REX_B
#define REX_BW (REX_PREFIX | REX_B | REX_W)
#define REX_BRW (REX_PREFIX | REX_B | REX_R | REX_W)
if (rex_prefix & REX_B)
if (r_m != NULL && (REX_B & rex_prefix) != 0)
if (x->d86_rex_prefix & REX_B)
x->d86_rex_prefix |= REX_B;
x->d86_rex_prefix |= REX_B;