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| author | H.J. Lu <hjl.tools@gmail.com> | 2017-08-04 11:14:19 -0700 |
|---|---|---|
| committer | H.J. Lu <hjl.tools@gmail.com> | 2017-08-04 11:14:33 -0700 |
| commit | d2cf37c0a2a375cf2fde69f1afbcc49e45368fc4 (patch) | |
| tree | 9de1e2154736c6c3c7990e2279b8ab6b02f714e2 /sysdeps | |
| parent | c8a0e6ec03b2646842fe3c7c59955f55175f3669 (diff) | |
| download | glibc-d2cf37c0a2a375cf2fde69f1afbcc49e45368fc4.zip glibc-d2cf37c0a2a375cf2fde69f1afbcc49e45368fc4.tar.gz glibc-d2cf37c0a2a375cf2fde69f1afbcc49e45368fc4.tar.bz2 | |
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.
Diffstat (limited to 'sysdeps')
| -rw-r--r-- | sysdeps/x86/cpu-features.c | 7 |
1 files changed, 5 insertions, 2 deletions
diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c index 1d087ea..6f90084 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; |