In MacroAssembler::JumpIfNotBothSequentialAsciiStrings a custom mask

src/ia32/macro-assembler-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 2780 matching lines @@
 
   // Load instance type for both strings.
   mov(scratch1, FieldOperand(object1, HeapObject::kMapOffset));
   mov(scratch2, FieldOperand(object2, HeapObject::kMapOffset));
   movzx_b(scratch1, FieldOperand(scratch1, Map::kInstanceTypeOffset));
   movzx_b(scratch2, FieldOperand(scratch2, Map::kInstanceTypeOffset));
 
   // Check that both are flat ASCII strings.
   const int kFlatAsciiStringMask =
       kIsNotStringMask | kStringRepresentationMask | kStringEncodingMask;
-  const int kFlatAsciiStringTag = ASCII_STRING_TYPE;
+  // Use ASCII_INTERNALIZED_STRING_TYPE because it's internalized bit is zero.
+  // Since we don't include that bit in the mask above, the test below will
+  // succeed for ASCII_STRING_TYPE and ASCII_INTERNALIZED_STRING_TYPE.
+  STATIC_ASSERT(kInternalizedTag == 0);
+  const int kFlatAsciiStringTag = ASCII_INTERNALIZED_STRING_TYPE;
   // Interleave bits from both instance types and compare them in one check.
   ASSERT_EQ(0, kFlatAsciiStringMask & (kFlatAsciiStringMask << 3));
   and_(scratch1, kFlatAsciiStringMask);
   and_(scratch2, kFlatAsciiStringMask);
   lea(scratch1, Operand(scratch1, scratch2, times_8, 0));
   cmp(scratch1, kFlatAsciiStringTag | (kFlatAsciiStringTag << 3));
   j(not_equal, failure);
 }
 
 
@@ skipping 372 matching lines @@
   j(greater, &no_memento_available);
   cmp(MemOperand(scratch_reg, -AllocationMemento::kSize),
       Immediate(Handle<Map>(isolate()->heap()->allocation_memento_map())));
   bind(&no_memento_available);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32