Re: on coding a portable R6RS package supporting Guile and its FFI

[Ludovic Courtès]

Andy Wingo <address@hidden> skribis:

> On Tue 05 Feb 2013 23:31, address@hidden (Ludovic Courtès) writes:
>
>> This would be less of a problem if we had “immediate pointers” [0].
>> Would the retagging in 2.1 allow this?
>
> The tc3 itag allocation in wip-retagging is:
>
>   * tc3:
>   *   000:  A heap object with a tag word on the heap
>   *   001:  Some other immediate
>   *   010:  A pair
>   *   011:  Small integer (odd)
>   *   100:  (Unallocated tc3.)
>   *   101:  (Unallocated tc3.)
>   *   110:  A struct
>   *   111:  Small integer (even)
>
> And SCM_IMP is:
>
>   #define SCM_IMP(x)          (1 & SCM_UNPACK (x))
>
> So perhaps 101 could be an immediate pointer.  Dunno.  Pretty nasty
> stuff!

Would be cool, I think.  Maybe we should make an auction for unused tc3
tags?  :-)

>> (At FOSDEM, Luca Saiu rightfully noted that we could use 4-bit tags
>> instead of 3-bit tags on 64-bit arches, which would give us the needed
>> room here.)
>
> GC_malloc still returns eight-byte aligned memory hunks on 64-bit, no?

Oh right:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (use-modules (system foreign))
scheme@(guile-user)> (pointer->procedure '* (dynamic-func "GC_malloc" 
(dynamic-link)) (list size_t))
$2 = #<procedure 1b60ac0 (_)>
scheme@(guile-user)> ,use (srfi srfi-1)
scheme@(guile-user)> ,use (srfi srfi-26)
scheme@(guile-user)> (unfold (cut > <> 100) (lambda (i) (logand 
(pointer-address ($2 (* i 2)) ) #x15)) 1+ 0)
$7 = (0 0 16 0 16 0 16 0 16 0 0 0 0 0 0 0 0 16 0 0 16 0 0 16 0 0 0 0 0 0 0 0 0 
16 0 16 0 16 0 16 0 0 0 0 0 0 0 0 0 0 16 0 16 0 16 0 16 0 0 0 0 0 0 0 0 16 0 16 
0 16 0 16 0 0 0 0 0 0 0 0 0 16 0 16 0 16 0 16 0 0 0 0 0 0 0 0 0 16 0 16 0)
--8<---------------cut here---------------end--------------->8---

If needed, we could just round up all the addresses returned.

And with 16 cases, we could even put single-precision floats in a word,
chars, and who knows what!

The downside is that we’d have to maintain two tag systems.

Ludo’.