In MacroAssembler::JumpIfNotBothSequentialAsciiStrings a custom mask

src/ia32/full-codegen-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 4027 matching lines @@
   // string_length: Sum of string lengths, as a smi.
   // elements: FixedArray of strings.
 
   // Check that the separator is a flat ASCII string.
   __ mov(string, separator_operand);
   __ JumpIfSmi(string, &bailout);
   __ mov(scratch, FieldOperand(string, HeapObject::kMapOffset));
   __ movzx_b(scratch, FieldOperand(scratch, Map::kInstanceTypeOffset));
   __ and_(scratch, Immediate(
       kIsNotStringMask | kStringEncodingMask | kStringRepresentationMask));
-  __ cmp(scratch, ASCII_STRING_TYPE);
+  __ cmp(scratch, kStringTag | kOneByteStringTag | kSeqStringTag);
   __ j(not_equal, &bailout);
 
   // Add (separator length times array_length) - separator length
   // to string_length.
   __ mov(scratch, separator_operand);
   __ mov(scratch, FieldOperand(scratch, SeqOneByteString::kLengthOffset));
   __ sub(string_length, scratch);  // May be negative, temporarily.
   __ imul(scratch, array_length_operand);
   __ j(overflow, &bailout);
   __ add(string_length, scratch);
@@ skipping 857 matching lines @@
   *stack_depth = 0;
   *context_length = 0;
   return previous_;
 }
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32