Re: [PATCH 1/2] ivopts: Revert computation of address cost complexity.

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Dec 16, 2022 at 12:37 PM Dimitrije Milosevic
<Dimitrije.Milosevic@syrmia.com> wrote:
>
>
> Hi Richard,
>
> > The only documentation on complexity I find is
> >
> >   int64_t cost;         /* The runtime cost.  */
> >   unsigned complexity;  /* The estimate of the complexity of the code for
> >                            the computation (in no concrete units --
> >                            complexity field should be larger for more
> >                            complex expressions and addressing modes).  */
> >
> > and complexity is used as tie-breaker only when cost is equal.  Given that
> > shouldn't unsupported addressing modes have higher complexity?  I'll note
> > that there's nothing "unsupported", each "unsupported" address computation
> > is lowered into supported pieces.  "unsupported" maybe means that
> > "cost" isn't fully covered by address-cost and compensation stmts might
> > be costed in quantities not fully compatible with that?
>
> Correct, that's what I was aiming for initially - before f9f69dd that was the case,
> "unsupported" addressing modes had higher complexities.
> Also, that's what I meant by "unsupported" as well, thanks.
>
> > That said, "complexity" seems to only complicate things :/  We do have the
> > tie-breaker on preferring less IVs.  complexity was added in
> > r0-85562-g6e8c65f6621fb0 as part of fixing PR34711.
>
> I agree that the complexity part is just (kind of) out there, not really strongly
> defined. I'm not sure how to feel about merging complexity into the cost part
> of an address cost, though.
>
> > If it's really only about the "complexity" value then each
> > compensation step should
> > add to the complexity?
>
> That could be the way to go. Also worth verifying is that we compensate for
> each case of an unsupported addressing mode.

Yes.  Also given complexity is only a tie-breaker we should cost the
compensation
somehow, but then complexity doesn't look necessary ...

Meh.

