Reverting r12937.

src/x64/code-stubs-x64.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 4747 matching lines @@
   __ 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.
   // rcx: first instance type AND second instance type.
   // r8: first instance type.
   // r9: second instance type.
   __ testb(rcx, Immediate(kAsciiDataHintMask));
   __ j(not_zero, &ascii_data);
+  __ xor_(r8, r9);
+  STATIC_ASSERT(kOneByteStringTag != 0 && kAsciiDataHintTag != 0);
+  __ andb(r8, Immediate(kOneByteStringTag | kAsciiDataHintTag));
+  __ cmpb(r8, Immediate(kOneByteStringTag | kAsciiDataHintTag));
+  __ j(equal, &ascii_data);
   // Allocate a two byte cons string.
   __ AllocateTwoByteConsString(rcx, rdi, 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.
   // rax: first string
   // rbx: length of resulting flat string as smi
@@ skipping 1716 matching lines @@
 #endif
 
   __ Ret();
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64