Re: [FFmpeg-devel] [PATCH] configure: Enable -fno-common for Darwin targets, avoid linker warnings

[Marvin Scholz <epirat07@gmail.com>]

On 5 May 2025, at 10:45, Martin Storsjö wrote:

> On Fri, 2 May 2025, Martin Storsjö wrote:
>
>> On Tue, 29 Apr 2025, Martin Storsjö wrote:
>>
>>> Since GCC 10 and llvm.org Clang 11, -fno-common is the default.
>>> However Apple's Xcode Clang hasn't followed suit yet, and still
>>> defaults to -fcommon.
>>>
>>> Compiling with -fcommon causes uninitialized global variables to
>>> be treated as "common" (which allows multiple object files to have
>>> similar definitions).
>>>
>>> Common variables seem to have the issue that their intended alignment
>>> isn't signaled, so the linker assumes that they may need alignment
>>> according to their full size.
>>>
>>> With large global tables, this can lead to linker warnings like
>>> this, with Xcode 16.3:
>>>
>>>    ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment
>>>
>>> This can be reproduced with a small snippet like this:
>>>
>>>    char table[16385];
>>>    int main(int argc, char* argv[]) { return 0; }
>>>
>>> Compiling with -fno-common avoids this issue and warning, and
>>> matches the default behaviour of other compilers. (Compiling with
>>> -fno-common also avoids the risk of accidentally accepting
>>> duplicate definitions of global variables, as long as they are
>>> uninitialized.)
>>> ---
>>> configure | 7 +++++++
>>> 1 file changed, 7 insertions(+)
>>
>> Will push soon.
>
> Pushed. I'll go ahead and backport this to a couple release branches as well.

Good idea, thanks for the warning fix.

>
> // Martin
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