Re: lto issues

[Zbigniew Jędrzejewski-Szmek <zbyszek@xxxxxxxxx>]

On Tue, Aug 06, 2019 at 09:34:36AM +0200, Michael Biebl wrote:
> Am Di., 6. Aug. 2019 um 09:26 Uhr schrieb Zbigniew Jędrzejewski-Szmek
> <zbyszek@xxxxxxxxx>:
> >
> > On Sat, Aug 03, 2019 at 07:03:47PM +0200, Michael Biebl wrote:
> > > Hi,
> > >
> > > today I tried compiling systemd v242 (on Debian sid) once using lto
> > > (-Db_lto=true) and once without lto (-Db_lto=false).
> > >
> > > The lto build took approximately twice as long on my laptop (using
> > > dpkg-buildpackage, which introduces a bit of overhead):
> > >
> > > lto:
> > > real 11m22,605s
> > > user 37m9,675s
> > > sys 2m51,041s
> > >
> > > nolto:
> > > real 6m35,615s
> > > user 18m51,782s
> > > sys 2m12,934s
> > >
> > > That's kinda expected. What suprised me though is that using lto
> > > produced larger binaries:
> >
> > I built systemd in F31 (-Doptimization=2 -Db_lto=true/false, and I saw
> > a big increase in binary sizes *before stripping*. After stripping,
> > binaries with lto=true are smaller:
> >
> > $ ls -l build-rawhide{,-lto}/{systemd,src/shared/libsystemd-shared-243.so}
> >   7116384 Aug  6 09:08 build-rawhide/systemd*
> >  11951256 Aug  6 09:07 build-rawhide/src/shared/libsystemd-shared-243.so*
> >   1594912 Aug  6 09:12 build-rawhide-lto/systemd*
> >   3167096 Aug  6 09:11 build-rawhide-lto/src/shared/libsystemd-shared-243.so*
> > $ strip build-rawhide{,-lto}/{systemd,src/shared/libsystemd-shared-243.so}
> > $ ls -l build-rawhide{,-lto}/{systemd,src/shared/libsystemd-shared-243.so}
> >   1439640 Aug  6 09:19 build-rawhide/systemd*
> >   2806456 Aug  6 09:19 build-rawhide/src/shared/libsystemd-shared-243.so*
> >   1370008 Aug  6 09:19 build-rawhide-lto/systemd*
> >   2806288 Aug  6 09:19 build-rawhide-lto/src/shared/libsystemd-shared-243.so*
> 
> 
> The sizes I posted i.e. the debdiff is after stripping.
> 
> gcc --version
> gcc (Debian 8.3.0-19) 8.3.0
> Copyright (C) 2018 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> 
> ld --version
> GNU ld (GNU Binutils for Debian) 2.32.51.20190727
> Copyright (C) 2019 Free Software Foundation, Inc.
> This program is free software; you may redistribute it under the terms of
> the GNU General Public License version 3 or (at your option) a later version.
> 
> So with the toolchain I have, mostly has downsides. The only benefit
> it seems to have is that it optimizes unnecessary library dependencies
> away (see how the udev subpackage does not depend on libcap2 (>=
> 1:2.10), libidn2-0 (>= 0.6)

In Fedora, lto seems to have good returns. The final package size was
~10% smaller. We disabled it for a while because there were linking
failures, but I think it's all resolved now.

I forgot to mention. In my test:
gcc-9.1.1-2.fc31.1.x86_64
binutils-2.32-19.fc31.x86_64

Zbyszek
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