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Date: Thu, 20 Dec 2018 23:46:00 -0000
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Aaron Sawdey <acsawdey@linux.ibm.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>, David Edelsohn <dje.gcc@gmail.com>,        Bill Schmidt <wschmidt@linux.ibm.com>
Subject: Re: [PATCH][rs6000] avoid using unaligned vsx or lxvd2x/stxvd2x for memcpy/memmove inline expansion
Message-ID: <20181220234402.GX3803@gate.crashing.org>
References: <0a17416b-57a0-99e7-2e7e-90a63da66fe6@linux.ibm.com> <20181220095119.GP3803@gate.crashing.org> <30fd466c-43c7-86aa-81f2-181a9d9ca7fc@linux.ibm.com>
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On Thu, Dec 20, 2018 at 05:34:54PM -0600, Aaron Sawdey wrote:
> On 12/20/18 3:51 AM, Segher Boessenkool wrote:
> > On Wed, Dec 19, 2018 at 01:53:05PM -0600, Aaron Sawdey wrote:
> >> Because of POWER9 dd2.1 issues with certain unaligned vsx instructions
> >> to cache inhibited memory, here is a patch that keeps memmove (and memcpy)
> >> inline expansion from doing unaligned vector or using vector load/store
> >> other than lvx/stvx. More description of the issue is here:
> >>
> >> https://patchwork.ozlabs.org/patch/814059/
> >>
> >> OK for trunk if bootstrap/regtest ok?
> > 
> > Okay, but see below.
> > 
> [snip]
> > 
> > This is extraordinarily clumsy :-)  Maybe something like:
> > 
> > static rtx
> > gen_lvx_v4si_move (rtx dest, rtx src)
> > {
> >   gcc_assert (!(MEM_P (dest) && MEM_P (src));
> >   gcc_assert (GET_MODE (dest) == V4SImode && GET_MODE (src) == V4SImode);
> >   if (MEM_P (dest))
> >     return gen_altivec_stvx_v4si_internal (dest, src);
> >   else if (MEM_P (src))
> >     return gen_altivec_lvx_v4si_internal (dest, src);
> >   else
> >     gcc_unreachable ();
> > }
> > 
> > (Or do you allow VOIDmode for src as well?)  Anyway, at least get rid of
> > the useless extra variable.
> 
> I think this should be better:

The gcc_unreachable at the end catches the non-mem to non-mem case.

> static rtx
> gen_lvx_v4si_move (rtx dest, rtx src)
> {
>   gcc_assert ((MEM_P (dest) && !MEM_P (src)) || (MEM_P (src) && !MEM_P(dest)));

But if you prefer this, how about

{
  gcc_assert (MEM_P (dest) ^ MEM_P (src));
  gcc_assert (GET_MODE (dest) == V4SImode && GET_MODE (src) == V4SImode);

  if (MEM_P (dest))
    return gen_altivec_stvx_v4si_internal (dest, src);
  else
    return gen_altivec_lvx_v4si_internal (dest, src);
}

:-)


Segher