Improve array to string conversion.

src/x64/codegen-x64.cc

Index: src/x64/codegen-x64.cc
diff --git a/src/x64/codegen-x64.cc b/src/x64/codegen-x64.cc
index 67d1d90b6c09fa4d52be6d01c8e640d051b8c2c3..d0d0ee9dc81626da0479231433aacac635eca212 100644
--- a/src/x64/codegen-x64.cc
+++ b/src/x64/codegen-x64.cc
@@ -606,6 +606,46 @@ void StringCharLoadGenerator::Generate(MacroAssembler* masm,
 }
 
 
+void SeqStringSetCharGenerator::Generate(MacroAssembler* masm,
+                                         bool one_byte_string,
+                                         Register string,
+                                         Register index,
+                                         Register value) {
+  if (FLAG_debug_code) {
+    __ Check(masm->CheckSmi(index), "Non-smi index");
+    __ Check(masm->CheckSmi(value), "Non-smi value");
+
+    __ SmiCompare(index, FieldOperand(string, String::kLengthOffset));
+    __ Check(less, "Index is too large");
+
+    __ SmiCompare(index, Smi::FromInt(0));
+    __ Check(greater_equal, "Index is negative");
+
+    __ push(value);
+    __ movq(value, FieldOperand(string, HeapObject::kMapOffset));
+    __ movzxbq(value, FieldOperand(value, Map::kInstanceTypeOffset));
+
+    __ andb(value, Immediate(kStringRepresentationMask | kStringEncodingMask));
+    static const uint32_t one_byte_seq_type = kSeqStringTag | kOneByteStringTag;
+    static const uint32_t two_byte_seq_type = kSeqStringTag | kTwoByteStringTag;
+    __ cmpq(value, Immediate(one_byte_string ? one_byte_seq_type
+                                             : two_byte_seq_type));
+    __ Check(equal, "Unexpected string type");
+    __ pop(value);
+  }
+
+  __ SmiToInteger32(value, value);
+  __ SmiToInteger32(index, index);
+  if (one_byte_string) {
+    __ movb(FieldOperand(string, index, times_1, SeqString::kHeaderSize),
+            value);
+  } else {
+    __ movw(FieldOperand(string, index, times_2, SeqString::kHeaderSize),
+            value);
+  }
+}
+
+
 void MathExpGenerator::EmitMathExp(MacroAssembler* masm,
                                    XMMRegister input,
                                    XMMRegister result,