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From: Manolis Tsamis <manolis.tsamis@vrull.eu>
To: Jeff Law <jeffreyalaw@gmail.com>
Cc: gcc-patches@gcc.gnu.org,
	Philipp Tomsich <philipp.tomsich@vrull.eu>,
	 Richard Biener <richard.guenther@gmail.com>,
	Jakub Jelinek <jakub@redhat.com>
Subject: Re: [PATCH v3] Implement new RTL optimizations pass: fold-mem-offsets.
Date: Tue, 18 Jul 2023 20:15:06 +0300	[thread overview]
Message-ID: <CAM3yNXrYQ7zLOrLv+CbRV49Cv-pDLWSZ35+mWAo4BXz04d1eFA@mail.gmail.com> (raw)
In-Reply-To: <1420ed26-9c30-b09e-f45c-54c89516814e@gmail.com>

On Fri, Jul 14, 2023 at 8:35 AM Jeff Law <jeffreyalaw@gmail.com> wrote:
>
>
>
> On 7/13/23 09:05, Manolis Tsamis wrote:
> > In this version I have made f-m-o able to also eliminate constant
> > moves in addition to the add constant instructions.
> > This increases the number of simplified/eliminated instructions and is
> > a good addition for RISC style ISAs where these are more common.
> >
> > This has led to pr52146.c failing in x86, which I haven't been able to
> > find a way to fix.
> > This involves directly writing to a constant address with -mx32
> >
> > The code
> >          movl    $-18874240, %eax
> >          movl    $0, (%eax)
> >
> > is 'optimized' to
> >          movl    $0, %eax
> >          movl    $0, -18874240(%eax)
> >
> > Which is actually
> >          movl    $0, -18874240
> >
> > which is wrong per the ticket.
> > The fix for the ticket involved changes to legitimate_address_p which
> > f-m-o does call but it doesn't reject due to the existence of (%eax)
> > which in turn is actually zero.
> > I believe this is not strictly an f-m-o issue since the pass calls all
> > the required functions to test whether the newly synthesized memory
> > instruction is valid.
> >
> > Any ideas on how to solve this issue is appreciated.
> I wonder if costing might be useful here.  I would expect the 2nd
> sequence is the most costly of the three if address costing models are
> reasonably accurate.
>
> Another way would be to look at the length of the memory reference insn.
>   If it's larger, then it's likely more costly.
>
> That's what I've got off the top of my head.
>

I could test whether the cost function prefers the version that we
want, but that would be a workaround I would like to avoid. It may
also be the case that this reproduces with a different sequence where
the unwanted code is actually more profitable.

I was trying to find out whether the original fix can be extended in a
way that solves this, because having an address that is reported as
legitimate but is actually not could also create issues elsewhere.
But I don't yet have a suggestion on how to fix it yet.

Thanks,
Manolis

> Jeff

  reply	other threads:[~2023-07-18 17:15 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-07-13 14:13 Manolis Tsamis
2023-07-13 15:05 ` Manolis Tsamis
2023-07-14  5:35   ` Jeff Law
2023-07-18 17:15     ` Manolis Tsamis [this message]
2023-07-18 18:01       ` Jeff Law
2023-07-18 23:42         ` Vineet Gupta
2023-07-19  4:31           ` Jeff Law
2023-07-19  8:08             ` Manolis Tsamis
2023-07-19 14:16               ` Jeff Law
2023-07-20  6:18             ` Vineet Gupta
2023-07-20 21:59               ` Jeff Law
2023-08-07 14:44                 ` Manolis Tsamis
2023-08-07 17:13                   ` Jeff Law

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