Re: For a cheaper ‘bytevector->pointer’

[Andy Wingo <wingo@igalia.com>]

Hi :)

On Mon 25 Nov 2019 23:03, Ludovic Courtès <ludo@gnu.org> writes:

> Andy Wingo <wingo@igalia.com> skribis:
>
>> Honestly I would prefer not to do this.  If I understand correctly, the
>> problem is in FFI calls -- you have a bytevector and you want to pass it
>> as a pointer.  In that case the "right" optimization is to avoid the
>> scm_tc7_pointer altogether and instead having an unboxed raw pointer.
>> The idioms used in FFI are local enough that a compiler can do this.
>
> I agree!  I have a patch from the 2.0 era (attached), but it doesn’t
> work because all the tc3s are already taken.  I don’t think this has
> changed but I could well be missing something about the tag space.
> WDYT?

I was actually thinking about raw pointer values -- i.e. not
immediate-tagged values.  If you think about it these values are
generally live only between the bytevector->pointer and the FFI call --
the compiler is capable of safely unboxing values in spaces like that.
But this would work better with a more compiler-focussed FFI than with
the current "interpreted" FFI.

But, immediate pointers would be nice too; nicer, in some ways.  See
also Mark's fixrat work.

>> In the short term, what about allowing bytevectors as arguments
>> whereever a pointer is allowed?  Perhaps it's bad to expand the domain
>> of these functions but it may be the right trade-off.
>
> So in practice, every time there’s '* in the FFI, it’d accept a
> bytevector, right?

That was the idea :)

> I would prefer immediate pointers if that’s possible, and then one of
> the two other solutions.

In that case I am not sure what a good solution is.  Having to add an
additional 2-word internal displacement is a bit unfortunate, if that's
the case!

Andy