diff --git a/lib/CodeGen/CGExpr.cpp b/lib/CodeGen/CGExpr.cpp
index 9def554..3275327 100644
--- a/lib/CodeGen/CGExpr.cpp
+++ b/lib/CodeGen/CGExpr.cpp
@@ -745,6 +745,122 @@
   }
 }
 
+/// Given an object of the given canonical type, can we safely copy a
+/// value out of it based on its initializer?
+static bool isConstantEmittableObjectType(QualType type) {
+  assert(type.isCanonical());
+  assert(!type->isReferenceType());
+
+  // Must be const-qualified but non-volatile.
+  Qualifiers qs = type.getLocalQualifiers();
+  if (!qs.hasConst() || qs.hasVolatile()) return false;
+
+  // Otherwise, all object types satisfy this except C++ classes with
+  // mutable subobjects or non-trivial copy/destroy behavior.
+  if (const RecordType *RT = dyn_cast<RecordType>(type))
+    if (const CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(RT->getDecl()))
+      if (RD->hasMutableFields() || !RD->isTrivial())
+        return false;
+
+  return true;
+}
+
+/// Can we constant-emit a load of a reference to a variable of the
+/// given type?  This is different from predicates like
+/// Decl::isUsableInConstantExpressions because we do want it to apply
+/// in situations that don't necessarily satisfy the language's rules
+/// for this (e.g. C++'s ODR-use rules).  For example, we want to able
+/// to do this with const float variables even if those variables
+/// aren't marked 'constexpr'.
+enum ConstantEmissionKind {
+  CEK_None,
+  CEK_AsReferenceOnly,
+  CEK_AsValueOrReference,
+  CEK_AsValueOnly
+};
+static ConstantEmissionKind checkVarTypeForConstantEmission(QualType type) {
+  type = type.getCanonicalType();
+  if (const ReferenceType *ref = dyn_cast<ReferenceType>(type)) {
+    if (isConstantEmittableObjectType(ref->getPointeeType()))
+      return CEK_AsValueOrReference;
+    return CEK_AsReferenceOnly;
+  }
+  if (isConstantEmittableObjectType(type))
+    return CEK_AsValueOnly;
+  return CEK_None;
+}
+
+/// Try to emit a reference to the given value without producing it as
+/// an l-value.  This is actually more than an optimization: we can't
+/// produce an l-value for variables that we never actually captured
+/// in a block or lambda, which means const int variables or constexpr
+/// literals or similar.
+CodeGenFunction::ConstantEmission
+CodeGenFunction::tryEmitAsConstant(ValueDecl *value, Expr *refExpr) {
+  // The value needs to be an enum constant or a constant variable.
+  ConstantEmissionKind CEK;
+  if (isa<ParmVarDecl>(value)) {
+    CEK = CEK_None;
+  } else if (VarDecl *var = dyn_cast<VarDecl>(value)) {
+    CEK = checkVarTypeForConstantEmission(var->getType());
+  } else if (isa<EnumConstantDecl>(value)) {
+    CEK = CEK_AsValueOnly;
+  } else {
+    CEK = CEK_None;
+  }
+  if (CEK == CEK_None) return ConstantEmission();
+
+  // We evaluate use an on-stack DeclRefExpr because the constant
+  // evaluator (quite reasonably) ignores BlockDeclRefExprs.
+  DeclRefExpr stackRef(value, refExpr->getType(), refExpr->getValueKind(),
+                       refExpr->getExprLoc());
+
+  // If it's okay to evaluate as a 
+  Expr::EvalResult result;
+  bool resultIsReference;
+  QualType resultType;
+
+  // It's best to evaluate all the way as an r-value if that's permitted.
+  if (CEK != CEK_AsReferenceOnly &&
+      stackRef.EvaluateAsRValue(result, getContext())) {
+    resultIsReference = false;
+    resultType = refExpr->getType();
+
+  // Otherwise, try to evaluate as an l-value.
+  } else if (CEK != CEK_AsValueOnly &&
+             stackRef.EvaluateAsLValue(result, getContext())) {
+    resultIsReference = true;
+    resultType = value->getType();
+
+  // Failure.
+  } else {
+    return ConstantEmission();
+  }
+
+  // In any case, if the initializer has side-effects, abandon ship.
+  if (result.HasSideEffects)
+    return ConstantEmission();
+
+  // Emit as a constant.
+  llvm::Constant *C = CGM.EmitConstantValue(result.Val, resultType, this);
+
+  // Make sure we emit a debug reference to the global variable.
+  // This should probably fire even for 
+  if (isa<VarDecl>(value)) {
+    if (!getContext().DeclMustBeEmitted(cast<VarDecl>(value)))
+      EmitDeclRefExprDbgValue(&stackRef, C);
+  } else {
+    assert(isa<EnumConstantDecl>(value));
+    EmitDeclRefExprDbgValue(&stackRef, C);
+  }
+
+  // If we emitted a reference constant, we need to dereference that.
+  if (resultIsReference)
+    return ConstantEmission::forReference(C);
+
+  return ConstantEmission::forValue(C);
+}
+
 llvm::Value *CodeGenFunction::EmitLoadOfScalar(LValue lvalue) {
   return EmitLoadOfScalar(lvalue.getAddress(), lvalue.isVolatile(),
                           lvalue.getAlignment().getQuantity(),
diff --git a/lib/CodeGen/CGExprAgg.cpp b/lib/CodeGen/CGExprAgg.cpp
index 899577c..f7c640a 100644
--- a/lib/CodeGen/CGExprAgg.cpp
+++ b/lib/CodeGen/CGExprAgg.cpp
@@ -109,7 +109,28 @@
   }
 
