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POManually optimize a nested loop
 

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    <p>I'm working on a homework assignment where I must manually optimize a nested loop (my program will be compiled with optimizations disabled). The goal of the assignment is to run the entire program in less than 6 seconds (extra credit for less than 4.5 seconds).</p> <p>I'm only allowed to change a small block of code, and the starting point is such:</p> <pre><code> for (j=0; j &lt; ARRAY_SIZE; j++) { sum += array[j]; } </code></pre> <p>Where <code>ARRAY_SIZE</code> is 9973. This loop is contained within another loop that is run 200,000 times. This particular version runs in 16 seconds.</p> <p>What I've done so far is change the implementation to unroll the loop and use pointers as my iterator:</p> <p>(These declarations are not looped over 200,000 times)</p> <pre><code> register int unroll_length = 16; register int *unroll_end = array + (ARRAY_SIZE - (ARRAY_SIZE % unroll_length)); register int *end = array + (ARRAY_SIZE -1); register int *curr_end; curr_end = end; while (unroll_end != curr_end) { sum += *curr_end; curr_end--; } do { sum += *curr_end + *(curr_end-1) + *(curr_end-2) + *(curr_end-3) + *(curr_end-4) + *(curr_end-5) + *(curr_end-6) + *(curr_end-7) + *(curr_end-8) + *(curr_end-9) + *(curr_end-10) + *(curr_end-11) + *(curr_end-12) + *(curr_end-13) + *(curr_end-14) + *(curr_end-15); } while ((curr_end -= unroll_length) != array); sum += *curr_end; </code></pre> <p>Using these techniques, I was able to get the execution down to 5.5 seconds, which will give me full credit. However; I sure do want to earn the extra credit, but I'm also curious what additional optimizations I can make that I might be overlooking?</p> <p>Edit #1 (Adding outer loop)</p> <pre><code> srand(time(NULL)); for(j = 0; j &lt; ARRAY_SIZE; j++) { x = rand() / (int)(((unsigned)RAND_MAX + 1) / 14); array[j] = x; checksum += x; } for (i = 0; i &lt; N_TIMES; i++) { // inner loop goes here if (sum != checksum) printf("Checksum error!\n"); sum = 0; } </code></pre>
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    1. CODoes the array being summed change each time? It would be helpful to see the outer loop too.
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    2. COThe array does not change. I've added the outer loop to the question.
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    3. COare you allowed to unroll the number generation loop as well? Also, what happens if you unroll the loop more than 10 times (say, 20)?
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      1. USKevin
 

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