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

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

[Sergei Trofimovich]

> 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

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

[Sergei Trofimovich]

On Sun, 19 Jan 2020 22:36:51 +0000
Sergei Trofimovich <slyfox@gentoo.org> 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

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

[Joonas Niilola]

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On 5/2/20 2:14 PM, Sergei Trofimovich wrote:
> 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!
>
Thanks for the heads-up!

-- juippis



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