Re: [PATCH] Tame path_range_query::compute_imports

[Aldy Hernandez <aldyh@redhat.com>]

On Tue, Aug 16, 2022 at 1:38 PM Richard Biener <rguenther@suse.de> wrote:
>
> On Tue, 16 Aug 2022, Aldy Hernandez wrote:
>
> > On Mon, Aug 15, 2022 at 11:53 AM Richard Biener <rguenther@suse.de> wrote:
> > >
> > > The remaining issue I have with the path_range_query is that
> > > we re-use the same instance in the back threader but the
> > > class doesn't provide any way to "restart", aka give m_path
> > > a lifetime.  The "start a new path" API seems to essentially
> > > be compute_ranges (), but there's no convenient way to end.
> > > It might be more appropriate to re-instantiate the path_range_query,
> > > though that comes at a cost.  Or abstract an actual query, like
> > > adding a
> >
> > Yes, compute_ranges() is the way to start a new path.  It resets exit
> > dependencies, the path, relations, etc.  I think it would be clearer
> > to name it set_path (or reset_path if we want to share nomenclature
> > with the path_oracle).
> >
> > Instantiating a new path_range_query per path is fine, as long as you
> > allocate the ranger it uses yourself, instead of letting
> > path_range_query allocate it.  Instantiating a new ranger does have a
> > cost, and it's best to let path_range_query re-use a ranger from path
> > to path.  This is why path_range_query is (class) global in the
> > backwards threader.  Andrew mentioned last year making the ranger
> > start-up 0-cost, but it still leaves the internal caching the ranger
> > will do from path to path (well, the stuff outside the current path,
> > cause the stuff inside the path is irrelevant since it'll get
> > recalculated).
> >
> > However, why can't you use compute_ranges (or whatever we rename it to ;-))??
>
> I've added
>
>    auto_bb_flag m_on_path;
>
> to the path query and at set_path time set m_on_path on each BB so
> the m_path->contains () linear walks go away.  But I need to clear
> the flag for which I would need something like finish_path (),
> doing it just at the point we deallocate the path query object
> or when we set the next path via compute_ranges doesn't look right
> (and in fact it doesn't work out-of-the-box without adjusting the
> lifetime of the path query object).
>
> So a more incremental thing would be to add such finish_path ()
> or to make the whole path query object single-shot, thus remove
> compute_ranges and instead use the CTOR for this.
>
> Probably not too important (for short paths).

On a high level, I wonder if this matters since we don't allow long
paths for other performance reasons you've already tackled.  But OTOH,
I've always been a little uncomfortable with contains_p linear search,
so if you think this makes a difference, go right ahead :).

I'm fine with either the finish_path() or the single-shot thing you
speak of.  Although making path query inmutable makes things cleaner
in the long run.  I like it!  My guess is that the non-ranger
instantiation penalty would be minimal.  I'd even remove the default
(auto-allocated) ranger from path_range_query, to make it obvious that
you need to manage that yourself and avoid folks shooting themselves
in the foot.

Wanna have a go at it?  If you'd rather not, I can work on it.

Aldy