Re: [PATCH] x86-64: Add Avoid_Short_Distance_REP_MOVSB

[Noah Goldstein via Libc-alpha <libc-alpha@sourceware.org>]

On Mon, Jul 26, 2021 at 11:11 PM H.J. Lu via Libc-alpha <
libc-alpha@sourceware.org> wrote:

> On Mon, Jul 26, 2021 at 7:15 PM Carlos O'Donell <carlos@redhat.com> wrote:
> >
> > On 7/26/21 8:00 AM, H.J. Lu via Libc-alpha wrote:
> > > commit 3ec5d83d2a237d39e7fd6ef7a0bc8ac4c171a4a5
> > > Author: H.J. Lu <hjl.tools@gmail.com>
> > > Date:   Sat Jan 25 14:19:40 2020 -0800
> > >
> > >     x86-64: Avoid rep movsb with short distance [BZ #27130]
> > > introduced some regressions on Intel processors without Fast Short REP
> > > MOV (FSRM).  Add Avoid_Short_Distance_REP_MOVSB to avoid rep movsb with
> > > short distance only on Intel processors with FSRM.  bench-memmove-large
> > > on Skylake server shows that cycles of __memmove_evex_unaligned_erms
> are
> > > improved for the following data size:
> > >
> > >                                   before    after    Improvement
> > > length=4127, align1=3, align2=0:  479.38    343.00      28%
> > > length=4223, align1=9, align2=5:  405.62    335.50      17%
> > > length=8223, align1=3, align2=0:  786.12    495.00      37%
> > > length=8319, align1=9, align2=5:  256.69    170.38      33%
> > > length=16415, align1=3, align2=0: 1436.88   839.50      41%
> > > length=16511, align1=9, align2=5: 1375.50   840.62      39%
> > > length=32799, align1=3, align2=0: 2890.00   1850.62     36%
> > > length=32895, align1=9, align2=5: 2891.38   1948.62     32%
> > >
> > > There are no regression on Ice Lake server.
> >
> > At this point we're waiting on Noah to provide feedback on the
> performance
> > results given the alignment nop insertion you provided as a follow-up
> patch
>

The results with the padding look good!


>
> We are testing 25 byte nop padding now:


>
> https://gitlab.com/x86-glibc/glibc/-/commit/de8985640a568786a59576716db54e0749d420e8
>
> How did you come to the exact padding choice used?


> > (unless you can confirm this yourself).
> >
> > Looking forward to a v2 the incorporates the alignment fix (pending
> Noah's
> > comments), and my suggestions below.

