Simplify StringCharCodeAt in non-crankshaft codegen.

src/x64/lithium-codegen-x64.cc

 // Copyright 2011 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 3362 matching lines @@
                  Heap::kEmptyStringRootIndex);
   __ j(not_equal, deferred->entry());
   __ movq(string, FieldOperand(string, ConsString::kFirstOffset));
 
   __ bind(&indirect_string_loaded);
   __ movq(result, FieldOperand(string, HeapObject::kMapOffset));
   __ movzxbl(result, FieldOperand(result, Map::kInstanceTypeOffset));
 
   // Check whether the string is sequential. The only non-sequential
   // shapes we support have just been unwrapped above.
+  // Note that if the original string is a cons or slice with an external
+  // string as underlying string, we pass that unpacked underlying string with
+  // the updated index to the runtime function.
   __ bind(&check_sequential);
   STATIC_ASSERT(kSeqStringTag == 0);
   __ testb(result, Immediate(kStringRepresentationMask));
   __ j(not_zero, deferred->entry());
 
   // Dispatch on the encoding: ASCII or two-byte.
   Label ascii_string;
   STATIC_ASSERT((kStringEncodingMask & kAsciiStringTag) != 0);
   STATIC_ASSERT((kStringEncodingMask & kTwoByteStringTag) == 0);
   __ testb(result, Immediate(kStringEncodingMask));
@@ skipping 900 matching lines @@
   RegisterEnvironmentForDeoptimization(environment);
   ASSERT(osr_pc_offset_ == -1);
   osr_pc_offset_ = masm()->pc_offset();
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64