x86-64: strlen-evex performance performance degradation compared to strlen-avx2

[Sunil Pandey]

On Mon, Apr 29, 2024 at 1:20 PM H.J. Lu <hjl.tools@gmail.com> wrote:

> On Mon, Apr 29, 2024 at 10:42 AM Sunil Pandey <skpgkp2@gmail.com> wrote:
> >
> >
> >
> > On Sun, Apr 28, 2024 at 9:17 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >>
> >> On Sun, Apr 28, 2024 at 9:13 AM Sunil Pandey <skpgkp2@gmail.com> wrote:
> >> >
> >> >
> >> >
> >> > On Sat, Apr 27, 2024 at 7:13 PM abush wang <abushwangs@gmail.com>
> wrote:
> >> >>
> >> >> Actually, I was handling performance issue from  libmicro in our
> distro OS.
> >> >> I found that the performance degradation of localtime_r benchmark
> from libmicro is blame to strlen.
> >> >> So I abstracted this test case.
> >> >>
> >> >
> >> > Can you consistently reproduce strlen perf behaviour by running
> multiple times back-to-back?
> >> >
> >> > You can see high swing from run
> >>
> >> Hi Sunil,
> >>
> >> Intel(R) Xeon(R) Gold 6133 CPU @ 2.50GHz is SKX.  Please add this test
> to
> >> benchtests/bench-strlen.c and check its performance on SKX.
> >>
> >> --
> >> H.J.
> >
> >
> > I collected the glibc micro-benchmark data for the string length in
> question.
> >
> > 2.38 evex data:
> >
> > length=4, alignment=4:         4.40
> > length=4, alignment=0:         4.29
> > length=4, alignment=0:         3.64
> > length=4, alignment=7:         3.64
> > length=4, alignment=2:         3.64
> >
> > 2.28 evex data:
> >
> > Length    4, alignment  4: 6.46875
> > Length    4, alignment  0: 6.5
> > Length    4, alignment  0: 6.53125
> > Length    4, alignment  7: 6.46875
> > Length    4, alignment  2: 6.53125
> >
> > Data collected on Machine: Intel(R) Xeon(R) Gold 6148 CPU @ 2.40GHz
> >
> > 2.38 perf numbers are better than 2.28 as expected.
>
> 1. Please compare AVX2 vs EVEX strlen on glibc master branch.
> 2. Please check strlen on strings of length == 4 and alignments = 0, 1, 2,
> 3.
>
> --
> H.J.
>

Data from master branch:

Data collected on Machine: Intel(R) Xeon(R) Gold 6148 CPU @ 2.40GHz

                      __strlen_evex __strlen_avx2
=======================================================
 length=4, alignment=0:         5.00        5.11
 length=4, alignment=1:         4.92        4.80
 length=4, alignment=2:         4.82        4.62
 length=4, alignment=3:         4.62        4.92
 length=4, alignment=4:         4.44        4.44
 length=4, alignment=5:         4.59        4.29
 length=4, alignment=6:         4.39        4.29
 length=4, alignment=7:         4.14        4.14
 length=4, alignment=8:         4.19        4.00
 length=4, alignment=9:         4.00        4.00
length=4, alignment=10:         4.31        3.87
length=4, alignment=11:         3.96        3.87
length=4, alignment=12:         3.86        3.75
length=4, alignment=13:         3.75        3.75
length=4, alignment=14:         3.64        3.64
length=4, alignment=15:         3.64        3.72
length=4, alignment=16:         3.64        3.53
length=4, alignment=17:         3.63        3.53
length=4, alignment=18:         4.12        3.53
length=4, alignment=19:         3.43        3.43
length=4, alignment=20:         3.43        3.43
length=4, alignment=21:         3.33        3.33
length=4, alignment=22:         3.33        3.42
length=4, alignment=23:         3.33        3.33
length=4, alignment=24:         3.33        3.33
length=4, alignment=25:         3.33        3.33
length=4, alignment=26:         3.96        3.33
length=4, alignment=27:         3.33        3.41
length=4, alignment=28:         3.33        3.33
length=4, alignment=29:         3.41        3.33
length=4, alignment=30:         3.33        3.41
length=4, alignment=31:         3.33        3.33

--Sunil
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