Fwd: enabling link time optimizations in package builds

Roland Clobus rclobus at rclobus.nl
Fri Jun 17 09:05:42 UTC 2022

Hello reproducible-builds members,

would enabling LTO cause reproducible issues?
If I remember correctly, Bernhard mentioned some issues, which got 
'solved' by using less parallel builds (-j1 or -j4?).

With kind regards,

-------- Forwarded Message --------
Subject: enabling link time optimizations in package builds
Resent-Date: Fri, 17 Jun 2022 08:28:51 +0000 (UTC)
Resent-From: debian-devel at lists.debian.org
Date: Fri, 17 Jun 2022 10:18:43 +0200
From: Matthias Klose <doko at debian.org>
To: debian-devel at lists.debian.org <debian-devel at lists.debian.org>
CC: debian-ports at lists.debian.org <debian-ports at lists.debian.org>

Link time optimizations are an optimization that helps with a single 
digit percent number optimizing both for smaller size, and better speed. 
  These optimizations are available for some time now in GCC.  Link time 
optimizations are also at least turned on in other distros like Fedora, 
OpenSuse (two years) and Ubuntu (one year).

Details at https://wiki.debian.org/ToolChain/LTO

The proposal is to turn on LTO by default on most 64bit release 
architectures. Not proposing to do this on 32bit architectures because 
of the limited address space at link time, and up to now nobody tested 
LTO on 32bit archs.  In test rebuilds, there were 373 packages (dd-list 
in the wiki page) found not to build with link time optimizations for 
various reasons.  These range from easily fixable issues in symbols 
files to some upstream issues.  The idea is to fix as many of these as 
possible, and then change the packaging for the others to just turn off 
LTO in the package build.

To explicitly turn on LTO for a package build:

   export DEB_BUILD_MAINT_OPTIONS=optimize=+lto

to explicitly disable LTO:

   export DEB_BUILD_MAINT_OPTIONS=optimize=-lto

The idea is to file wishlist bug reports for those 373 packages and then 
see how far we get, and if it's feasible to already turn on LTO for 
bookworm.  If not, it should be turned on by default for the following 


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