-    Int32Ty,
-    getPointerType(VoidPtrTy),
-    getPointerType(VoidPtrTy),
-    getPointerType(VoidPtrTy),
-    Ty
-  };
-
-  StringRef FieldNames[] = {
-    "__isa",
-    "__forwarding",
-    "__flags",
-    "__size",
-    "__copy_helper",
-    "__destroy_helper",
-    "__byref_variable_layout",
-    DeclName,
-  };
-  bool ByrefKnownLifetime = getByrefLifetime(Ty, Lifetime, HasByrefExtendedLayout);
-  for (size_t i = 0; i < 8; ++i) {
-    if (!HasCopyAndDispose && i >=4 && i <= 5)
-      continue;
-    if ((!ByrefKnownLifetime || !HasByrefExtendedLayout) && i == 6)
-      continue;
-    FieldDecl *Field = FieldDecl::Create(*this, T, SourceLocation(),
-                                         SourceLocation(),
-                                         &Idents.get(FieldNames[i]),
-                                         FieldTypes[i], /*TInfo=*/0,
-                                         /*BitWidth=*/0, /*Mutable=*/false,
-                                         ICIS_NoInit);
-    Field->setAccess(AS_public);
-    T->addDecl(Field);
-  }
-
-  T->completeDefinition();
-
-  return getPointerType(getTagDeclType(T));
-}
-
 TypedefDecl *ASTContext::getObjCInstanceTypeDecl() {
   if (!ObjCInstanceTypeDecl)
     ObjCInstanceTypeDecl = TypedefDecl::Create(*this, 
diff --git a/lib/CodeGen/CGBlocks.cpp b/lib/CodeGen/CGBlocks.cpp
index 06fd624..3302221 100644
--- a/lib/CodeGen/CGBlocks.cpp
+++ b/lib/CodeGen/CGBlocks.cpp
@@ -1922,9 +1922,8 @@
     
   // int32_t __size;
   types.push_back(Int32Ty);
-
-  bool HasCopyAndDispose =
-       (Ty->isObjCRetainableType()) || getContext().getBlockVarCopyInits(D);
+  // Note that this must match *exactly* the logic in buildByrefHelpers.
+  bool HasCopyAndDispose = getContext().BlockRequiresCopying(Ty, D);
   if (HasCopyAndDispose) {
     /// void *__copy_helper;
     types.push_back(Int8PtrTy);
diff --git a/lib/CodeGen/CGDebugInfo.cpp b/lib/CodeGen/CGDebugInfo.cpp
index 8a66dff..96cfd28 100644
--- a/lib/CodeGen/CGDebugInfo.cpp
+++ b/lib/CodeGen/CGDebugInfo.cpp
@@ -2229,7 +2229,7 @@
 
 // EmitTypeForVarWithBlocksAttr - Build up structure info for the byref.  
 // See BuildByRefType.
-llvm::DIType CGDebugInfo::EmitTypeForVarWithBlocksAttr(const ValueDecl *VD,
+llvm::DIType CGDebugInfo::EmitTypeForVarWithBlocksAttr(const VarDecl *VD,
                                                        uint64_t *XOffset) {
 
   SmallVector<llvm::Value *, 5> EltTys;
@@ -2248,7 +2248,7 @@
   EltTys.push_back(CreateMemberType(Unit, FType, "__flags", &FieldOffset));
   EltTys.push_back(CreateMemberType(Unit, FType, "__size", &FieldOffset));
 
-  bool HasCopyAndDispose = CGM.getContext().BlockRequiresCopying(Type);
+  bool HasCopyAndDispose = CGM.getContext().BlockRequiresCopying(Type, VD);
   if (HasCopyAndDispose) {
     FType = CGM.getContext().getPointerType(CGM.getContext().VoidTy);
     EltTys.push_back(CreateMemberType(Unit, FType, "__copy_helper",
diff --git a/lib/CodeGen/CGDebugInfo.h b/lib/CodeGen/CGDebugInfo.h
index efa6b86..99f29e8 100644
--- a/lib/CodeGen/CGDebugInfo.h
+++ b/lib/CodeGen/CGDebugInfo.h
@@ -243,7 +243,7 @@
 
   // EmitTypeForVarWithBlocksAttr - Build up structure info for the byref.  
   // See BuildByRefType.
-  llvm::DIType EmitTypeForVarWithBlocksAttr(const ValueDecl *VD, 
+  llvm::DIType EmitTypeForVarWithBlocksAttr(const VarDecl *VD,
                                             uint64_t *OffSet);
 
   /// getContextDescriptor - Get context info for the decl.
diff --git a/lib/Rewrite/Frontend/RewriteModernObjC.cpp b/lib/Rewrite/Frontend/RewriteModernObjC.cpp
index 4b56b37..19acb94 100644
--- a/lib/Rewrite/Frontend/RewriteModernObjC.cpp
+++ b/lib/Rewrite/Frontend/RewriteModernObjC.cpp
@@ -5053,7 +5053,7 @@
   // Add void *__Block_byref_id_object_copy; 
   // void *__Block_byref_id_object_dispose; if needed.
   QualType Ty = ND->getType();
-  bool HasCopyAndDispose = Context->BlockRequiresCopying(Ty);
+  bool HasCopyAndDispose = Context->BlockRequiresCopying(Ty, ND);
   if (HasCopyAndDispose) {
     ByrefType += " void (*__Block_byref_id_object_copy)(void*, void*);\n";
     ByrefType += " void (*__Block_byref_id_object_dispose)(void*);\n";
diff --git a/lib/Rewrite/Frontend/RewriteObjC.cpp b/lib/Rewrite/Frontend/RewriteObjC.cpp
index a6dcc6b..982c730 100644
--- a/lib/Rewrite/Frontend/RewriteObjC.cpp
+++ b/lib/Rewrite/Frontend/RewriteObjC.cpp
@@ -4309,7 +4309,7 @@
   // Add void *__Block_byref_id_object_copy; 
   // void *__Block_byref_id_object_dispose; if needed.
   QualType Ty = ND->getType();
-  bool HasCopyAndDispose = Context->BlockRequiresCopying(Ty);
+  bool HasCopyAndDispose = Context->BlockRequiresCopying(Ty, ND);
   if (HasCopyAndDispose) {
     ByrefType += " void (*__Block_byref_id_object_copy)(void*, void*);\n";
     ByrefType += " void (*__Block_byref_id_object_dispose)(void*);\n";
diff --git a/test/CodeGenObjC/block-byref-variable-layout.m b/test/CodeGenObjC/block-byref-variable-layout.m
index 1e8bcbc..6030661 100644
--- a/test/CodeGenObjC/block-byref-variable-layout.m
+++ b/test/CodeGenObjC/block-byref-variable-layout.m
@@ -1,5 +1,14 @@
 // RUN: %clang_cc1 -fblocks -fobjc-arc -fobjc-runtime-has-weak -triple x86_64-apple-darwin -O0 -emit-llvm %s -o - | FileCheck %s
 
+// rdar://12759433
+@class NSString;
+
+void Test12759433() {
+ __block __unsafe_unretained NSString *uuByref = (__bridge NSString *)(void*)0x102030405060708;
+ void (^block)() = ^{ uuByref = 0; };
+ block();
+}
+// CHECK: %struct.__block_byref_uuByref = type { i8*, %struct.__block_byref_uuByref*, i32, i32, [[ZERO:%.*]]* }
 int main() {
   __block __weak id wid;
   __block long XXX;