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Re: [PATCH] x86-64: Use _dl_runtime_resolve_opt only with AVX512F

• From: "H.J. Lu" <hjl dot tools at gmail dot com>
• To: GNU C Library <libc-alpha at sourceware dot org>, Libc-stable Mailing List <libc-stable at sourceware dot org>
• Date: Fri, 4 Aug 2017 06:21:39 -0700
• Subject: Re: [PATCH] x86-64: Use _dl_runtime_resolve_opt only with AVX512F
• Authentication-results: sourceware.org; auth=none
• References: <20170802151213.GA12009@gmail.com>

On Wed, Aug 2, 2017 at 8:12 AM, H.J. Lu <hongjiu.lu@intel.com> wrote:> On AVX machines with XGETBV (ECX == 1) like Skylake processors,>> (gdb) disass _dl_runtime_resolve_avx_opt> Dump of assembler code for function _dl_runtime_resolve_avx_opt:>    0x0000000000015890 <+0>:     push   %rax>    0x0000000000015891 <+1>:     push   %rcx>    0x0000000000015892 <+2>:     push   %rdx>    0x0000000000015893 <+3>:     mov    $0x1,%ecx>    0x0000000000015898 <+8>:     xgetbv>    0x000000000001589b <+11>:    mov    %eax,%r11d>    0x000000000001589e <+14>:    pop    %rdx>    0x000000000001589f <+15>:    pop    %rcx>    0x00000000000158a0 <+16>:    pop    %rax>    0x00000000000158a1 <+17>:    and    $0x4,%r11d>    0x00000000000158a5 <+21>:    bnd je 0x16200 <_dl_runtime_resolve_sse_vex>> End of assembler dump.>> is slower than:>> (gdb) disass _dl_runtime_resolve_avx_slow> Dump of assembler code for function _dl_runtime_resolve_avx_slow:>    0x0000000000015850 <+0>:     vorpd  %ymm0,%ymm1,%ymm8>    0x0000000000015854 <+4>:     vorpd  %ymm2,%ymm3,%ymm9>    0x0000000000015858 <+8>:     vorpd  %ymm4,%ymm5,%ymm10>    0x000000000001585c <+12>:    vorpd  %ymm6,%ymm7,%ymm11>    0x0000000000015860 <+16>:    vorpd  %ymm8,%ymm9,%ymm9>    0x0000000000015865 <+21>:    vorpd  %ymm10,%ymm11,%ymm10>    0x000000000001586a <+26>:    vpcmpeqd %xmm8,%xmm8,%xmm8>    0x000000000001586f <+31>:    vorpd  %ymm9,%ymm10,%ymm10>    0x0000000000015874 <+36>:    vptest %ymm10,%ymm8>    0x0000000000015879 <+41>:    bnd jae 0x158b0 <_dl_runtime_resolve_avx>>    0x000000000001587c <+44>:    vzeroupper>    0x000000000001587f <+47>:    bnd jmpq 0x16200 <_dl_runtime_resolve_sse_vex>> End of assembler dump.> (gdb)>> since xgetbv takes much more cycles than single cycle operations like> vpord/vvpcmpeq/ptest.  _dl_runtime_resolve_opt should be used only with> AVX512 where AVX512 instructions lead to lower CPU frequency on Skylake> server.>> Any comments or objections?>> H.J.> --->         [BZ #21871]>         * sysdeps/x86/cpu-features.c (init_cpu_features): Set>         bit_arch_Use_dl_runtime_resolve_opt only with AVX512F.> --->  sysdeps/x86/cpu-features.c | 7 +++++-->  1 file changed, 5 insertions(+), 2 deletions(-)>> diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c> index 1d087ea732..6f900840d4 100644> --- a/sysdeps/x86/cpu-features.c> +++ b/sysdeps/x86/cpu-features.c> @@ -244,10 +244,13 @@ init_cpu_features (struct cpu_features *cpu_features)>           |= bit_arch_Prefer_No_AVX512;>>        /* To avoid SSE transition penalty, use _dl_runtime_resolve_slow.> -         If XGETBV suports ECX == 1, use _dl_runtime_resolve_opt.  */> +         If XGETBV suports ECX == 1, use _dl_runtime_resolve_opt.> +        Use _dl_runtime_resolve_opt only with AVX512F since it is> +        slower than _dl_runtime_resolve_slow with AVX.  */>        cpu_features->feature[index_arch_Use_dl_runtime_resolve_slow]>         |= bit_arch_Use_dl_runtime_resolve_slow;> -      if (cpu_features->max_cpuid >= 0xd)> +      if (CPU_FEATURES_ARCH_P (cpu_features, AVX512F_Usable)> +         && cpu_features->max_cpuid >= 0xd)>         {>           unsigned int eax;>> --> 2.13.3>I am checking it in today and will backport it to 2.26/2.25/2.24 branchesnext week.-- H.J.

• References:
  • [PATCH] x86-64: Use _dl_runtime_resolve_opt only with AVX512F
    • From: H.J. Lu