Introduce read buffer for external strings when using charAt (ia32).

src/x64/macro-assembler-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 4203 matching lines @@
   // If it's external, the length is just ExternalString::kSize.
   // Otherwise it's String::kHeaderSize + string->length() * (1 or 2).
   Label not_external;
   // External strings are the only ones with the kExternalStringTag bit
   // set.
   ASSERT_EQ(0, kSeqStringTag & kExternalStringTag);
   ASSERT_EQ(0, kConsStringTag & kExternalStringTag);
   testb(instance_type, Immediate(kExternalStringTag));
   j(zero, &not_external, Label::kNear);
   movq(length, Immediate(ExternalString::kSize));
+  testb(instance_type, Immediate(kBufferedStringTag));
+  j(zero, &is_data_object, Label::kNear);
+  movq(length, Immediate(ExternalString::kExtendedSize));
   jmp(&is_data_object, Label::kNear);
 
   bind(&not_external);
   // Sequential string, either ASCII or UC16.
   ASSERT(kAsciiStringTag == 0x04);
   and_(length, Immediate(kStringEncodingMask));
   xor_(length, Immediate(kStringEncodingMask));
   addq(length, Immediate(0x04));
   // Value now either 4 (if ASCII) or 8 (if UC16), i.e. char-size shifted by 2.
   imul(length, FieldOperand(value, String::kLengthOffset));
   shr(length, Immediate(2 + kSmiTagSize + kSmiShiftSize));
   addq(length, Immediate(SeqString::kHeaderSize + kObjectAlignmentMask));
   and_(length, Immediate(~kObjectAlignmentMask));
 
   bind(&is_data_object);
   // Value is a data object, and it is white.  Mark it black.  Since we know
   // that the object is white we can make it black by flipping one bit.
   or_(Operand(bitmap_scratch, MemoryChunk::kHeaderSize), mask_scratch);
 
   and_(bitmap_scratch, Immediate(~Page::kPageAlignmentMask));
   addl(Operand(bitmap_scratch, MemoryChunk::kLiveBytesOffset), length);
 
   bind(&done);
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64