Improve array to string conversion.

src/ia32/codegen-ia32.cc

Index: src/ia32/codegen-ia32.cc
diff --git a/src/ia32/codegen-ia32.cc b/src/ia32/codegen-ia32.cc
index 2f5553c475f0560af1b2df8b902b847e714a7b2e..9ada4a5db9751ffac627d79d5c3605ddc8426368 100644
--- a/src/ia32/codegen-ia32.cc
+++ b/src/ia32/codegen-ia32.cc
@@ -793,6 +793,49 @@ void StringCharLoadGenerator::Generate(MacroAssembler* masm,
 }
 
 
+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));
 }