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cperformancememset

Is memset() more efficient than for loop in C?

Is memset() more efficient than for loop.

Considering this code:

char x[500];
memset(x,0,sizeof(x));

And this one:

char x[500];
for(int i = 0 ; i < 500 ; i ++) x[i] = 0;

Which one is more efficient and why? Is there any special instruction in hardware to do block level initialization.

Solution

  • Most certainly, memset will be much faster than that loop. Note how you treat one character at a time, but those functions are so optimized that set several bytes at a time, even using, when available, MMX and SSE instructions.

    I think the paradigmatic example of these optimizations, that go unnoticed usually, is the GNU C library strlen function. One would think that it has at least O(n) performance, but it actually has O(n/4) or O(n/8) depending on the architecture (yes, I know, in big O() will be the same, but you actually get an eighth of the time). How? Tricky, but nicely: strlen.