[rb-general] got a reproducible gcc

[Bernhard M. Wiedemann]

Hi,

over the past weeks, I made great progress towards reproducible gcc-8 builds

Apart from this 8 year old libtool patch
https://gcc.gnu.org/ml/gcc-patches/2018-06/msg01115.html


and embedded mtime timestamps in ada-related files
that could be avoided with a single touch -d at 1

https://build.opensuse.org/package/view_file/home:bmwiedemann:reproducible:test:gccnoprofilevalues/gcc8/reproducible.patch?expand=1


There is the issue with 'profiledbootstrap'
described in general at
https://github.com/bmwiedemann/theunreproduciblepackage/tree/master/pgo
(including a surprisingly simple example)

and our related bug
https://bugzilla.opensuse.org/show_bug.cgi?id=1040589

One interesting problem with PGO is that you can avoid irreproducibility
by disabling it, but then you lose 8% performance (measured for gcc8).
That is a hard trade-off.


But now I found that disabling ASLR together with non-parallelized
profiling and gcc CFLAGS: -fprofile-update=atomic -fprofile-generate

still gives a reproducible build with PGO enabled.
maybe the -fprofile-update=atomic CFLAGS is not actually needed.


I hope in the future our (SUSE (maybe with upstream)) compiler-guys will
find ways to make the profiling code even more deterministic.
E.g. there seems to be strange merging code for profile counters that is
not commutative so a+b != b+a so that ordering matters.

All my testing is using our highly normalized build-env
(same user, build-path, umask, locale, timezone)
and likely it requires this patch https://savannah.gnu.org/bugs/?52076
to keep the make order stable

With all that, it is already possible to build bit-identical
profile-optimized gcc8 packages.

Ciao
Bernhard M.