清除一个小整数数组 : memset vs. for 循环

Question

有两种方法可以将整数/ float 组清零:

memset(array, 0, sizeof(int)*arraysize);

或:

for (int i=0; i <arraysize; ++i)
    array[i]=0;

显然，memset 对于大的arraysize 更快。然而，什么时候 memset 的开销实际上大于 for 循环的开销？例如，对于大小为 5 的数组——哪个最好？第一个、第二个，甚至可能是未滚动的版本:

array[0] = 0;
array[1] = 0;
array[2] = 0;
array[3] = 0;
array[4] = 0;

Answer 1

很有可能，memset() 将被您的编译器内联(大多数编译器将其视为“内部”，这基本上意味着它是内联的，除非可能处于最低优化或除非明确禁用)。

例如，这里有一些release notes from GCC 4.3 :

Code generation of block move (memcpy) and block set (memset) was rewritten. GCC can now pick the best algorithm (loop, unrolled loop, instruction with rep prefix or a library call) based on the size of the block being copied and the CPU being optimized for. A new option -minline-stringops-dynamically has been added. With this option string operations of unknown size are expanded such that small blocks are copied by in-line code, while for large blocks a library call is used. This results in faster code than -minline-all-stringops when the library implementation is capable of using cache hierarchy hints. The heuristic choosing the particular algorithm can be overwritten via -mstringop-strategy. Newly also memset of values different from 0 is inlined.

编译器可能会对您提供的替代示例做类似的事情，但我敢打赌它不太可能。

而且它是 grep 的，一眼就能看出启动的意图(并不是说循环特别难以理解)。