   // l-values.
-  void VisitDeclRefExpr(DeclRefExpr *DRE) { EmitAggLoadOfLValue(DRE); }
+  void emitDeclRef(ValueDecl *VD, Expr *refExpr) {
+    // For aggregates, we should always be able to emit the variable
+    // as an l-value unless it's a reference.  This is due to the fact
+    // that we can't actually ever see a normal l2r conversion on an
+    // aggregate in C++, and in C there's no language standard
+    // actively preventing us from listing variables in the captures
+    // list of a block.
+    if (VD->getType()->isReferenceType()) {
+      if (CodeGenFunction::ConstantEmission result
+            = CGF.tryEmitAsConstant(VD, refExpr)) {
+        EmitFinalDestCopy(refExpr, result.getReferenceLValue(CGF, refExpr));
+        return;
+      }
+    }
+
+    EmitAggLoadOfLValue(refExpr);
+  }
+  void VisitDeclRefExpr(DeclRefExpr *E) { emitDeclRef(E->getDecl(), E); }
+  void VisitBlockDeclRefExpr(BlockDeclRefExpr *E) {
+    emitDeclRef(E->getDecl(), E);
+  }
+
   void VisitMemberExpr(MemberExpr *ME) { EmitAggLoadOfLValue(ME); }
   void VisitUnaryDeref(UnaryOperator *E) { EmitAggLoadOfLValue(E); }
   void VisitStringLiteral(StringLiteral *E) { EmitAggLoadOfLValue(E); }
@@ -117,9 +138,6 @@
   void VisitArraySubscriptExpr(ArraySubscriptExpr *E) {
     EmitAggLoadOfLValue(E);
   }
-  void VisitBlockDeclRefExpr(const BlockDeclRefExpr *E) {
-    EmitAggLoadOfLValue(E);
-  }
   void VisitPredefinedExpr(const PredefinedExpr *E) {
     EmitAggLoadOfLValue(E);
   }
diff --git a/lib/CodeGen/CGExprComplex.cpp b/lib/CodeGen/CGExprComplex.cpp
index d2a7600..7be8844 100644
--- a/lib/CodeGen/CGExprComplex.cpp
+++ b/lib/CodeGen/CGExprComplex.cpp
@@ -111,8 +111,24 @@
   }
 
