Mailing List Archive

CFLAGS=-fno-common related breakage is incoming
> What is happening?

gcc-10 is coming soon. It will be more disruptive than gcc-9.

One of the major changes is the switch from C{,XX}FLAGS=-fcommon
to C{,XX}FLAGS=-fno-common by default: https://gcc.gnu.org/PR85678

It's a planned change and not a gcc regression. It will expose some
warts on old code and unblock minor optimisations accessing globals.

The change has already happened in gcc trunk.

> Is my package affected? Should I do anything?

You can check proactively if your packages are affected.

Add -fno-common to your make.conf's C{,XX}FLAGS and
see if things still build.

The typical symptom is a linker failure on multiple definitions
for some global variable:

ld: a.o:(.bss+0x0): multiple definition of `a'; main.o:(.data+0x0): first defined here

> How to fix it?

https://wiki.gentoo.org/wiki/Gcc_10_porting_notes/fno_common contains
some examples. Ideally code will need a few 'extern' additions and maybe
moving variable definitions.

Example of proposed openrc fix:
https://github.com/OpenRC/openrc/pull/348

Adding 'append-flags -fcommon' might work as a temporary workaround.

> Can I help?

Glad you asked! We will need to gather failed packages and fix them
upstream and downstream. Here is what you can do:

1. Add -fno-common to your make.conf's C{,XX}FLAGS
2. Build packages you maintain
3. Fix a bug upstream (or report a failure).
4. Pull a fix downstream (or file a bug and add it to the tracker).

> What is already known to be broken? Can I look at example fixes?

See https://wiki.gentoo.org/wiki/Gcc_10_porting_notes/fno_common
for an artificial example.

Gentoo tracker bug of known issues:
https://bugs.gentoo.org/705764
15 bugs so far.

SUSE tracker bug of known issues:
https://bugzilla.suse.com/show_bug.cgi?id=1160244
95 bugs so far. A good source of packages to check against the
ones you care about.

> What does -fcommon do?

Look up -fcommon in https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html

> I have no idea why my package broke. The error does not make sense.

Feel free to CC toolchain@ on a bug you observe and we'll try to sort it out.

--

Sergei