fancy GC question

[Markus Mottl <mottl@miss.wu-wien.ac.at>]

Hello,

I am not sure whether the following "trick" is really safe, but it seems
to work:

When I allocate an integer array in OCaml, which is always boxed, both
the pointers to and the elements are obviously contiguous in memory.
One could exploit this in C-interfaces under the restriction that the
array is never changed by the OCaml-runtime, e.g.:

  int *ar = (int *) &Field(v_ar, 0);

And then one can read/write directly into the integer array without
having to follow an indirection (an intermediate pointer) by treating
"ar" as a normal C array.

But is this really always safe if only C writes to the array? What about
e.g. heap compactions and other GC-actions? Can other effects mess up the
fact that the pointers map continuously on a contiguous chunk of memory
(of integers)?

If yes, this would, of course, require the traditional use of the
"Field"-macro for every access. Otherwise, one could squeeze out a bit
more performance in some (probably rare) cases.

- Markus Mottl

-- 
Markus Mottl, mottl@miss.wu-wien.ac.at, http://miss.wu-wien.ac.at/~mottl