Improve array to string conversion.

src/arm/codegen-arm.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 503 matching lines @@
   // Two-byte string.
   __ ldrh(result, MemOperand(string, index, LSL, 1));
   __ jmp(&done);
   __ bind(&ascii);
   // Ascii string.
   __ ldrb(result, MemOperand(string, index));
   __ bind(&done);
 }
 
 
+void SeqStringSetCharGenerator::Generate(MacroAssembler* masm,
+                                         bool one_byte_string,
+                                         Register string,
+                                         Register index,
+                                         Register value) {
+  if (FLAG_debug_code) {
+    __ tst(index, Operand(kSmiTagMask));
+    __ Check(eq, "Non-smi index");
+    __ tst(value, Operand(kSmiTagMask));
+    __ Check(eq, "Non-smi value");
+
+    __ ldr(ip, FieldMemOperand(string, String::kLengthOffset));
+    __ cmp(index, ip);
+    __ Check(lt, "Index is too large");
+
+    __ cmp(index, Operand(Smi::FromInt(0)));
+    __ Check(ge, "Index is negative");
+
+    __ ldr(ip, FieldMemOperand(string, HeapObject::kMapOffset));
+    __ ldrb(ip, FieldMemOperand(ip, Map::kInstanceTypeOffset));
+
+    __ and_(ip, ip, Operand(kStringRepresentationMask | kStringEncodingMask));
+    static const uint32_t one_byte_seq_type = kSeqStringTag | kOneByteStringTag;
+    static const uint32_t two_byte_seq_type = kSeqStringTag | kTwoByteStringTag;
+    __ cmp(ip, Operand(one_byte_string ? one_byte_seq_type
+                                       : two_byte_seq_type));
+    __ Check(eq, "Unexpected string type");
+  }
+
+  __ add(ip,
+         string,
+         Operand(SeqString::kHeaderSize - kHeapObjectTag));
+  STATIC_ASSERT(kSmiTagSize == 1);
+  __ SmiUntag(value, value);
+  if (one_byte_string) {
+    __ strb(value, MemOperand(ip, index, LSR, 1));
+  } else {
+    __ strh(value, MemOperand(ip, index));

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

Please add a comment here as well to indicate that LSR 1 takes care of
smi-untag; and maybe a STATIC_ASSERT to document reliance on this fact.

+  }
+}
+
+
 static MemOperand ExpConstant(int index, Register base) {
   return MemOperand(base, index * kDoubleSize);
 }
 
 
 void MathExpGenerator::EmitMathExp(MacroAssembler* masm,
                                    DoubleRegister input,
                                    DoubleRegister result,
                                    DoubleRegister double_scratch1,
                                    DoubleRegister double_scratch2,
@@ skipping 119 matching lines @@
     patcher.masm()->add(r0, pc, Operand(-8));
     patcher.masm()->ldr(pc, MemOperand(pc, -4));
     patcher.masm()->dd(reinterpret_cast<uint32_t>(stub->instruction_start()));
   }
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_ARM