Re: V2 [PATCH] x86-64: Avoid rep movsb with short distance [BZ #27130]

Re: V2 [PATCH] x86-64: Avoid rep movsb with short distance [BZ #27130]

[H.J. Lu]

On Mon, Jan 4, 2021 at 7:47 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * H. J. Lu:
>
> > On Mon, Jan 4, 2021 at 7:22 AM Florian Weimer <fweimer@redhat.com> wrote:
> >>
> >> * H. J. Lu via Libc-alpha:
> >>
> >> >  1:
> >> > +# if AVOID_SHORT_DISTANCE_REP_MOVSB
> >> > +     movq    %rsi, %rcx
> >> > +     subq    %rdi, %rcx
> >> > +2:
> >> > +/* Avoid "rep movsb" if RCX, the distance between source and destination,
> >> > +   is N*4GB + [1..63] with N >= 0.  */
> >> > +     cmpl    $63, %ecx
> >> > +     jbe     L(more_2x_vec)  /* Avoid "rep movsb" if ECX <= 63.  */
> >> > +# endif
> >> >       mov     %RDX_LP, %RCX_LP
> >> >       rep movsb
> >> >  L(nop):
> >>
> >> Why not use _LP names here?  I think the %ecx comparison at least can
> >> give false results on x86-64 (64-bit).
> >>
> >
> > This is done on purpose since we want to avoid "rep movsb" for distances of
> > N*4GB + [1..63] with N >= 0 which include 0x100000003.
>
> Ah, and the comment is quite clear (the commit subject less so).

It isn't easy to describe it with so few letters.

> I tried to make sense of the assembler code, and I think the change is
> okay because L(movsb) is only reached when there is more to copy than
> twice the vector size.
>

That is correct.  I am checking it in.  I will backport it to release branches
next week.

Thanks.

-- 
H.J.