>
> Kind regards,
> Dimitrije
>
> From: Richard Biener <richard.guenther@gmail.com>
> Sent: Friday, December 16, 2022 10:58 AM
> To: Dimitrije Milosevic <Dimitrije.Milosevic@Syrmia.com>
> Cc: Jeff Law <jeffreyalaw@gmail.com>; gcc-patches@gcc.gnu.org <gcc-patches@gcc.gnu.org>; Djordje Todorovic <Djordje.Todorovic@syrmia.com>
> Subject: Re: [PATCH 1/2] ivopts: Revert computation of address cost complexity.
>
> On Thu, Dec 15, 2022 at 4:26 PM Dimitrije Milosevic
> <Dimitrije.Milosevic@syrmia.com> wrote:
> >
> > Hi Richard,
> >
> > Sorry for the delayed response, I couldn't find the time to fully focus on this topic.
> >
> > > I'm not sure this is accurate but at least the cost of using an unsupported
> > > addressing mode should be at least that of the compensating code to
> > > mangle it to a supported form.
> >
> > I'm pretty sure IVOPTS does not filter out candidates which aren't supported by
> > the target architecture. It does, however, adjust the cost for a subset of those.
> > The adjustment code modifies only the cost part of the address cost (which
> > consists of a cost and a complexity).
> > Having said this, I'd propose two approaches:
> >     1. Cover all cases of unsupported addressing modes (if needed, I'm not entirely
> >         sure they aren't already covered), leaving complexity for unsupported
> >         addressing modes zero.
>
> The only documentation on complexity I find is
>
>   int64_t cost;         /* The runtime cost.  */
>   unsigned complexity;  /* The estimate of the complexity of the code for
>                            the computation (in no concrete units --
>                            complexity field should be larger for more
>                            complex expressions and addressing modes).  */
>
> and complexity is used as tie-breaker only when cost is equal.  Given that
> shouldn't unsupported addressing modes have higher complexity?  I'll note
> that there's nothing "unsupported", each "unsupported" address computation
> is lowered into supported pieces.  "unsupported" maybe means that
> "cost" isn't fully covered by address-cost and compensation stmts might
> be costed in quantities not fully compatible with that?
>
> That said, "complexity" seems to only complicate things :/  We do have the
> tie-breaker on prefering less IVs.  complexity was added in
> r0-85562-g6e8c65f6621fb0 as part of fixing PR34711.
>
> >     2. Revert the complexity calculation (which my initial patch does), leaving
> >         everything else as it is.
> >     3. A combination of both - if the control path gets into the adjustment code, we
> >         use the reverted complexity calculation.
>
> If it's really only about the "complexity" value then each
> compensation step should
> add to the complexity?
>
> > I'd love to get feedback regarding this, so I could focus on a concrete approach.
> >
> > Kind regards,
> > Dimitrije
> >
> > From: Richard Biener <richard.guenther@gmail.com>
> > Sent: Monday, November 7, 2022 2:35 PM
> > To: Dimitrije Milosevic <Dimitrije.Milosevic@Syrmia.com>
> > Cc: Jeff Law <jeffreyalaw@gmail.com>; gcc-patches@gcc.gnu.org <gcc-patches@gcc.gnu.org>; Djordje Todorovic <Djordje.Todorovic@syrmia.com>
> > Subject: Re: [PATCH 1/2] ivopts: Revert computation of address cost complexity.
> >
> > On Wed, Nov 2, 2022 at 9:40 AM Dimitrije Milosevic
> > <Dimitrije.Milosevic@syrmia.com> wrote:
> > >
> > > Hi Jeff,
> > >
> > > > This is exactly what I was trying to get to.   If the addressing mode
> > > > isn't supported, then we shouldn't be picking it as a candidate.  If it
> > > > is, then we've probably got a problem somewhere else in this code and
> > > > this patch is likely papering over it.
> >
> > I'm not sure this is accurate but at least the cost of using an unsupported
> > addressing mode should be at least that of the compensating code to
> > mangle it to a supported form.
> >
> > > I'll take a deeper look into the candidate selection algorithm then. Will
> > > get back to you.
> >
> > Thanks - as said the unfortunate situation is that both the original author and
> > the one who did the last bigger reworks of the code are gone.
> >
> > Richard.
> >
> > > Regards,
> > > Dimitrije
> > >
> > > ________________________________________
> > > From: Jeff Law <jeffreyalaw@gmail.com>
> > > Sent: Tuesday, November 1, 2022 7:46 PM
> > > To: Richard Biener; Dimitrije Milosevic
> > > Cc: gcc-patches@gcc.gnu.org; Djordje Todorovic
> > > Subject: Re: [PATCH 1/2] ivopts: Revert computation of address cost complexity.
> > >
> > >
> > > On 10/28/22 01:00, Richard Biener wrote:
> > > > On Fri, Oct 28, 2022 at 8:43 AM Dimitrije Milosevic
> > > > <Dimitrije.Milosevic@syrmia.com> wrote:
> > > >> Hi Jeff,
> > > >>
> > > >>> THe part I don't understand is, if you only have BASE+OFF, why does
> > > >>> preventing the calculation of more complex addressing modes matter?  ie,
> > > >>> what's the point of computing the cost of something like base + off +
> > > >>> scaled index when the target can't utilize it?
> > > >> Well, the complexities of all addressing modes other than BASE + OFFSET are
> > > >> equal to 0. For targets like Mips, which only has BASE + OFFSET, it would still
> > > >> be more complex to use a candidate with BASE + INDEX << SCALE + OFFSET
> > > >> than a candidate with BASE + INDEX, for example, as it has to compensate
> > > >> the lack of other addressing modes somehow. If complexities for both of
> > > >> those are equal to 0, in cases where complexities decide which candidate is
> > > >> to be chosen, a more complex candidate may be picked.
> > > > But something is wrong then - it shouldn't ever pick a candidate with
> > > > an addressing
> > > > mode that isn't supported?  So you say that the cost of expressing
> > > > 'BASE + INDEX << SCALE + OFFSET' as 'BASE + OFFSET' is not computed
> > > > accurately?
> > >
> > > This is exactly what I was trying to get to.   If the addressing mode
> > > isn't supported, then we shouldn't be picking it as a candidate.  If it
> > > is, then we've probably got a problem somewhere else in this code and
> > > this patch is likely papering over it.
> > >
> > >
> > > Jeff
> > >