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Hi Mats & others,
<br>the idea is actually not mine; I copied it almost verbatim from ParcPlace
Smalltalk (now Cincom VisualWorks). VW has the following combinations:
<p>00 object pointer
<br>01 character
<br>10 forbidden
<br>11 small integer
<p>This has the big advantage that the tag bits have individual semantics:
bit 0 is the "immediate" bit, and bit 1 is the "small integer" bit. So
to check whether something is a SmallInt, you just have to check one bit.
In a similar vein, to check whether an oop is a legal pointer into memory
and can be dereferenced, you also have to check just one bit. Since both
of these operations are fairly common in the VM, they should be as cheap
as possible, and on most architectures multi-bit tests are a bit more expensive
than single-bit tests.
<br>In Squeak, there is the additional complication that the 10 tag bit
combination is used by the GC machinery to detect object headers when walking
through pointer objects. It works by scanning from the end of each pointer
object and writes the 10 tag combination into the last header word first.
When it finds that tag, it knows that it has reached the beginning of the
object. The tag can then be reconstructed from other header information.
<br>What my approach does differently from ParcPlace's is that 01 is just
the tag for immediate non-integer objects. What you put into that 30-bit
space is pretty much your business, but there are some constraints:
<ul>
<li>
It better had to be immutable data...</li>
<li>
In an object table implementation, these objects can not participate in
a become: operation. For Squeak with its direct pointers it does not matter.</li>
</ul>
Since I'm currently focusing on getting SCAN into gear, I would like if
somebody else could take over work on the 2-bit front. The old code is
there to pilfer, and I would certainly engage in discussion about what
should be implemented and what not. Specifically the networking aspects
of Squeak need to be considered, which is radically different from the
situation when I first implemented the stuff. Perhaps the 3D graphics guys
should have a word, too.
<p>Cheers,
<br>Hans-Martin</html>