Ported SubStringStub to X64.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index ce2848c059b887b03b2e33feff270c14d76516cf..8d4a8f2e2aae443c7664caef92047dd5901dd38a 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -207,6 +207,9 @@ class MacroAssembler: public Assembler {
   // Are both values tagged smis.
   Condition CheckBothSmi(Register first, Register second);
 
+  // Are both values tagged smis.
+  Condition CheckBothPositiveSmi(Register first, Register second);
+
   // Are either value a tagged smi.
   Condition CheckEitherSmi(Register first, Register second);
 
@@ -248,6 +251,10 @@ class MacroAssembler: public Assembler {
   // Jump if either or both register are not smi values.
   void JumpIfNotBothSmi(Register src1, Register src2, Label* on_not_both_smi);
 
+  // Jump if either or both register are not positive smi values.
+  void JumpIfNotBothPositiveSmi(Register src1, Register src2,
+                                Label* on_not_both_smi);
+
   // Operations on tagged smi values.
 
   // Smis represent a subset of integers. The subset is always equivalent to
@@ -452,6 +459,15 @@ class MacroAssembler: public Assembler {
   // Always use unsigned comparisons: above and below, not less and greater.
   void CmpInstanceType(Register map, InstanceType type);
 
+  // Check if the object in register heap_object is a string. Afterwards the
+  // register map contains the object map and the register instance_type
+  // contains the instance_type. The registers map and instance_type can be the
+  // same in which case it contains the instance type afterwards. Either of the
+  // registers map and instance_type can be the same as heap_object.
+  Condition IsObjectStringType(Register heap_object,
+                               Register map,
+                               Register instance_type);
+
   // FCmp is similar to integer cmp, but requires unsigned
   // jcc instructions (je, ja, jae, jb, jbe, je, and jz).
   void FCmp();