More compact string representation on 64 bit.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
diff --git a/runtime/vm/assembler_x64.cc b/runtime/vm/assembler_x64.cc
index da4021a02e37d1b3bb1ba953afc69fe733d2f6ea..24ffd1c5cab5e13a60eeda61ea605e760715a8bb 100644
--- a/runtime/vm/assembler_x64.cc
+++ b/runtime/vm/assembler_x64.cc
@@ -3624,12 +3624,12 @@ void Assembler::EmitGenericShift(bool wide,
 
 
 void Assembler::LoadClassId(Register result, Register object) {
-  ASSERT(RawObject::kClassIdTagPos == kBitsPerInt32);
-  ASSERT(RawObject::kClassIdTagSize == kBitsPerInt32);
-  ASSERT(sizeof(classid_t) == sizeof(uint32_t));
+  ASSERT(RawObject::kClassIdTagPos == 16);
+  ASSERT(RawObject::kClassIdTagSize == 16);
+  ASSERT(sizeof(classid_t) == sizeof(uint16_t));
   const intptr_t class_id_offset =
       Object::tags_offset() + RawObject::kClassIdTagPos / kBitsPerByte;
-  movl(result, FieldAddress(object, class_id_offset));
+  movzxw(result, FieldAddress(object, class_id_offset));
 }
 
 
@@ -3659,9 +3659,9 @@ void Assembler::SmiUntagOrCheckClass(Register object,
                                      intptr_t class_id,
                                      Label* is_smi) {
   ASSERT(kSmiTagShift == 1);
-  ASSERT(RawObject::kClassIdTagPos == kBitsPerInt32);
-  ASSERT(RawObject::kClassIdTagSize == kBitsPerInt32);
-  ASSERT(sizeof(classid_t) == sizeof(uint32_t));
+  ASSERT(RawObject::kClassIdTagPos == 16);
+  ASSERT(RawObject::kClassIdTagSize == 16);
+  ASSERT(sizeof(classid_t) == sizeof(uint16_t));
   const intptr_t class_id_offset =
       Object::tags_offset() + RawObject::kClassIdTagPos / kBitsPerByte;
 
@@ -3670,7 +3670,7 @@ void Assembler::SmiUntagOrCheckClass(Register object,
   j(NOT_CARRY, is_smi, kNearJump);
   // Load cid: can't use LoadClassId, object is untagged. Use TIMES_2 scale
   // factor in the addressing mode to compensate for this.
-  movl(TMP, Address(object, TIMES_2, class_id_offset));
+  movzxw(TMP, Address(object, TIMES_2, class_id_offset));
   cmpl(TMP, Immediate(class_id));
 }