__BIT_FLS
#define BIT_FLS(_s, p) __BIT_FLS(_s, p)
#define CPU_FLS(p) __BIT_FLS(CPU_SETSIZE, p)
#define DOMAINSET_FLS(p) __BIT_FLS(DOMAINSET_SETSIZE, p)
id = __BIT_FLS(i, set) - 1;