Re: [PATCH, i386]: Do not limit the cost of moves to/from XMM register to minimum 8.

[Hongtao Liu <crazylht at gmail dot com>]

On Fri, Aug 30, 2019 at 2:18 PM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> On Fri, Aug 30, 2019 at 2:08 AM Hongtao Liu <crazylht@gmail.com> wrote:
> >
> > On Fri, Aug 30, 2019 at 2:09 AM Uros Bizjak <ubizjak@gmail.com> wrote:
> > >
> > > 2019-08-28  Uroš Bizjak  <ubizjak@gmail.com>
> > >
> > >     * config/i386/i386.c (ix86_register_move_cost): Do not
> > >     limit the cost of moves to/from XMM register to minimum 8.
> > >
> > > Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.
> > >
> > > Actually committed as r274994 with the wrong ChangeLog.
> > >
> > > Uros.
> >
> > There is 11% regression in 548.exchange_r of SPEC2017.
> >
> > Reason for the regression:
> > For 548.exchange_r, a lot of movements between gpr and xmm are
> > generated as expected,
> > and it reduced  clocksticks by 3%.
>
> This is OK, and expected from the patch.
>
> > But  however maybe too many xmm registers are used,
> > a frequency reduction issue is triggered(average frequency reduced by 13%).
> > So totally it takes more time.
>
> This is a secondary effect that is currently not modelled by the compiler.
>
> However, I expected that SSE <-> int moves in x86-tune-cost.h will
> have to be retuned. Up to now, both directions were limited to minimum
> 8, so any value lower than 8 was ignored. However, minimum was set to
> work-around certain limitation in reload, which is not needed anymore.
>
> You can simply set the values of SSE <-> int moves to 8 (which is an
> arbitrary value!) to restore the previous behaviour, but I think that
> a more precise cost value should be determined, probably a different
> one for each direction. But until register pressure effects are
> modelled, any artificially higher value will represent a workaround
> and not the true reg-reg move cost.
>
> Uros.

Yes,  we're still working on skylake_cost model.

-- 
BR,
Hongtao