Hi Christoph,

While we wait for an expert's answer, I allow myself to add some observations

from my own investigations of the runtime system (hopefully correct):

* OCaml uses very different calling convention than the standard C convention.

When calling C code (for example, when the GC is triggered) we need to pass the content of

the registers as used by OCaml (this includes things like the allocation pointer, exception

pointer, etc) to the C code so that they can be adjusted as necessary.  Since these

registers need to be handled according the C calling convention, we need to save their values somewhere.

The simplest way to do this is to pass them to the C code using global variables and restore those

globals into the registers when returning from the C code back into OCaml code.

As far as I know the global variables are only updated when passing the OCaml-C boundary.

* Having said that one could bundle all the globals in one record so as to allow multiple

OCaml processes (with separate heaps) running on the same process.  I think I even remember

seeing some experiments in this direction ...

(Corrections welcome!)

Thanks!

Cheers,

Nicolas

On Fri, Jan 6, 2017 at 9:10 AM, Christoph Höger <christoph.hoeger@tu-berlin.de> wrote:

Dear all,

after investigating the interaction of native code and the runtime
environment (in particular the GC), I am puzzled about the static
storage of some data (e.g. the young_pointer, global roots etc):

* if I am not mistaken, each function obtains the young pointer in a
register (%rax on x86)

* the same value is stored globally in a variable allocated by the
executable

* several other variables are allocated that way

I wonder why this is necessary. If the generated code uses one register
anyway, why not put a pointer to the necessary global data structures in
there as well? (say, in the first element of the minor heap).

I am probably missing something here, but at first glance this strategy
prevents concurrent ocaml execution in one process and at the same time
it seems to be fixable, right?

thanks for any comments,

Christoph
--
Christoph Höger

Technische Universität Berlin
Fakultät IV - Elektrotechnik und Informatik
Übersetzerbau und Programmiersprachen

Sekr. TEL12-2, Ernst-Reuter-Platz 7, 10587 Berlin

Tel.: +49 (30) 314-24890
E-Mail: christoph.hoeger@tu-berlin.de