Ported SubStringStub to X64.

src/x64/codegen-x64.h

Index: src/x64/codegen-x64.h
diff --git a/src/x64/codegen-x64.h b/src/x64/codegen-x64.h
index 766ca139de4f14bd89363ff4085f30add9465f29..af7c804daf018f90c2bf0a571d9a44c61a8bdecd 100644
--- a/src/x64/codegen-x64.h
+++ b/src/x64/codegen-x64.h
@@ -713,6 +713,29 @@ class GenericBinaryOpStub: public CodeStub {
 };
 
 
+class StringStubBase: public CodeStub {
+ public:
+  // Generate code for copying characters using a simple loop. This should only
+  // be used in places where the number of characters is small and the
+  // additional setup and checking in GenerateCopyCharactersREP adds too much
+  // overhead. Copying of overlapping regions is not supported.
+  void GenerateCopyCharacters(MacroAssembler* masm,
+                              Register dest,
+                              Register src,
+                              Register count,
+                              bool ascii);
+
+  // Generate code for copying characters using the rep movs instruction.
+  // Copies rcx characters from rsi to rdi. Copying of overlapping regions is
+  // not supported.
+  void GenerateCopyCharactersREP(MacroAssembler* masm,
+                                 Register dest,     // Must be rdi.
+                                 Register src,      // Must be rsi.
+                                 Register count,    // Must be rcx.
+                                 bool ascii);
+};
+
+
 // Flag that indicates how to generate code for the stub StringAddStub.
 enum StringAddFlags {
   NO_STRING_ADD_FLAGS = 0,
@@ -720,7 +743,7 @@ enum StringAddFlags {
 };
 
 
-class StringAddStub: public CodeStub {
+class StringAddStub: public StringStubBase {
  public:
   explicit StringAddStub(StringAddFlags flags) {
     string_check_ = ((flags & NO_STRING_CHECK_IN_STUB) == 0);
@@ -732,17 +755,23 @@ class StringAddStub: public CodeStub {
 
   void Generate(MacroAssembler* masm);
 
-  void GenerateCopyCharacters(MacroAssembler* masm,
-                              Register desc,
-                              Register src,
-                              Register count,
-                              bool ascii);
-
   // Should the stub check whether arguments are strings?
   bool string_check_;
 };
 
 
+class SubStringStub: public StringStubBase {
+ public:
+  SubStringStub() {}
+
+ private:
+  Major MajorKey() { return SubString; }
+  int MinorKey() { return 0; }
+
+  void Generate(MacroAssembler* masm);
+};
+
+
 class StringCompareStub: public CodeStub {
  public:
   explicit StringCompareStub() {}