请教一下大神线程初始化时栈地址为什么加4呢

static rt_err_t _rt_thread_init(struct rt_thread *thread,
const char *name,
void (*entry)(void *parameter),
void *parameter,
void stack_start,
rt_uint32_t stack_size,
rt_uint8_t priority,
rt_uint32_t tick)
{
/ init thread list */
rt_list_init(&(thread->tlist));
thread->entry = (void )entry;
thread->parameter = parameter;
/ stack init /
thread->stack_addr = stack_start;
thread->stack_size = stack_size;
/ init thread stack */
rt_memset(thread->stack_addr, '#', thread->stack_size);
#ifdef ARCH_CPU_STACK_GROWS_UPWARD
thread->sp = (void *)rt_hw_stack_init(thread->entry, thread->parameter,
(void *)((char *)thread->stack_addr),
(void *)rt_thread_exit);
#else
thread->sp = (void *)rt_hw_stack_init(thread->entry, thread->parameter,
(void *)((char *)thread->stack_addr + thread->stack_size - 4),
(void *)rt_thread_exit);
#endif

根据宏定义的不同，栈初始化函数的参数不一样。一个是stackaddr，一个是stackaddr+stacksize-4.
在rt_hw_stack_init函数里，
stk = stack_addr + sizeof(rt_uint32_t);
说明前面减去的4，在这里被加回来了。
那看条件编译的第一个分支，栈向上生长的话，没有减4，相当于在函数里单纯的加了个4，为什么要加个4呢