__align_down
pcb = (struct pcb *)__align_down(td->td_kstack + td->td_kstack_pages *
thread0.td_pcb = (struct pcb *)__align_down(thread0.td_kstack +
pcb = (struct pcb *)__align_down(td2->td_kstack +
#define rounddown2(x, y) __align_down(x, y) /* if y is power of two */
#define STACKALIGN(p) (__align_down(p, STACKALIGNBYTES + 1))