diff --git a/lib/CodeGen/CGCall.cpp b/lib/CodeGen/CGCall.cpp
index dc3e702..846e1aa 100644
--- a/lib/CodeGen/CGCall.cpp
+++ b/lib/CodeGen/CGCall.cpp
@@ -907,6 +907,7 @@
   // Name the struct return argument.
   if (CGM.ReturnTypeUsesSRet(FI)) {
     AI->setName("agg.result");
+    AI->addAttr(llvm::Attribute::NoAlias);
     ++AI;
   }
 
diff --git a/lib/CodeGen/CGClass.cpp b/lib/CodeGen/CGClass.cpp
index 8e46fcd..8bdfbf1 100644
--- a/lib/CodeGen/CGClass.cpp
+++ b/lib/CodeGen/CGClass.cpp
@@ -401,7 +401,8 @@
   AggValueSlot AggSlot =
     AggValueSlot::forAddr(V, Qualifiers(),
                           AggValueSlot::IsDestructed,
-                          AggValueSlot::DoesNotNeedGCBarriers);
+                          AggValueSlot::DoesNotNeedGCBarriers,
+                          AggValueSlot::IsNotAliased);
 
   CGF.EmitAggExpr(BaseInit->getInit(), AggSlot);
   
@@ -441,7 +442,8 @@
       AggValueSlot Slot =
         AggValueSlot::forAddr(Dest, LHS.getQuals(),
                               AggValueSlot::IsDestructed,
-                              AggValueSlot::DoesNotNeedGCBarriers);
+                              AggValueSlot::DoesNotNeedGCBarriers,
+                              AggValueSlot::IsNotAliased);
       
       CGF.EmitAggExpr(MemberInit->getInit(), Slot);
     }
@@ -1330,7 +1332,8 @@
   AggValueSlot AggSlot =
     AggValueSlot::forAddr(ThisPtr, Qualifiers(),
                           AggValueSlot::IsDestructed,
-                          AggValueSlot::DoesNotNeedGCBarriers);
+                          AggValueSlot::DoesNotNeedGCBarriers,
+                          AggValueSlot::IsNotAliased);
 
   EmitAggExpr(Ctor->init_begin()[0]->getInit(), AggSlot);
 
diff --git a/lib/CodeGen/CGDecl.cpp b/lib/CodeGen/CGDecl.cpp
index 609542c..5fa99a5 100644
--- a/lib/CodeGen/CGDecl.cpp
+++ b/lib/CodeGen/CGDecl.cpp
@@ -1046,7 +1046,8 @@
     // TODO: how can we delay here if D is captured by its initializer?
     EmitAggExpr(init, AggValueSlot::forLValue(lvalue,
                                               AggValueSlot::IsDestructed,
-                                         AggValueSlot::DoesNotNeedGCBarriers));
+                                         AggValueSlot::DoesNotNeedGCBarriers,
+                                              AggValueSlot::IsNotAliased));
   }
 }
 
diff --git a/lib/CodeGen/CGDeclCXX.cpp b/lib/CodeGen/CGDeclCXX.cpp
index f4bed79..e4c327b 100644
--- a/lib/CodeGen/CGDeclCXX.cpp
+++ b/lib/CodeGen/CGDeclCXX.cpp
@@ -47,7 +47,8 @@
     CGF.EmitComplexExprIntoAddr(Init, DeclPtr, lv.isVolatile());
   } else {
     CGF.EmitAggExpr(Init, AggValueSlot::forLValue(lv,AggValueSlot::IsDestructed,
-                                          AggValueSlot::DoesNotNeedGCBarriers));
+                                          AggValueSlot::DoesNotNeedGCBarriers,
+                                                  AggValueSlot::IsNotAliased));
   }
 }
 
diff --git a/lib/CodeGen/CGExpr.cpp b/lib/CodeGen/CGExpr.cpp
index da892c7..1ed24cc 100644
--- a/lib/CodeGen/CGExpr.cpp
+++ b/lib/CodeGen/CGExpr.cpp
@@ -360,7 +360,8 @@
         = AggValueSlot::IsDestructed_t(InitializedDecl != 0);
       AggSlot = AggValueSlot::forAddr(ReferenceTemporary, Qualifiers(),
                                       isDestructed,
-                                      AggValueSlot::DoesNotNeedGCBarriers);
+                                      AggValueSlot::DoesNotNeedGCBarriers,
+                                      AggValueSlot::IsNotAliased);
     }
     
     if (InitializedDecl) {
diff --git a/lib/CodeGen/CGExprAgg.cpp b/lib/CodeGen/CGExprAgg.cpp
index 4a151fc..c42c87b 100644
--- a/lib/CodeGen/CGExprAgg.cpp
+++ b/lib/CodeGen/CGExprAgg.cpp
@@ -35,11 +35,18 @@
   AggValueSlot Dest;
   bool IgnoreResult;
 
