Re: [PATCH 1/2] powerpc: Add 64bit optimised memcmp

[Joakim Tjernlund <Joakim.Tjernlund@transmode.se>]

On Mon, 2015-01-12 at 11:55 +1100, Anton Blanchard wrote:
> Hi David,
>=20
> > The unrolled loop (deleted) looks excessive.
> > On a modern cpu with multiple execution units you can usually
> > manage to get the loop overhead to execute in parallel to the
> > actual 'work'.
> > So I suspect that a much simpler 'word at a time' loop will be almost a=
s fast - especially in the case where the code isn't
> > already in the cache and the compare is relatively short.
>=20
> I'm always keen to keep things as simple as possible, but your loop is ov=
er 50% slower. Once the loop hits a steady state you are going to run into =
front end issues with instruction fetch on POWER8.
>=20

Out of curiosity, does preincrement make any difference(or can gcc do that =
for you nowadays)?

         a1 =3D *a;
         b1 =3D *b;
         while {
                 a2 =3D *++a;
                 b2 =3D *++b;
                 if (a1 !=3D a2)
                 break;
                 a1 =3D *++a;
                 b1 =3D *++b;
         } while (a2 !=3D a1);

 Jocke

> Anton
>=20
> > Try something based on:
> >         a1 =3D *a++;
> >         b1 =3D *b++;
> >         while {
> >                 a2 =3D *a++;
> >                 b2 =3D *b++;
> >                 if (a1 !=3D a2)
> >                 break;
> >                 a1 =3D *a++;
> >                 b1 =3D *b++;
> >         } while (a2 !=3D a1);
> >=20
> >         David
> >=20
>=20
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