递归函数可以通过内联? [英] Can a recursive function be inline?
问题描述
inline int factorial(int n)
{
if(!n) return 1;
else return n*factorial(n-1);
}
当我在读这,发现上面code会导致无限汇编如果不通过编译器正确处理。
As I was reading this, found that the above code would lead to "infinite compilation" if not handled by compiler correctly.
编译器如何决定是否内联函数或不?
How does the compiler decide whether to inline a function or not ?
推荐答案
首先,在一个函数在线
规范只是一个提示。编译器可以(而且经常)完全忽略了在线
预选赛的presence与否。随着中说,一个编译器的可以的内联递归函数,就像它可以展开一个无限循环。它只是放置一个上限水平,它会展开功能。
First, the inline
specification on a function is just a hint. The compiler can (and often does) completely ignore the presence or absence of an inline
qualifier. With that said, a compiler can inline a recursive function, much as it can unroll an infinite loop. It simply has to place a limit on the level to which it will "unroll" the function.
这是优化编译器可能会将这个code:
An optimizing compiler might turn this code:
inline int factorial(int n)
{
if (n <= 1)
{
return 1;
}
else
{
return n * factorial(n - 1);
}
}
int f(int x)
{
return factorial(x);
}
这个code:
int factorial(int n)
{
if (n <= 1)
{
return 1;
}
else
{
return n * factorial(n - 1);
}
}
int f(int x)
{
if (x <= 1)
{
return 1;
}
else
{
int x2 = x - 1;
if (x2 <= 1)
{
return x * 1;
}
else
{
int x3 = x2 - 1;
if (x3 <= 1)
{
return x * x2 * 1;
}
else
{
return x * x2 * x3 * factorial(x3 - 1);
}
}
}
}
在这种情况下,我们基本上是内联函数的3倍。一些编译器的不的执行该优化。我记得MSVC ++有一个设置调整,将在递归函数(最多20个,我相信)进行内联的水平。
In this case, we've basically inlined the function 3 times. Some compilers do perform this optimization. I recall MSVC++ having a setting to tune the level of inlining that would be performed on recursive functions (up to 20, I believe).
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