In MacroAssembler::JumpIfNotBothSequentialAsciiStrings a custom mask

src/x64/macro-assembler-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 2235 matching lines @@
   // Load instance type for both strings.
   movq(scratch1, FieldOperand(first_object, HeapObject::kMapOffset));
   movq(scratch2, FieldOperand(second_object, HeapObject::kMapOffset));
   movzxbl(scratch1, FieldOperand(scratch1, Map::kInstanceTypeOffset));
   movzxbl(scratch2, FieldOperand(scratch2, Map::kInstanceTypeOffset));
 
   // Check that both are flat ASCII strings.
   ASSERT(kNotStringTag != 0);
   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;
 
   andl(scratch1, Immediate(kFlatAsciiStringMask));
   andl(scratch2, Immediate(kFlatAsciiStringMask));
   // Interleave the bits to check both scratch1 and scratch2 in one test.
   ASSERT_EQ(0, kFlatAsciiStringMask & (kFlatAsciiStringMask << 3));
   lea(scratch1, Operand(scratch1, scratch2, times_8, 0));
   cmpl(scratch1,
        Immediate(kFlatAsciiStringTag + (kFlatAsciiStringTag << 3)));
   j(not_equal, on_fail, near_jump);
 }
 
 
 void MacroAssembler::JumpIfInstanceTypeIsNotSequentialAscii(
     Register instance_type,
     Register scratch,
     Label* failure,
     Label::Distance near_jump) {
   if (!scratch.is(instance_type)) {
     movl(scratch, instance_type);
   }
 
   const int kFlatAsciiStringMask =
       kIsNotStringMask | kStringRepresentationMask | kStringEncodingMask;
 
   andl(scratch, Immediate(kFlatAsciiStringMask));
   cmpl(scratch, Immediate(kStringTag | kSeqStringTag | kOneByteStringTag));

[Yang, 2013/07/22 09:09:46]

I like it this way. Less error prone apparently.

[mvstanton, 2013/07/22 09:40:17]

Indeed, way better. thx.

   j(not_equal, failure, near_jump);
 }
 
 
 void MacroAssembler::JumpIfBothInstanceTypesAreNotSequentialAscii(
     Register first_object_instance_type,
     Register second_object_instance_type,
     Register scratch1,
     Register scratch2,
     Label* on_fail,
@@ skipping 2398 matching lines @@
   j(greater, &no_memento_available);
   CompareRoot(MemOperand(scratch_reg, -AllocationMemento::kSize),
               Heap::kAllocationMementoMapRootIndex);
   bind(&no_memento_available);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64