+  /// We want to use 'dest' as the return slot except under two
+  /// conditions:
+  ///   - The destination slot requires garbage collection, so we
+  ///     need to use the GC API.
+  ///   - The destination slot is potentially aliased.
+  bool shouldUseDestForReturnSlot() const {
+    return !(Dest.requiresGCollection() || Dest.isPotentiallyAliased());
+  }
+
   ReturnValueSlot getReturnValueSlot() const {
-    // If the destination slot requires garbage collection, we can't
-    // use the real return value slot, because we have to use the GC
-    // API.
-    if (Dest.requiresGCollection()) return ReturnValueSlot();
+    if (!shouldUseDestForReturnSlot())
+      return ReturnValueSlot();
 
     return ReturnValueSlot(Dest.getAddr(), Dest.isVolatile());
   }
@@ -69,7 +76,7 @@
   void EmitFinalDestCopy(const Expr *E, LValue Src, bool Ignore = false);
   void EmitFinalDestCopy(const Expr *E, RValue Src, bool Ignore = false);
 
-  void EmitGCMove(const Expr *E, RValue Src);
+  void EmitMoveFromReturnSlot(const Expr *E, RValue Src);
 
   AggValueSlot::NeedsGCBarriers_t needsGC(QualType T) {
     if (CGF.getLangOptions().getGCMode() && TypeRequiresGCollection(T))
@@ -179,23 +186,27 @@
   return Record->hasObjectMember();
 }
 
-/// \brief Perform the final move to DestPtr if RequiresGCollection is set.
+/// \brief Perform the final move to DestPtr if for some reason
+/// getReturnValueSlot() didn't use it directly.
 ///
 /// The idea is that you do something like this:
 ///   RValue Result = EmitSomething(..., getReturnValueSlot());
-///   EmitGCMove(E, Result);
-/// If GC doesn't interfere, this will cause the result to be emitted
-/// directly into the return value slot.  If GC does interfere, a final
-/// move will be performed.
-void AggExprEmitter::EmitGCMove(const Expr *E, RValue Src) {
-  if (Dest.requiresGCollection()) {
-    CharUnits size = CGF.getContext().getTypeSizeInChars(E->getType());
-    llvm::Type *SizeTy = CGF.ConvertType(CGF.getContext().getSizeType());
-    llvm::Value *SizeVal = llvm::ConstantInt::get(SizeTy, size.getQuantity());
-    CGF.CGM.getObjCRuntime().EmitGCMemmoveCollectable(CGF, Dest.getAddr(),
-                                                    Src.getAggregateAddr(),
-                                                    SizeVal);
+///   EmitMoveFromReturnSlot(E, Result);
+///
+/// If nothing interferes, this will cause the result to be emitted
+/// directly into the return value slot.  Otherwise, a final move
+/// will be performed.
+void AggExprEmitter::EmitMoveFromReturnSlot(const Expr *E, RValue Src) {
+  if (shouldUseDestForReturnSlot()) {
+    // Logically, Dest.getAddr() should equal Src.getAggregateAddr().
+    // The possibility of undef rvalues complicates that a lot,
+    // though, so we can't really assert.
+    return;
   }
+
+  // Otherwise, do a final copy, 
+  assert(Dest.getAddr() != Src.getAggregateAddr());
+  EmitFinalDestCopy(E, Src, /*Ignore*/ true);
 }
 
 /// EmitFinalDestCopy - Perform the final copy to DestPtr, if desired.
@@ -316,7 +327,7 @@
     LValue LV = CGF.EmitLValue(E->getSubExpr());
     assert(LV.isPropertyRef());
     RValue RV = CGF.EmitLoadOfPropertyRefLValue(LV, getReturnValueSlot());
-    EmitGCMove(E, RV);
+    EmitMoveFromReturnSlot(E, RV);
     break;
   }
 
@@ -381,12 +392,12 @@
   }
 
   RValue RV = CGF.EmitCallExpr(E, getReturnValueSlot());
-  EmitGCMove(E, RV);
+  EmitMoveFromReturnSlot(E, RV);
 }
 
 void AggExprEmitter::VisitObjCMessageExpr(ObjCMessageExpr *E) {
   RValue RV = CGF.EmitObjCMessageExpr(E, getReturnValueSlot());
-  EmitGCMove(E, RV);
+  EmitMoveFromReturnSlot(E, RV);
 }
 
