Speed up charCodeAt on very large cons strings, by insisting on...

src/x64/codegen-x64.cc

Index: src/x64/codegen-x64.cc
===================================================================
--- src/x64/codegen-x64.cc	(revision 3315)
+++ src/x64/codegen-x64.cc	(working copy)
@@ -3778,6 +3778,12 @@
   __ j(not_equal, &slow_case);
 
   // ConsString.
+  // Check that the right hand side is the empty string (ie if this is really a
+  // flat string in a cons string).  If that is not the case we would rather go
+  // to the runtime system now, to flatten the string.
+  __ movq(temp.reg(), FieldOperand(object.reg(), ConsString::kSecondOffset));
+  __ CompareRoot(temp.reg(), Heap::kEmptyStringRootIndex);

[Lasse Reichstein, 2009/11/17 10:08:20]

Is a memory read really more efficient than an extra opcode that loads the value
as a constant?

+  __ j(not_equal, &slow_case);
   // Get the first of the two strings.
   __ movq(object.reg(), FieldOperand(object.reg(), ConsString::kFirstOffset));
   __ jmp(&try_again_with_new_string);