On Mon, Jul 26, 2021 at 9:06 PM Noah Goldstein wrote: > > > > On Mon, Jul 26, 2021 at 11:11 PM H.J. Lu via Libc-alpha wrote: >> >> On Mon, Jul 26, 2021 at 7:15 PM Carlos O'Donell wrote: >> > >> > On 7/26/21 8:00 AM, H.J. Lu via Libc-alpha wrote: >> > > commit 3ec5d83d2a237d39e7fd6ef7a0bc8ac4c171a4a5 >> > > Author: H.J. Lu >> > > 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? I first replaced the 9 byte instructions: andl $X86_STRING_CONTROL_AVOID_SHORT_DISTANCE_REP_MOVSB, __x86_string_control(%rip) jz 3f with a 9-byte NOP and reproduced the regression on Tiger Lake. It confirmed that the code layout caused the regression. I first tried adding ".p2align 4" to branch targets and they made no differences. Then I started adding different size of nops after andl $X86_STRING_CONTROL_AVOID_SHORT_DISTANCE_REP_MOVSB, __x86_string_control(%rip) jz 3f movq %rdi, %rcx subq %rsi, %rcx jmp 2f with ".nops N". I started with N == 1 and doubled N in each step. I noticed that improvement started at N == 32. I started bisecting between 16 and 32: 1. 24 and 32 are good. 2. 24 and 28 are good. 3. 25 is the best overall. >> >> > (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. Here is the v2 patch: 1. Add a 25-byte NOP padding after JMP for Avoid_Short_Distance_REP_MOVSB, which improves bench-memcpy-random performance on Tiger Lake by ~30%. 2. Update comments for __x86_string_control. -- H.J.