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

["Martin Storsjö" <martin@martin.st>]

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.

// Martin
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