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Re: Tagging
by Jason Johnson-5
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Thanks Bryce and Andrew for your quick and informative responses. Comments/questions below.
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On 8/4/07, bryce@... <bryce@...> wrote:
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.
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.
I see, I didn't realize the bottom 2 bits had to be zero. That answers a lot of my questions. :)
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?
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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