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

[Dimitrije Milosevic <Dimitrije.Milosevic@Syrmia.com>]

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.
    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.
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
>