https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111096
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
#define FIXED_REGISTERS \
{ \
0, 0, 0, 0, 0, 0, 0, 0, /* R0 - R7 */ \
0, 0, 0, 0, 0, 0, 0, 0, /* R8 - R15 */ \
0, 0, 0, 0, 0, 0, 0, 0, /* R16 - R23 */ \
0, 0, 0, 0, 0, 1, 0, 1, /* R24 - R30, SP */ \
It is specifically marked as a fixed register ...
