Improve array to string conversion.

src/arm/codegen-arm.cc

Index: src/arm/codegen-arm.cc
diff --git a/src/arm/codegen-arm.cc b/src/arm/codegen-arm.cc
index af2f21623160a9aa192c142115547dd1e8d45c0b..81e23e334b42a5777d71db010a3deb1621b670ef 100644
--- a/src/arm/codegen-arm.cc
+++ b/src/arm/codegen-arm.cc
@@ -521,6 +521,48 @@ void StringCharLoadGenerator::Generate(MacroAssembler* masm,
 }
 
 
+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);
 }