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From: "H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>
To: "Carlos O'Donell" <carlos@redhat.com>
Cc: GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [PATCH] x86-64: Add Avoid_Short_Distance_REP_MOVSB
Date: Mon, 26 Jul 2021 20:11:05 -0700	[thread overview]
Message-ID: <CAMe9rOr+Df=dbNsCgtyj71B-tEkPT6e7ocnPt_6QAfK5wZZ_6A@mail.gmail.com> (raw)
In-Reply-To: <d0177979-6420-52c0-517d-232112307118@redhat.com>

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

We are testing 25 byte nop padding now:

https://gitlab.com/x86-glibc/glibc/-/commit/de8985640a568786a59576716db54e0749d420e8

> (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.

  reply	other threads:[~2021-07-27  3:12 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-26 12:00 [PATCH] x86-64: Add Avoid_Short_Distance_REP_MOVSB H.J. Lu via Libc-alpha
2021-07-26 17:20 ` Noah Goldstein via Libc-alpha
2021-07-26 18:50   ` H.J. Lu via Libc-alpha
2021-07-27  2:15 ` Carlos O'Donell via Libc-alpha
2021-07-27  3:11   ` H.J. Lu via Libc-alpha [this message]
2021-07-27  4:05     ` Noah Goldstein via Libc-alpha
2021-07-27 16:05       ` [PATCH v2] " H.J. Lu via Libc-alpha
2021-07-27 19:12         ` Noah Goldstein via Libc-alpha
2021-07-27 19:22           ` [PATCH v3] " H.J. Lu via Libc-alpha
2021-07-27 19:50             ` Noah Goldstein via Libc-alpha
2021-07-27 19:55               ` H.J. Lu via Libc-alpha
2021-07-28 15:08               ` H.J. Lu via Libc-alpha
2021-08-28  0:27 ` [PATCH] " Alexey Tourbin via Libc-alpha
2021-08-28  2:57   ` Noah Goldstein via Libc-alpha
2022-04-28  0:15     ` Sunil Pandey via Libc-alpha

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