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

src/x64/codegen-x64.cc

 // Copyright 2009 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 3760 matching lines @@
   __ Integer32ToSmi(temp.reg(), temp.reg());
   __ jmp(&end);
 
   // Handle non-flat strings.
   __ bind(&not_a_flat_string);
   __ and_(temp.reg(), Immediate(kStringRepresentationMask));
   __ cmpb(temp.reg(), Immediate(kConsStringTag));
   __ 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);
 
   __ bind(&slow_case);
   // Move the undefined value into the result register, which will
   // trigger the slow case.
   __ LoadRoot(temp.reg(), Heap::kUndefinedValueRootIndex);
 
   __ bind(&end);
@@ skipping 4046 matching lines @@
   masm.GetCode(&desc);
   // Call the function from C++.
   return FUNCTION_CAST<ModuloFunction>(buffer);
 }
 
 #endif
 
 #undef __
 
 } }  // namespace v8::internal