[systemd-devel] lto issues

[Michael Biebl]

Am Di., 6. Aug. 2019 um 09:26 Uhr schrieb Zbigniew Jędrzejewski-Szmek
<zbyszek at in.waw.pl>:
>
> 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)



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