Based on Stefan's ChatGPT result that matched what Andrew found in the source at "https://github.com/xerpi/SDL-3DS/blob/master/src/render/mmx.h", I checked this patch "0001-Use-m-constrain-instead-of-X-in-libzmpeg3-video-mmx..patch" into GIT.  I compared that source mmx.h with ours at libzmpeg3/video/mmx.h and am convinced that no other relevant changes would interfere with the mod changing X to m.  Did some testing not using ffmpeg, but probably not even applicable.

>
>         Can you try:
>
>         CFLAGS="-march=native -O3" CXXFLAGS="-march=native -O3" ./configure
>
>         gcc (Gentoo 15.2.1_p20251122 p4) 15.2.1 20251122
>
>
>         Both systems tested are AMD Ryzen 5, and older Athlon X4.
>
>     Of course you are right that it errs out with O3 (O2 obviously
>     works). Do you have a recommended fix? -- I can not code in C/C+ but
>     I can test. I tried looking online for a code solution that I could
>     understand and found nothing that made sense to me.

ChatGPT suggested something very simple just replace the =X with =m

#define mmx_r2m(op, reg, mem) \
     __asm__ __volatile__ ( \
         #op " %%" #reg ", %0" \
         : "=m" (mem) \
         : \
         : "memory")

May be, I think I moved some of it to =X because it was failing on i586 build 


this one uses "m" in those functions but I am not sure it was always correct.