https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78611
--- Comment #5 from Uroš Bizjak <ubizjak at gmail dot com> ---
I can confirm the slowdown with -mavx on ivybridge-E:
-msse4
real 0m0.278s
user 0m0.276s
sys 0m0.002s
-mavx
real 0m0.699s
user 0m0.696s
sys 0m0.003s
in -msse4 case, perf report annotates mtz_ with:
1.33 │ → callq logf@plt
│ addss 0x7acb(%rip),%xmm0 # 40b0cc
<options.236.5085+0x4c>
6.87 │ mulss 0x7ac7(%rip),%xmm0 # 40b0d0
<options.236.5085+0x50>
9.53 │ addss 0x7aa7(%rip),%xmm0 # 40b0b8
<options.236.5085+0x38>
7.54 │ cvttss %xmm0,%eax
7.76 │ cmp %r12d,%eax
and in -mavx case:
0.09 │ → callq logf@plt
│ vaddss 0x7a85(%rip),%xmm0,%xmm0 # 40b16c
<options.236.5085+0x4c>
59.65 │ vmulss 0x7a81(%rip),%xmm0,%xmm0 # 40b170
<options.236.5085+0x50>
2.92 │ vaddss 0x7a61(%rip),%xmm0,%xmm0 # 40b158
<options.236.5085+0x38>
2.04 │ vcvtts %xmm0,%eax
2.65 │ cmp %r12d,%eax
Something happens with vmulss, but I doubt it is the compiler fault.
