attribute const

最新推荐文章于 2024-04-19 16:41:58 发布

最新推荐文章于 2024-04-19 16:41:58 发布 · 190 阅读

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本文探讨了使用__attribute__((const))标记函数的方法，该方法允许编译器优化重复调用的函数，尤其是那些数学函数，它们没有静态状态或副作用，且返回值完全由输入参数决定。文章还讨论了此属性使用的限制条件。

This attribute marks the function as considering only its numeric parameters. This is mainly intended for the compiler to optimize away repeated calls to a function that the compiler knows will return the same value repeatedly. It applies mostly to math functions that have no static state or side effects, and whose return is solely determined by the inputs.

In this highly-contrived example, the compiler normally must call the square() function in every loop even though we know that it's going to return the same value each time:

extern int square(int n) __attribute__((const));

...
	for (i = 0; i < 100; i++ )
	{
		total += square(5) + i;
	}

By adding __attribute__((const)), the compiler can choose to call the function just once and cache the return value.

In virtually every case, const can't be used on functions that take pointers, because the function is not considering just the function parameters but also the data the parameters point to, and it will almost certainly break the code very badly in ways that will be nearly impossible to track down.

Furthermore, the functions so tagged cannot have any side effects or static state, so things like getchar() or time() would behave very poorly under these circumstances.