Re: [gentoo-dev] CFLAGS=-fno-common related breakage is incoming

[Sergei Trofimovich]

On Sun, 19 Jan 2020 22:36:51 +0000
Sergei Trofimovich <slyfox@g.o> wrote:

> > 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.

With Toralf's help we now have rough estimate of broken packages. It's about
450 yet unfixed ones:
    https://bugs.gentoo.org/showdependencytree.cgi?id=705764&hide_resolved=1

gcc-10 will be released soon. Maybe in a week.

Please look at the broken list and fix your packages.

Thanks!

-- 

  Sergei