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 ...