> Does it compile with -O2, -march=native ?
>
> May be aggressive optimizations broke some old calling conventions
> between asm and C?

No, I think it is GCC 15 which is more strict on register usage. The
compile fix is rather trivial. But I need a way to check this codepath.

Just FYI, compiling with whatever the default optimization level (used in CinGG) with gcc version 15.2.1 on Fedora 42 does not report this error. Although I do not know how to check the codepath to make sure slice/mmx is utilized, it could just be loading/playing/rendering not using ffmpeg, but rather "use ffmpeg last" for loading and rendering "generic mpeg2".