Make smi.toString() do its work faster using native string helpers.

runtime/lib/integers.cc

Index: runtime/lib/integers.cc
diff --git a/runtime/lib/integers.cc b/runtime/lib/integers.cc
index 84dc9d6205fa1c82c40e82ebc78b3aff4ceb2af8..3bdd21c62b2fd529561623b25169e367f93c9dd1 100644
--- a/runtime/lib/integers.cc
+++ b/runtime/lib/integers.cc
@@ -344,6 +344,37 @@ DEFINE_NATIVE_ENTRY(Smi_bitLength, 1) {
   return Smi::New(result);
 }
 
+DEFINE_NATIVE_ENTRY(Smi_toString, 1) {
+  const Smi& operand = Smi::CheckedHandle(arguments->NativeArgAt(0));
+  if (FLAG_trace_intrinsified_natives) {
+    OS::Print("Smi_toString: %s\n", operand.ToCString());
+  }
+  // Maximal number of digits of a 63 bit number is 19. Add one for minus sign.

[Lasse Reichstein Nielsen, 2013/09/23 11:25:18]

I decided against using OS::SNPrint(characters, 0, "%ld", signed_value) because
I assume it will have some kind of overhead in parsing the format.
We should try that too, and see what's faster.

And I should make some timings of this change as well.

+  const size_t kMaxLength = 20;
+  uint8_t characters[kMaxLength];
+  uint8_t* end = characters + kMaxLength;
+  uint8_t* cursor = end;
+  bool negative = false;
+  int64_t signed_value = operand.AsInt64Value();
+  uint64_t value;
+  if (signed_value < 0) {
+    negative = true;
+    value = -signed_value;
+  } else {
+    value = signed_value;
+  }
+  do {
+    uint64_t digit = value % 10;
+    value = (value - digit) / 10;
+    *--cursor = '0' + digit;
+  } while (value);
+  if (negative) {
+    *--cursor = '-';
+  }
+  intptr_t length = static_cast<intptr_t>(end - cursor);
+  return OneByteString::New(cursor, length, Heap::kNew);
+}
+
 
 // Mint natives.