Improve array to string conversion.

src/ia32/codegen-ia32.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 775 matching lines @@
   // Load the byte into the result register.
   __ bind(&ascii);
   __ movzx_b(result, FieldOperand(string,
                                   index,
                                   times_1,
                                   SeqOneByteString::kHeaderSize));
   __ bind(&done);
 }
 
 
+void SeqStringSetCharGenerator::Generate(MacroAssembler* masm,
+                                         bool one_byte_string,
+                                         Register string,
+                                         Register index,
+                                         Register value) {
+  if (FLAG_debug_code) {
+    __ test(index, Immediate(kSmiTagMask));
+    __ Check(zero, "Non-smi index");
+    __ test(value, Immediate(kSmiTagMask));
+    __ Check(zero, "Non-smi value");
+
+    __ cmp(index, FieldOperand(string, String::kLengthOffset));
+    __ Check(less, "Index is too large");
+
+    __ cmp(index, Immediate(Smi::FromInt(0)));
+    __ Check(greater_equal, "Index is negative");
+
+    __ push(value);
+    __ mov(value, FieldOperand(string, HeapObject::kMapOffset));
+    __ movzx_b(value, FieldOperand(value, Map::kInstanceTypeOffset));
+
+    __ and_(value, Immediate(kStringRepresentationMask | kStringEncodingMask));
+    static const uint32_t one_byte_seq_type = kSeqStringTag | kOneByteStringTag;
+    static const uint32_t two_byte_seq_type = kSeqStringTag | kTwoByteStringTag;
+    __ cmp(value, Immediate(one_byte_string ? one_byte_seq_type
+                                            : two_byte_seq_type));
+    __ Check(equal, "Unexpected string type");
+    __ pop(value);
+  }
+
+  STATIC_ASSERT(kSmiTagSize == 1);
+  __ SmiUntag(value);
+  if (one_byte_string) {
+    __ SmiUntag(index);
+    __ mov_b(FieldOperand(string, index, times_1, SeqString::kHeaderSize),
+             value);
+  } else {
+    __ mov_w(FieldOperand(string, index, times_1, SeqString::kHeaderSize),

[Toon Verwaest, 2012/12/05 14:29:12]

Can we add a comment that this works because of the Smi-tagging? Or at least a
STATIC_ASSERT that the tagbit is 0 and shifts by 1?

+             value);
+  }
+}
+
+
 static Operand ExpConstant(int index) {
   return Operand::StaticVariable(ExternalReference::math_exp_constants(index));
 }
 
 
 void MathExpGenerator::EmitMathExp(MacroAssembler* masm,
                                    XMMRegister input,
                                    XMMRegister result,
                                    XMMRegister double_scratch,
                                    Register temp1,
@@ skipping 102 matching lines @@
     Code* stub = GetCodeAgeStub(age, parity);
     CodePatcher patcher(sequence, young_length);
     patcher.masm()->call(stub->instruction_start(), RelocInfo::NONE);
   }
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32