 void AggExprEmitter::VisitObjCPropertyRefExpr(ObjCPropertyRefExpr *E) {
@@ -600,6 +611,7 @@
     CGF.EmitAggExpr(E, AggValueSlot::forLValue(LV,
                                                AggValueSlot::IsDestructed,
                                       AggValueSlot::DoesNotNeedGCBarriers,
+                                               AggValueSlot::IsNotAliased,
                                                Dest.isZeroed()));
   } else if (LV.isSimple()) {
     CGF.EmitScalarInit(E, /*D=*/0, LV, /*Captured=*/false);
diff --git a/lib/CodeGen/CGObjC.cpp b/lib/CodeGen/CGObjC.cpp
index 83a42fb..179dc75 100644
--- a/lib/CodeGen/CGObjC.cpp
+++ b/lib/CodeGen/CGObjC.cpp
@@ -820,7 +820,8 @@
                                     LoadObjCSelf(), Ivar, 0);
       EmitAggExpr(IvarInit->getInit(),
                   AggValueSlot::forLValue(LV, AggValueSlot::IsDestructed,
-                                          AggValueSlot::DoesNotNeedGCBarriers));
+                                          AggValueSlot::DoesNotNeedGCBarriers,
+                                          AggValueSlot::IsNotAliased));
     }
     // constructor returns 'self'.
     CodeGenTypes &Types = CGM.getTypes();
diff --git a/lib/CodeGen/CGStmt.cpp b/lib/CodeGen/CGStmt.cpp
index b7b127f..461a173 100644
--- a/lib/CodeGen/CGStmt.cpp
+++ b/lib/CodeGen/CGStmt.cpp
@@ -786,7 +786,8 @@
   } else {
     EmitAggExpr(RV, AggValueSlot::forAddr(ReturnValue, Qualifiers(),
                                           AggValueSlot::IsDestructed,
-                                          AggValueSlot::DoesNotNeedGCBarriers));
+                                          AggValueSlot::DoesNotNeedGCBarriers,
+                                          AggValueSlot::IsNotAliased));
   }
 
   EmitBranchThroughCleanup(ReturnBlock);
diff --git a/lib/CodeGen/CGValue.h b/lib/CodeGen/CGValue.h
index dda9693..c448949 100644
--- a/lib/CodeGen/CGValue.h
+++ b/lib/CodeGen/CGValue.h
@@ -347,9 +347,14 @@
   /// be set.
   bool ZeroedFlag : 1;
 
+  /// AliasedFlag - This generally defaults to false, but can be true
+  /// if the memory is known not to be aliased.
+  bool AliasedFlag : 1;
+
 public:
-  enum IsZeroed_t { IsNotZeroed, IsZeroed };
+  enum IsAliased_t { IsNotAliased, IsAliased };
   enum IsDestructed_t { IsNotDestructed, IsDestructed };
+  enum IsZeroed_t { IsNotZeroed, IsZeroed };
   enum NeedsGCBarriers_t { DoesNotNeedGCBarriers, NeedsGCBarriers };
 
   /// ignored - Returns an aggregate value slot indicating that the
@@ -358,7 +363,10 @@
     AggValueSlot AV;
     AV.Addr = 0;
     AV.Quals = Qualifiers();
-    AV.LifetimeFlag = AV.RequiresGCollection = AV.ZeroedFlag = 0;
+    AV.LifetimeFlag = AV.RequiresGCollection = AV.ZeroedFlag = false;
+
+    // If there's ever an address here, it will be a temporary.
+    AV.AliasedFlag = false;
     return AV;
   }
 
@@ -375,6 +383,7 @@
   static AggValueSlot forAddr(llvm::Value *addr, Qualifiers quals,
                               IsDestructed_t isDestructed,
                               NeedsGCBarriers_t needsGC,
+                              IsAliased_t isAliased = IsAliased,
                               IsZeroed_t isZeroed = IsNotZeroed) {
     AggValueSlot AV;
     AV.Addr = addr;
@@ -382,14 +391,16 @@
     AV.LifetimeFlag = isDestructed;
     AV.RequiresGCollection = needsGC;
     AV.ZeroedFlag = isZeroed;
+    AV.AliasedFlag = isAliased;
     return AV;
   }
 
   static AggValueSlot forLValue(LValue LV, IsDestructed_t isDestructed,
                                 NeedsGCBarriers_t needsGC,
+                                IsAliased_t isAliased = IsAliased,
                                 IsZeroed_t isZeroed = IsNotZeroed) {
     return forAddr(LV.getAddress(), LV.getQuals(),
-                   isDestructed, needsGC, isZeroed);
+                   isDestructed, needsGC, isAliased, isZeroed);
   }
 
   IsDestructed_t isLifetimeExternallyManaged() const {
@@ -421,6 +432,10 @@
     return Addr == 0;
   }
 
+  IsAliased_t isPotentiallyAliased() const {
+    return IsAliased_t(AliasedFlag);
+  }
+
   RValue asRValue() const {
     return RValue::getAggregate(getAddr(), isVolatile());
   }
diff --git a/lib/CodeGen/CodeGenFunction.h b/lib/CodeGen/CodeGenFunction.h
index 2d47301..36dcffc 100644
--- a/lib/CodeGen/CodeGenFunction.h
+++ b/lib/CodeGen/CodeGenFunction.h
@@ -1466,7 +1466,8 @@
   AggValueSlot CreateAggTemp(QualType T, const Twine &Name = "tmp") {
     return AggValueSlot::forAddr(CreateMemTemp(T, Name), T.getQualifiers(),
                                  AggValueSlot::IsNotDestructed,
-                                 AggValueSlot::DoesNotNeedGCBarriers);
+                                 AggValueSlot::DoesNotNeedGCBarriers,
+                                 AggValueSlot::IsNotAliased);
   }
 
   /// Emit a cast to void* in the appropriate address space.