Re: scratch/igc: Implications of MPS being asynchronous

[Stefan Kangas]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Stefan Kangas <stefankangas@gmail.com>
>> Date: Sun, 12 Jan 2025 10:17:30 +0000
>> Cc: pipcet@protonmail.com, gerd@gnu.org
>>
>> In Bug#75322, Pip Cet writes:
>>
>> > The MPS-based GC in scratch/igc does not allow or require application
>> > C code to make "no GC here" assumptions.
>>
>> This is stated quite unambiguously also in the MPS guide:
>>
>>      "The MPS is asynchronous: this means that it might be scanning,
>>      moving, or collecting, at any point in time (potentially, between
>>      any pair of instructions in your program)."  (Chapter 2)
>>
>> This makes it clear what it means not to "allow" C code to make "no GC
>> here" assumptions.  But I don't understand what it means that it is not
>> "required" to make such assumptions.  Could someone clarify that part
>> please?
>
> The above text is misleading: "asynchronous" doesn't seem to be
> relevant to the part of MPS we are using, since its GC runs on our
> threads, not on a separate thread.

My understanding is that this applies also when we run MPS on the same
thread.  Is that wrong?

> And saying that "no GC here" assumptions are always false is too
> radical to my palate, and AFAIU is at least not accurate.

I'm not sure what "radical" means in this context.  If it is true, we
should write it, and if not, we shouldn't.