Re: [PATCH] Refactor back_threader_profitability

[Aldy Hernandez <aldyh@redhat.com>]

I just have a few high level comments.

On Tue, Aug 16, 2022 at 4:05 PM Richard Biener <rguenther@suse.de> wrote:
>
> The following refactors profitable_path_p in the backward threader,
> splitting out parts that can be computed once the exit block is known,
> parts that contiguously update and that can be checked allowing
> for the path to be later identified as FSM with larger limits,
> possibly_profitable_path_p, and final checks done when the whole
> path is known, profitable_path_p.

I thought we were removing references to FSM, as they were leftovers
from some previous incarnation.  For that matter, I don't think I ever
understood what they are, so if we're gonna keep them, could you
comment what makes FSM threads different from other threads?

In your possibly_profitable_path_p function, could you document a bit
better what's the difference between profitable_path_p and
possibly_profitable_path_p?

>
> I've removed the back_threader_profitability instance from the
> back_threader class and instead instantiate it once per path
> discovery.  I've kept the size compute non-incremental to simplify
> the patch and not worry about unwinding.
>
> There's key changes to previous behavior - namely we apply
> the param_max_jump_thread_duplication_stmts early only when
> we know the path cannot become an FSM one (multiway + thread through
> latch) but make sure to elide the path query when we we didn't
> yet discover that but are over this limit.  Similarly the
> speed limit is now used even when we did not yet discover a
> hot BB on the path.  Basically the idea is to only stop path
> discovery when we know the path will never become profitable
> but avoid the expensive path range query when we know it's
> currently not.
>
> I've done a few cleanups, merging functions, on the way.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu.
>
> Statistics show an overall slight increase in threading but
> looking at different files theres noise up and down.  That's
> somewhat expected since we now are applying the "more correct"
> limits in the end.  Unless I made big mistakes of course.
>
> The next thing cost-wise would be to raise the backwards
> threading limit to the limit of DOM so we don't get
> artificial high counts for that.

The DOM threader has limits?  I thought most of those limits were just
due to the fact that it couldn't determine long enough paths?  Either
way, I like that we're merging the necessary forward threader bits
here, in preparation for its demise ;-).

Looks good.

Thanks.
Aldy