   // l-values.
-  ComplexPairTy VisitDeclRefExpr(const Expr *E) { return EmitLoadOfLValue(E); }
-  ComplexPairTy VisitBlockDeclRefExpr(const Expr *E) { return EmitLoadOfLValue(E); }
+  ComplexPairTy emitDeclRef(ValueDecl *VD, Expr *refExpr) {
+    if (CodeGenFunction::ConstantEmission result
+          = CGF.tryEmitAsConstant(VD, refExpr)) {
+      if (result.isReference())
+        return EmitLoadOfLValue(result.getReferenceLValue(CGF, refExpr));
+
+      llvm::ConstantStruct *pair =
+        cast<llvm::ConstantStruct>(result.getValue());
+      return ComplexPairTy(pair->getOperand(0), pair->getOperand(1));
+    }
+    return EmitLoadOfLValue(refExpr);
+  }
+  ComplexPairTy VisitDeclRefExpr(DeclRefExpr *E) {
+    return emitDeclRef(E->getDecl(), E);
+  }
+  ComplexPairTy VisitBlockDeclRefExpr(BlockDeclRefExpr *E) {
+    return emitDeclRef(E->getDecl(), E);
+  }
   ComplexPairTy VisitObjCIvarRefExpr(ObjCIvarRefExpr *E) {
     return EmitLoadOfLValue(E);
   }
diff --git a/lib/CodeGen/CGExprScalar.cpp b/lib/CodeGen/CGExprScalar.cpp
index 9da4934..a6ba798 100644
--- a/lib/CodeGen/CGExprScalar.cpp
+++ b/lib/CodeGen/CGExprScalar.cpp
@@ -211,48 +211,24 @@
     // Otherwise, assume the mapping is the scalar directly.
     return CGF.getOpaqueRValueMapping(E).getScalarVal();
   }
-    
+
   // l-values.
-  Value *VisitDeclRefExpr(DeclRefExpr *E) {
-    VarDecl *VD = dyn_cast<VarDecl>(E->getDecl());
-    if (!VD && !isa<EnumConstantDecl>(E->getDecl()))
-      return EmitLoadOfLValue(E);
-    if (VD && !VD->isUsableInConstantExpressions(CGF.getContext()))
-      return EmitLoadOfLValue(E);
-
-    // This is an enumerator or a variable which is usable in constant
-    // expressions. Try to emit its value instead.
-    Expr::EvalResult Result;
-    bool IsReferenceConstant = false;
-    QualType EvalTy = E->getType();
-    if (!E->EvaluateAsRValue(Result, CGF.getContext())) {
-      // If this is a reference, try to determine what it is bound to.
-      if (!E->getDecl()->getType()->isReferenceType() ||
-          !E->EvaluateAsLValue(Result, CGF.getContext()))
-        return EmitLoadOfLValue(E);
-
-      IsReferenceConstant = true;
-      EvalTy = E->getDecl()->getType();
+  Value *emitDeclRef(ValueDecl *VD, Expr *refExpr) {
+    if (CodeGenFunction::ConstantEmission result
+          = CGF.tryEmitAsConstant(VD, refExpr)) {
+      if (result.isReference())
+        return EmitLoadOfLValue(result.getReferenceLValue(CGF, refExpr));
+      return result.getValue();
     }
-
-    assert(!Result.HasSideEffects && "Constant declref with side-effect?!");
-
-    llvm::Constant *C = CGF.CGM.EmitConstantValue(Result.Val, EvalTy, &CGF);
-
-    // Make sure we emit a debug reference to the global variable.
-    if (VD) {
-      if (!CGF.getContext().DeclMustBeEmitted(VD))
-        CGF.EmitDeclRefExprDbgValue(E, C);
-    } else {
-      assert(isa<EnumConstantDecl>(E->getDecl()));
-      CGF.EmitDeclRefExprDbgValue(E, C);
-    }
-
-    if (IsReferenceConstant)
-      return EmitLoadOfLValue(CGF.MakeNaturalAlignAddrLValue(C, E->getType()));
-
-    return C;
+    return EmitLoadOfLValue(refExpr);
   }
+  Value *VisitDeclRefExpr(DeclRefExpr *E) {
+    return emitDeclRef(E->getDecl(), E);
+  }
+  Value *VisitBlockDeclRefExpr(BlockDeclRefExpr *E) {
+    return emitDeclRef(E->getDecl(), E);
+  }
+
   Value *VisitObjCSelectorExpr(ObjCSelectorExpr *E) {
     return CGF.EmitObjCSelectorExpr(E);
   }
@@ -304,8 +280,6 @@
 
   Value *VisitStmtExpr(const StmtExpr *E);
 
-  Value *VisitBlockDeclRefExpr(const BlockDeclRefExpr *E);
-
   // Unary Operators.
   Value *VisitUnaryPostDec(const UnaryOperator *E) {
     LValue LV = EmitLValue(E->getSubExpr());
@@ -1272,11 +1246,6 @@
     .getScalarVal();
 }
 
-Value *ScalarExprEmitter::VisitBlockDeclRefExpr(const BlockDeclRefExpr *E) {
-  LValue LV = CGF.EmitBlockDeclRefLValue(E);
-  return CGF.EmitLoadOfLValue(LV).getScalarVal();
-}
-
 //===----------------------------------------------------------------------===//
 //                             Unary Operators
 //===----------------------------------------------------------------------===//
diff --git a/lib/CodeGen/CodeGenFunction.h b/lib/CodeGen/CodeGenFunction.h
index 833eee2..aeeb18c 100644
--- a/lib/CodeGen/CodeGenFunction.h
+++ b/lib/CodeGen/CodeGenFunction.h
@@ -2107,6 +2107,36 @@
   LValue EmitMaterializeTemporaryExpr(const MaterializeTemporaryExpr *E);
   LValue EmitOpaqueValueLValue(const OpaqueValueExpr *e);
 
+  class ConstantEmission {
+    llvm::PointerIntPair<llvm::Constant*, 1, bool> ValueAndIsReference;
+    ConstantEmission(llvm::Constant *C, bool isReference)
+      : ValueAndIsReference(C, isReference) {}
+  public:
+    ConstantEmission() {}
+    static ConstantEmission forReference(llvm::Constant *C) {
+      return ConstantEmission(C, true);
+    }
+    static ConstantEmission forValue(llvm::Constant *C) {
+      return ConstantEmission(C, false);
+    }
+
+    operator bool() const { return ValueAndIsReference.getOpaqueValue() != 0; }
+
+    bool isReference() const { return ValueAndIsReference.getInt(); }
+    LValue getReferenceLValue(CodeGenFunction &CGF, Expr *refExpr) const {
+      assert(isReference());
+      return CGF.MakeNaturalAlignAddrLValue(ValueAndIsReference.getPointer(),
+                                            refExpr->getType());
+    }
+
+    llvm::Constant *getValue() const {
+      assert(!isReference());
+      return ValueAndIsReference.getPointer();
+    }
+  };
+
+  ConstantEmission tryEmitAsConstant(ValueDecl *VD, Expr *refExpr);
+
   RValue EmitPseudoObjectRValue(const PseudoObjectExpr *e,
                                 AggValueSlot slot = AggValueSlot::ignored());
   LValue EmitPseudoObjectLValue(const PseudoObjectExpr *e);