[FFmpeg-devel] [PATCH] Add -fno-common to PE32 CFLAGS

[Ramiro Polla]

Hi,

On Mon, Apr 13, 2009 at 5:31 PM, Baptiste Coudurier
<baptiste.coudurier at gmail.com> wrote:
> On 4/13/2009 12:48 PM, Baptiste Coudurier wrote:
>> On 4/8/2009 6:42 PM, Ramiro Polla wrote:
>>> Some time ago Art Clarke sent a patch to unconditionally add
>>> -fno-common to mingw32 builds because of a bug in GCC. Instead of
>>> applying the patch, the documentation was updated to reflect this bug
>>> and the workaround.
>>>
>>> The bug details can be found here:
>>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37216
>>>
>>> I finally took some time to investigate what combination of
>>> gcc+binutils triggers the bug. Just like Brian Dessent mentions in
>>> comment #6, the bug appears with binutils starting November 2007. The
>>> gcc version doesn't matter as long as it puts data in .comm. binutils
>>> 2.18 doesn't show this bug, but it has other shortcomings which make
>>> it not fit for building FFmpeg.
>>>
>>> With current mingw32 binutils floating around the web, all builds will
>>> crash at some point in some optimizations (maybe Baptiste can give us
>>> a sample that really crashes, I haven't seen them yet, but there are
>>> lots of reports).
>>>
>>> So is it ok to unconditionally include this flag with attached patch
>>> (and a very descriptive log message) before binutils is fixed?
>>
>> Yes, please many bug reports on windows because of this, shared libs are
>> just broken without it.
>>
>> You might remove it for OS/2 though according to Dave Yeo.
>
> I think you could also only add it for shared libs. I don't recall it
> was needed for static builds.

The bug on gcc+binutils shows itself independent of being in a .dll or
in the .exe itself, so I assume this bug doesn't appear on static
FFmpeg builds purely by luck.

I updated the patch removing the flag from OS/2 as suggested by Dave Yeo.

The commit message would be:
"win32, dos: Work around a bug in the GNU toolchain.
The bug[1] is exposed when gcc decides to put some data in a common
section (i.e. data that will be used by more than one object). It will
suggest the data to be aligned, but binutils >= 2.19 will fail to
properly align it. Thus if the data requires any alignment greater
than 4, the application will crash.
The workaround prevents gcc from putting data in a common section,
instead putting it in a properly aligned section.

[1] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37216"

I'll commit this if I get an explicit ok from the build system
maintainers, because I don't want them bugging us later on if they
don't like this hack. And if they're not ok with it, then please
suggest an alternative solution.

Ramiro Polla
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