[Bug dynamic-link/21871] _dl_runtime_resolve_avx_opt is slower than _dl_runtime_resolve_avx_slow

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21871

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=ac3c2bcce8694b5d26bf51f235ed682e353bb880

commit ac3c2bcce8694b5d26bf51f235ed682e353bb880
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Fri Jul 28 15:13:40 2017 -0700

    x86-64: Use _dl_runtime_resolve_opt only with AVX512F [BZ #21871]

    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.

        [BZ #21871]
        * sysdeps/x86/cpu-features.c (init_cpu_features): Set
        bit_arch_Use_dl_runtime_resolve_opt only with AVX512F.

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