Re: [Caml-list] How is the frametable handled when throwing exceptions?

["Nicolás Ojeda Bär" <nicolas.ojeda.bar@lexifi.com>]

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Hi Christoph,

No, the "frametable" is not a single static, global object (it would be
hard to make this work with separate compilation, etc).
Rather each function (both ML and C) is associated to a "frame descriptor"
detailing where to find live values (for ML functions, the frame descriptor
is associated to its function using the return address as a key).  When the
GC is triggered it traverses the stack (both ML and C) and collects the
full list of GC roots.  Of course there is also a list of "global" roots
which *are* registered in a single, global table.  For these roots indeed
one must deregister them explicitly (see caml_{register,remove}_global_root
and its "generational" friends).

Some pointers:

You can read the stack-walking code in

  https://github.com/ocaml/ocaml/blob/trunk/asmrun/roots.c#L438

When the GC is invoked when running ML code, this code depends on some
global pointers which are set up in the asm
glue, e.g.

  https://github.com/ocaml/ocaml/blob/trunk/asmrun/amd64.S

Finally frame descriptors are emitted in the native-code backends, e.g.:

  https://github.com/ocaml/ocaml/blob/trunk/asmcomp/amd64/emit.mlp#L241

Hope this helps,
Nicolas


On Sun, Feb 12, 2017 at 2:25 PM, Christoph Höger <
christoph.hoeger@celeraone.com> wrote:

> Nicolás,
>
> that is my understanding, too. But what I am missing is the
> _de_-registration of the roots. AFAIK, the frametable is a static, global
> object and when the stack is cut down, it then contains stale pointers. So
> either the frametable is useless (as a global object), because the GC
> traverses the call stack or there is some kind of deregstration going on
> somewhere.
>
> On Sat, Feb 11, 2017 at 3:10 PM, Nicolás Ojeda Bär <
> nicolas.ojeda.bar@lexifi.com> wrote:
>
>> Hi Christoph,
>>
>> Someone who knows more will be able to get a better answer, but I don't
>> think there is much of an issue here:
>> when using the C API to register local roots (CAMLparam, CAMLlocal, etc),
>> these roots are in effect
>> stored in the C stack.  When raising an exception the stack is _cut down_
>> and all those roots
>> are promptly forgotten (as they should be).
>>
>> Cheers,
>> Nicolas
>>
>>
>> On Sat, Feb 11, 2017 at 3:02 PM, Christoph Höger <
>> christoph.hoeger@celeraone.com> wrote:
>>
>>> Dear all,
>>>
>>> while studying camls native code generation and garbage collection
>>> strategy, I came upon Connor Benner's bachelor thesis from 2012, where he
>>> implemented a llvm backend for ocamlopt. One intriguing remark mentioned
>>> OCamls exception mechanism as basically consisting a pointer to the stack
>>> frame of the last exception handler in a special register (r14, when I
>>> recall correctly). Throwing an exception that becomes a mov/pop/ret
>>> operation. This however, seems to interfere with garbage collection: From
>>> the C-API, it seems that all local roots are stored in the frametable via
>>> some special macros (e.g. CAMLParam, CAMLlocal).
>>>
>>> When control just returns from a stack frame, how are the entries
>>> removed from the frametable?
>>>
>>> I would be glad, if someone could answer this or point me to the
>>> relevant documentation (if any exists).
>>>
>>> thanks,
>>>
>>> Christoph
>>>
>>
>>
>

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