>
> > > ---
> > >  sysdeps/x86/cacheinfo.h                                    | 7 +++++++
> > >  sysdeps/x86/cpu-features.c                                 | 5 +++++
> > >  .../x86/include/cpu-features-preferred_feature_index_1.def | 1 +
> > >  sysdeps/x86/sysdep.h                                       | 3 +++
> > >  sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S      | 5 +++++
> > >  5 files changed, 21 insertions(+)
> > >
> > > diff --git a/sysdeps/x86/cacheinfo.h b/sysdeps/x86/cacheinfo.h
> > > index eba8dbc4a6..174ea38f5b 100644
> > > --- a/sysdeps/x86/cacheinfo.h
> > > +++ b/sysdeps/x86/cacheinfo.h
> > > @@ -49,6 +49,9 @@ long int __x86_rep_stosb_threshold attribute_hidden
> = 2048;
> > >  /* Threshold to stop using Enhanced REP MOVSB.  */
> > >  long int __x86_rep_movsb_stop_threshold attribute_hidden;
> > >
> > > +/* String/memory function control.  */
> > > +int __x86_string_control attribute_hidden;
> >
> > Please expand comment.
> >
> > Suggest:
> >
> > /* A bit-wise OR of string/memory requirements for optimal performance
> >    e.g. X86_STRING_CONTROL_AVOID_SHORT_DISTANCE_REP_MOVSB.  These bits
> >    are used at runtime to tune implementation behavior.  */
> > int __x86_string_control attribute_hidden;
>
> I will fix it in the v2 patch.
>
> Thanks.
>
> > > +
> > >  static void
> > >  init_cacheinfo (void)
> > >  {
> > > @@ -71,5 +74,9 @@ init_cacheinfo (void)
> > >    __x86_rep_movsb_threshold = cpu_features->rep_movsb_threshold;
> > >    __x86_rep_stosb_threshold = cpu_features->rep_stosb_threshold;
> > >    __x86_rep_movsb_stop_threshold =
> cpu_features->rep_movsb_stop_threshold;
> > > +
> > > +  if (CPU_FEATURES_ARCH_P (cpu_features,
> Avoid_Short_Distance_REP_MOVSB))
> > > +    __x86_string_control
> > > +      |= X86_STRING_CONTROL_AVOID_SHORT_DISTANCE_REP_MOVSB;
> >
> > OK.
> >
> > >  }
> > >  #endif
> > > diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
> > > index 706a172ba9..645bba6314 100644
> > > --- a/sysdeps/x86/cpu-features.c
> > > +++ b/sysdeps/x86/cpu-features.c
> > > @@ -555,6 +555,11 @@ init_cpu_features (struct cpu_features
> *cpu_features)
> > >           cpu_features->preferred[index_arch_Prefer_AVX2_STRCMP]
> > >             |= bit_arch_Prefer_AVX2_STRCMP;
> > >       }
> > > +
> > > +      /* Avoid avoid short distance REP MOVSB on processor with
> FSRM.  */
> > > +      if (CPU_FEATURES_CPU_P (cpu_features, FSRM))
> > > +
>  cpu_features->preferred[index_arch_Avoid_Short_Distance_REP_MOVSB]
> > > +       |= bit_arch_Avoid_Short_Distance_REP_MOVSB;
> >
> > OK.
> >
> > >      }
> > >    /* This spells out "AuthenticAMD" or "HygonGenuine".  */
> > >    else if ((ebx == 0x68747541 && ecx == 0x444d4163 && edx ==
> 0x69746e65)
> > > diff --git
> a/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def
> b/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def
> > > index 133aab19f1..d7c93f00c5 100644
> > > --- a/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def
> > > +++ b/sysdeps/x86/include/cpu-features-preferred_feature_index_1.def
> > > @@ -33,3 +33,4 @@ BIT (Prefer_No_AVX512)
> > >  BIT (MathVec_Prefer_No_AVX512)
> > >  BIT (Prefer_FSRM)
> > >  BIT (Prefer_AVX2_STRCMP)
> > > +BIT (Avoid_Short_Distance_REP_MOVSB)
> >
> > OK.
> >
> > > diff --git a/sysdeps/x86/sysdep.h b/sysdeps/x86/sysdep.h
> > > index 51c069bfe1..35cb90d507 100644
> > > --- a/sysdeps/x86/sysdep.h
> > > +++ b/sysdeps/x86/sysdep.h
> > > @@ -57,6 +57,9 @@ enum cf_protection_level
> > >  #define STATE_SAVE_MASK \
> > >    ((1 << 1) | (1 << 2) | (1 << 3) | (1 << 5) | (1 << 6) | (1 << 7))
> > >
> >
> > Suggest adding:
> >
> > /* Constants for bits in __x86_string_control:  */
> >
> > > +/* Avoid short distance REP MOVSB.  */
> > > +#define X86_STRING_CONTROL_AVOID_SHORT_DISTANCE_REP_MOVSB    (1 << 0)
> >
> > OK.
> >
> > > +
> > >  #ifdef       __ASSEMBLER__
> > >
> > >  /* Syntactic details of assembler.  */
> > > diff --git a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
> b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
> > > index a783da5de2..9f02624375 100644
> > > --- a/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
> > > +++ b/sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
> > > @@ -325,12 +325,16 @@ L(movsb):
> > >       /* Avoid slow backward REP MOVSB.  */
> > >       jb      L(more_8x_vec_backward)
> > >  # if AVOID_SHORT_DISTANCE_REP_MOVSB
> > > +     andl    $X86_STRING_CONTROL_AVOID_SHORT_DISTANCE_REP_MOVSB,
> __x86_string_control(%rip)
> > > +     jz      3f
> >
> > OK.
> >
> > >       movq    %rdi, %rcx
> > >       subq    %rsi, %rcx
> > >       jmp     2f
> > >  # endif
> > >  1:
> > >  # if AVOID_SHORT_DISTANCE_REP_MOVSB
> > > +     andl    $X86_STRING_CONTROL_AVOID_SHORT_DISTANCE_REP_MOVSB,
> __x86_string_control(%rip)
> > > +     jz      3f
> >
> > OK.
> >
> > >       movq    %rsi, %rcx
> > >       subq    %rdi, %rcx
> > >  2:
> > > @@ -338,6 +342,7 @@ L(movsb):
> > >     is N*4GB + [1..63] with N >= 0.  */
> > >       cmpl    $63, %ecx
> > >       jbe     L(more_2x_vec)  /* Avoid "rep movsb" if ECX <= 63.  */
> > > +3:
> >
> > OK.
> >
> > >  # endif
> > >       mov     %RDX_LP, %RCX_LP
> > >       rep movsb
> > >
> >
> >
> > --
> > Cheers,
> > Carlos.
> >
>
>
> --
> H.J.
>