ex_reg            410 gc/mark.c            EXCEPTION_REGISTRATION ex_reg;
ex_reg            472 gc/mark.c            er.ex_reg.handler = mark_ex_handler;
ex_reg            473 gc/mark.c            asm volatile ("movl %%fs:0, %0" : "=r" (er.ex_reg.prev));
ex_reg            519 gc/mark.c            asm volatile ("mov %0, %%fs:0" : : "r" (er.ex_reg.prev));