[systemd-devel] lto issues

[Zbigniew Jędrzejewski-Szmek]

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*

Zbyszek