Tagging

[Jason Johnson]

Thanks Bryce and Andrew for your quick and informative responses.
Comments/questions below.

On 8/4/07, bryce at kampjes.demon.co.uk <bryce at kampjes.demon.co.uk> wrote:
>
>
> You don't need to shift the numbers for addition and subtraction, just
> make sure the bottom bits are 0. Even with the tag being 1, you
> only need to detag one argument and if that's a constant the
> detagging can be done at compile time.
>
> With multiplication and division only one argument needs to be
> shifted.



Interesting.  Well, I assume that in Smalltalk object access is much more
often then integer access, but does anyone have any data that proves this?
If it is the case then I guess that's the death of my MSB tagging idea. :)

Doesn't Strongtalk use 0 tagged SmallIntegers?  And if so, why?  I looked in
the source code a bit but I haven't figured out yet where that would be.


> It was used by some Lisp systems in the mid 80's. I think CMU Common
> Lisp from memory.
>
> There are two disadvantages of using the top bit:
> 1) You need to control the address space to make sure all objects
> live. Controlling the address space to that degree is unlikely to
> be something that can be done portably.
> 2) You loose half your address space.



Well, I suppose I could shift the pointers up one (which would make the
first bit zero) to reclaim the space, but as I believe it must be the case
that object access is much more common then number access, that would be a
bad trade-off.


Alignment. Words are aligned to 4 byte boundaries which is always
> required for performance and required by some CPU architectures.



I see, I didn't realize the bottom 2 bits had to be zero.  That answers a
lot of my questions. :)

There is a large literature on garbage collection. These day's I'd try
> citeseer as well as Google. Scanning PLDI and OOPSLA proceedings from
> the late 70s and 80s will provide plenty of information if you've
> either got ACM membership or access to a university library.
>
> Bryce



I have the "Garbage Collection" book from Jones & Lins, but I will try to
look on those sites as well.  I just need to go to the "ACM digital
library", no?
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