C loop optimization help for final assignment (with compiler optimization disabled)

Question

So for my final assignment in my Computer Systems class, we need to optimize these for loops to be faster than the original.

The basic grade is under 7 seconds and t

Answer 1

You may be on the right track, though you'll need to measure it to be certain (my normal advice to measure, not guess seems a little superfluous here since the whole point of the assignment is to measure).

Optimising compilers will probably not see much of a difference since they're pretty clever about that sort of stuff but, since we don't know what optimisation level it will be compiling at, you may get a substantial improvement.

To use pointers in the inner loop is a simple matter of first adding a pointer variable:

register double *pj;

then changing the loop to:

for (pj = &(array[0]); pj < &(array[ARRAY_SIZE]); j++) {
        sum += *j++;
        sum1 += *j++;
        sum2 += *j++;
        sum3 += *j++;
        sum4 += *j++;
        sum5 += *j++;
        sum6 += *j++;
        sum7 += *j++;
        sum8 += *j++;
        sum9 += *j;
    }

This keeps the amount of additions the same within the loop (assuming you're counting += and ++ as addition operators, of course) but basically uses pointers rather than array indexes.

With no optimisation¹ on my system, this drops it from 9.868 seconds (CPU time) to 4.84 seconds. Your mileage may vary.

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¹ With optimisation level -O3, both are reported as taking 0.001 seconds so, as mentioned, the optimisers are pretty clever. However, given you're seeing 5+ seconds, I'd suggest it wasn't been compiled with optimisation on.

As an aside, this is a good reason why it's usually advisable to write your code in a readable manner and let the compiler take care of getting it running faster. While my meager attempts at optimisation roughly doubled the speed, using -O3 made it run some ten thousand times faster :-)