[Bug lto/66305] -ffat-lto-objects create unreproducible objects

["lunar at debian dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66305

--- Comment #3 from lunar at debian dot org ---
Richard Biener:
> I think they become deterministic with -frandom-seed=0 for example.
> They are not deterministic to support partial linking of LTO objects as far
> as I know.

They are indeed reproducible with `-frandom-seed=0`. But I guess there's a
downside to that, right?

Andi:
> In theory it would be possible to use the hash of the full output path name or
> similar, which would be a bit more deterministic, but there are still
> ways this could break (e.g. if someone copies object files around)

Would using a hash over the section content work? In any cases, in the context
of Debian (this applies for FreeBSD as well), we have a canonical build path
so it would probably be fine to use it as the source of the hash.

I guess one could already do this without further help by giving the
build path to `-frandom-seed=`. This only would need some Makefile trickery.

> How about your "deterministic build" tools just learn to ignore that
> suffix?

I am not sure I understand what you are talking about. We want to get
byte-for-byte identical packages on each build (provided the same software
versions are used), so we need a given version of GCC to always produce the
same output.