Hello!

While using (system foreign) in a couple of projects I found myself
doing quite a bit of pointer arithmetic in Scheme:

--8<---------------cut here---------------start------------->8---
(define (foreign-array->list array-pointer element-count)
  (let ((array (pointer->bytevector array-pointer
                                    (* element-count (sizeof '*)))))
    (unfold (cut >= <> element-count)
            (lambda (element)
              (let ((start (* element (sizeof '*))))
                (bytevector->pointer array start)))
            1+
            0)))

(define (pointer+ array-pointer type index)
  (let ((offset (* index (align (sizeof type) (alignof type)))))
    (make-pointer (+ (pointer-address array-pointer) offset))))

(define (foreign-string-array->list array len)
  ;; Return a list of string comprising the LEN strings pointed to by the
  ;; elements of ARRAY, a pointer to an array of pointers.
  (unfold (cut < <> 0)
          (lambda (index)
            (let ((ptr (make-pointer (+ (pointer-address array)
                                        (* index (sizeof '*))))))
             (pointer->string (dereference-pointer ptr))))
          1-
          (- len 1)))
--8<---------------cut here---------------end--------------->8---

(Examples from
<https://gforge.inria.fr/plugins/scmgit/cgi-bin/gitweb.cgi?p=hubble/hubble.git;a=blob;f=modules/simgrid.scm>.)

The problem is that each ‘make-pointer’ call (and ‘dereference-pointer’,
etc.) conses.  This can make conversion to/from C quite inefficient.

In addition, 90% of the C pointers of interest are 8-byte aligned---that’s
on x86_64-linux-gnu, but it surely holds on most platforms, at least for
pointers returned by ‘malloc’.

So, here comes the idea of “immediate pointers”, which would fit in a
word.  A 3-bit tag is used, as for immediate numbers & co; pointers that
aren’t 8-byte aligned are still stored in an scm_tc7_pointer cell.

I experimented with it using ‘scm_tc3_unused’ (== 3).  Alas, that can’t
work, because it makes it impossible to use such an object as the ‘car’
of a pair:

  #define SCM_I_CONSP(x)  (!SCM_IMP (x) && ((1 & SCM_CELL_TYPE (x)) == 0))

I would welcome feedback and help from tag-savvy people.

Thanks,
Ludo’.