Why is the second loop over a static array in the BSS faster than the first?

Question

I have the following code that writes a global array with zeros twice, once forward and once backward.

#include 
#include 
#inc         


        

Answer 1

If you modify the second loop to be identical to the first the effect is the same, the second loop is faster:

int main() {
   int i;

   clock_t t = clock();
   for(i = 0; i < SIZE; i++)
       c[i] = 0;
   t = clock() - t;
   printf("%d\n\n", t);

   t = clock(); 
   for(i = 0; i < SIZE; i++)
      c[i] = 0;
   t = clock() - t;
   printf("%d\n\n", t);
}

This is due to the first loop loading the information into the cache and that information being readily accessible during the second loop

Results of the above:

317841
277270

Edit: Leeor brings up a good point, c does not fit in the cache. I have an Intel Core i7 processor: http://ark.intel.com/products/37147/Intel-Core-i7-920-Processor-8M-Cache-2_66-GHz-4_80-GTs-Intel-QPI

According to the link, this means the L3 cache is only 8 MB, or 8,388,608 bytes and c is 100,000,000 bytes