[m-rev.] for post-commit review: document .par grades

[Julien Fischer]

On Wed, 20 Aug 2025 at 13:10, Peter Wang <novalazy at gmail.com> wrote:

> I remembered something. On startup, LLDS .par grades will create a bunch
> of threads for Mercury engines (by default, one for each hardware thread),
> in preparation for running Mercury threads created with thread.spawn,
> or to execute parallel conjuctions. Those engine threads will often
> never be used because the program doesn't actually spawn non-native threads
> or contain parallel conjuctions.
>
> It might be possible to defer creating Mercury engine threads until they
> are actually needed, but it would need someone to make that effort.
> I don't think there is any interest now.
>
> So, as an easier option, it *would* make sense to introduce a .mt grade
> component for C grades that (unlike .par) supports multi-threading
> but not non-native threads or parallel conjuction.

So with this proposal, in the grade asm_fast.mt.gc, spawn/3 would act
identically to
spawn_native/3?

Julien.