Reverting r12937.

src/ia32/code-stubs-ia32.cc

 // Copyright 2012 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 5730 matching lines @@
   __ mov(eax, ecx);
   __ IncrementCounter(counters->string_add_native(), 1);
   __ ret(2 * kPointerSize);
   __ bind(&non_ascii);
   // At least one of the strings is two-byte. Check whether it happens
   // to contain only ASCII characters.
   // ecx: first instance type AND second instance type.
   // edi: second instance type.
   __ test(ecx, Immediate(kAsciiDataHintMask));
   __ j(not_zero, &ascii_data);
+  __ mov(ecx, FieldOperand(eax, HeapObject::kMapOffset));
+  __ movzx_b(ecx, FieldOperand(ecx, Map::kInstanceTypeOffset));
+  __ xor_(edi, ecx);
+  STATIC_ASSERT(kOneByteStringTag != 0 && kAsciiDataHintTag != 0);
+  __ and_(edi, kOneByteStringTag | kAsciiDataHintTag);
+  __ cmp(edi, kOneByteStringTag | kAsciiDataHintTag);
+  __ j(equal, &ascii_data);
   // Allocate a two byte cons string.
   __ AllocateTwoByteConsString(ecx, edi, no_reg, &call_runtime);
   __ jmp(&allocated);
 
   // We cannot encounter sliced strings or cons strings here since:
   STATIC_ASSERT(SlicedString::kMinLength >= ConsString::kMinLength);
   // Handle creating a flat result from either external or sequential strings.
   // Locate the first characters' locations.
   // eax: first string
   // ebx: length of resulting flat string as a smi
@@ skipping 1759 matching lines @@
   // Restore ecx.
   __ pop(ecx);
   __ ret(0);
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32