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

[Martin Storsjö]

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.

// Martin