diff --git a/src/codegen.h b/src/codegen.h
index 0576fbb..783bef0 100644
--- a/src/codegen.h
+++ b/src/codegen.h
@@ -75,6 +75,7 @@
 
 // Mode to overwrite BinaryExpression values.
 enum OverwriteMode { NO_OVERWRITE, OVERWRITE_LEFT, OVERWRITE_RIGHT };
+enum UnaryOverwriteMode { UNARY_OVERWRITE, UNARY_NO_OVERWRITE };
 
 // Types of uncatchable exceptions.
 enum UncatchableExceptionType { OUT_OF_MEMORY, TERMINATION };
@@ -414,21 +415,33 @@
 };
 
 
+enum NegativeZeroHandling {
+  kStrictNegativeZero,
+  kIgnoreNegativeZero
+};
+
+
 class GenericUnaryOpStub : public CodeStub {
  public:
-  GenericUnaryOpStub(Token::Value op, bool overwrite)
-      : op_(op), overwrite_(overwrite) { }
+  GenericUnaryOpStub(Token::Value op,
+                     UnaryOverwriteMode overwrite,
+                     NegativeZeroHandling negative_zero = kStrictNegativeZero)
+      : op_(op), overwrite_(overwrite), negative_zero_(negative_zero) { }
 
  private:
   Token::Value op_;
-  bool overwrite_;
+  UnaryOverwriteMode overwrite_;
+  NegativeZeroHandling negative_zero_;
 
-  class OverwriteField: public BitField<int, 0, 1> {};
-  class OpField: public BitField<Token::Value, 1, kMinorBits - 1> {};
+  class OverwriteField: public BitField<UnaryOverwriteMode, 0, 1> {};
+  class NegativeZeroField: public BitField<NegativeZeroHandling, 1, 1> {};
+  class OpField: public BitField<Token::Value, 2, kMinorBits - 2> {};
 
   Major MajorKey() { return GenericUnaryOp; }
   int MinorKey() {
-    return OpField::encode(op_) | OverwriteField::encode(overwrite_);
+    return OpField::encode(op_) |
+           OverwriteField::encode(overwrite_) |
+           NegativeZeroField::encode(negative_zero_);
   }
 
   void Generate(MacroAssembler* masm);