Linux sentinel写法
在linux驱动里我们经常能看到类似下面的写法:
static const struct of_device_id asensm6_of_match[] = {{ .compatible = DRIVER_COMPATIBLE },{ /* sentinel */ },
};static const struct of_device_id rockchip_pinctrl_dt_match[] = {{ .compatible = "rockchip,px30-pinctrl",.data = &px30_pin_ctrl },{ .compatible = "rockchip,rv1108-pinctrl",.data = &rv1108_pin_ctrl },{ .compatible = "rockchip,rv1126-pinctrl",.data = &rv1126_pin_ctrl },{ .compatible = "rockchip,rk1808-pinctrl",.data = &rk1808_pin_ctrl },{ .compatible = "rockchip,rk2928-pinctrl",.data = &rk2928_pin_ctrl },{ .compatible = "rockchip,rk3036-pinctrl",.data = &rk3036_pin_ctrl },{ .compatible = "rockchip,rk3066a-pinctrl",.data = &rk3066a_pin_ctrl },{ .compatible = "rockchip,rk3066b-pinctrl",.data = &rk3066b_pin_ctrl },{ .compatible = "rockchip,rk3128-pinctrl",.data = (void *)&rk3128_pin_ctrl },{ .compatible = "rockchip,rk3188-pinctrl",.data = &rk3188_pin_ctrl },{ .compatible = "rockchip,rk3228-pinctrl",.data = &rk3228_pin_ctrl },{ .compatible = "rockchip,rk3288-pinctrl",.data = &rk3288_pin_ctrl },{ .compatible = "rockchip,rk3308-pinctrl",.data = &rk3308_pin_ctrl },{ .compatible = "rockchip,rk3328-pinctrl",.data = &rk3328_pin_ctrl },{ .compatible = "rockchip,rk3368-pinctrl",.data = &rk3368_pin_ctrl },{ .compatible = "rockchip,rk3399-pinctrl",.data = &rk3399_pin_ctrl },{},
};
可以看到数组声明初始化时的尾部会有一个空白的{}
, 对于GCC
编译器来说它会被初始化为0
,也就是说{}
等价于{.a = 0, .b = 0}
, linux
的用意在于防止数组越界以及不需要去动态计算大小。
例如我们现在想遍历一个数组,那么终止条件就是判断当前是否为结束符,如果是结束符则遍历结束。
在声明中使用哨兵主要目的简化链表
与数组
的算法步骤,使得判断终止变得比较单一
了,无论什么条件只要遇到了哨兵那就意味着这个数组的遍历该结束
了,因为判断比较单一这样能够提升CPU分支预测
的命中率
,因为如果判断语句较为复杂会导致CPU分支预测失败率提高,当分支预测失败时CPU为了保证程序能够正常运行下去会重新回滚
去执行这就间接导致了这个代码被放入热路径
的概率会非常小。
此外在遍历数组时不需要去关心它的大小了,也不需要去计算
,只需要简单的判断是否指向了哨兵
元素就可以了,因为它判断的单一性,会让CPU很容易预测并将代码放入热路径(CPU缓存)里加快寻址与执行过程让循环代码变得更紧凑
。