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

[Florian Weimer <fweimer@redhat.com>]

* 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).

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.

Thanks,
Florian
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