Date: Wed, 25 Oct 2017 21:46:03 +0200
From: Natanael Copa <ncopa@alpinelinux.org>
To: =?ISO-8859-1?B?UHJ6ZW15cyphdyBQYXdlKmN6eWs=?= <przemoc@zoho.com>
Cc: <alpine-devel@lists.alpinelinux.org>
Subject: Re: [alpine-devel] force compile flag for musl?
Message-ID: <20171025214603.00169caf@ncopa-desktop.copa.dup.pw>
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	<15f542f8688.cf7ee9da1704.8790472834519953285@zoho.com>
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On Wed, 25 Oct 2017 17:38:57 +0200
Przemys*aw Pawe*czyk <przemoc@zoho.com> wrote:

> ---- On Wed, 25 Oct 2017 16:46:14 +0200 Natanael Copa <ncopa@alpinelinux.=
org> wrote ----=20
> > I wonder what you think about overriding the -Os compile flag for musl,=
=20
> > and hardcode it to -O2.  =20
>=20
> I would be very careful with such changes.
>=20
> There is misconception that the higher optimization level, the faster
> code is generated.  That is the general -Olevel idea, but not what is
> seen in practice.  Gains (or losses) from higher optimization levels
> vary between archs and obviously depend on the code that is being
> optimized.
>=20
> Smaller code, beside being smaller, is also more cache-friendly, so -Os
> can be faster than -O2 and often is.  OTOH higher optimization levels
> for x86-64 usually tend to give better results than on other archs.
>=20
> There is no rule.  It all depends on:
> - source code,
> - compiler,
> - platform.

As I understand -Os and -O2 are basically the same thing, with the
difference that -O2 enables some more alignments.

> > I think this makes sense since the functions in libc are so often
> > used that we want trade better performance at the cost of slightly
> > bigger binary.  =20
>=20
> This makes sense if we really get better performance with -O2 on all
> platforms AL supports.  And to be able to confirm that, it has te be
> measured.

You are right, of course.

I did a quick test with the code that made me think of -O2 in the first pla=
ce. It is from here:
https://superuser.com/questions/1219609/why-is-the-alpine-docker-image-over=
-50-slower-than-the-ubuntu-image

BENCHMARK=3D"import timeit; print(timeit.timeit('import json; json.dumps(li=
st(range(10000)))', number=3D5000))"

# with -O2

$ for i in $(seq 0 6); do python3 -c "$BENCHMARK"; done
8.284425613994244
8.354899992002174
8.359624709992204
8.392496702988865
8.3223694319895
8.285188248992199
8.294311116012977

# with -Os

$ for i in $(seq 0 6); do python3 -c "$BENCHMARK"; done
8.267152725020424
8.260578833986074
8.232940819987562
8.224149581015809
8.27815192801063
8.31035227200482
8.29849099801504

So it looks that for this specific workload, -Os is actually slightly
faster.

It just highlights the point that you need to actually measure before
trying to optimize.


> > =20
> > This means that we override whatever user as set CFLAGS to=20
> > in /etc/abuild.conf=20
> > =20
> > We already do this with zlib.  =20
>=20
> zlib is a different beast, because it's computational software.  It's
> much more natural to see gains from higher -Olevel in that kind of
> apps.

I'm glad to hear that I'm not totally stupid :)

>=20
> > =20
> > What do you think?  =20
>=20
> There were similar changes in aports for various applications over
> recent months, but I haven't seen even one proof behind them.
>=20
> Performance improvements are imporant, and they may come from simply
> bumping optimization level, but it should be verified, not blindly
> assumed.

Yeah, lets keep -Os and only change if its tested to have any impact.


>=20
> Regards,
> Przemek
>=20
>=20
> > =20
> > diff --git a/main/musl/APKBUILD b/main/musl/APKBUILD=20
> > index 1938bbb3ca..193002186d 100644=20
> > --- a/main/musl/APKBUILD=20
> > +++ b/main/musl/APKBUILD=20
> > @@ -54,6 +54,8 @@ build() {=20
> >         fi=20
> >  =20
> >         # note: not autotools=20
> > +       # force -O2 compile flag for better performance=20
> > +       CFLAGS=3D"-O2" \=20
> >         LDFLAGS=3D"$LDFLAGS -Wl,-soname,libc.musl-${CARCH}.so.1" \=20
> >         ./configure \=20
> >                 --build=3D$CBUILD \=20
> > =20
> > -nc  =20
>=20



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