Re: [Sbcl-devel] How To have my own integer types?

[Stas B. <sta...@gm...>]

My hint: don't do that.

On Thu, Jun 6, 2024 at 2:46 PM Philipp Marek via Sbcl-devel <
sbc...@li...> wrote:

> Hi everybody,
>
> in one of my projects I have a large number of small objects;
> for testing purposes, and to reduce memory usage and GC time
> I wrote a (simple) DEF-COLUMN-STRUCT that (pre)allocates an
> array for each slot in the structure, and provides the getter
> & setter functions (as well a GF that allows to reset the storage)
>
> There's also a "(typedef ,struct-name () `(unsigned-byte 27))",
> so that other structures, classes, and arrays can use "struct-name"
> for specialization..
>
>
> Most of that worked out quite nicely.
>
> + only 3 (specialized) arrays instead of millions of objects
> + preallocation (for approx. known number of entries) is easy
> + "allocation" via an atomic-incf is quick
> - no GC support (not needed here)
> ~ identification of "object" happens via the array index
> - no support for type dispatch
>
>
> The last two points is what I'd like to improve now --
> by having (a few) own number types that are distinct from FIXNUM
> etc., so that
>
>    (etypecase input
>      (fixnum ...)
>      (struct-name ...)
>
> and
>
>    (defmethod print-object ((obj struct-name) ...) ...)
>
> work as "expected".
>
>
> Constraints/wishes:
> - CHARACTER is not big enough -- there are already ~50M structure
>    instances right now, and that'll get bigger over time.
>    character is limited to ~1M.
> - If 4 tag bits are taken up, but the "ID" (index) of the object
>    could still be 28 bits long, I hope that references via other
>    vectors would resolve to a 32bit element type -- that would save
>    some memory again.
>
>
> I read doc/internals/objects-in-memory.texinfo about
> sb-vm:n-fixnum-tag-bits, and looked at the referenced
> src/compiler/generic/early-objdef.lisp.
>
> The latter lists a few OTHER-IMMEDIATE-x-LOWTAG and PADx-LOWTAG
> that seem to be available for this purpose.
>
>
> So, my questions are:
>
> - Is it necessary/helpful to increase the n-fixnum-tag-bits via
>    src/compiler/generic/early-vm.lisp for this purpose, or are lowtags
>    already available?
>
> - Is there already some (hidden, reserved, internal) infrastructure
>    that allows to define a new (semi-numeric) type that is different
>    from FIXNUM etc.?
>
> - If now, is there some code that could be moved into a contrib?
>    Yeah, I'm aware that this would be a highly restricted usecase --
>    only one user per image to avoid conflicting tag bits and so on.
>
>
> Thanks for any hints!
>
>
> _______________________________________________
> Sbcl-devel mailing list
> Sbc...@li...
> https://lists.sourceforge.net/lists/listinfo/sbcl-devel
>