unroll nested for loops in C++
- by Hristo
How would I unroll the following nested loops?
for(k = begin; k != end; ++k) {
for(j = 0; j < Emax; ++j) {
for(i = 0; i < N; ++i) {
if (j >= E[i]) continue;
array[k] += foo(i, tr[k][i], ex[j][i]);
}
}
}
I tried the following, but my output isn't the same, and it should be:
for(k = begin; k != end; ++k) {
for(j = 0; j < Emax; ++j) {
for(i = 0; i+4 < N; i+=4) {
if (j >= E[i]) continue;
array[k] += foo(i, tr[k][i], ex[j][i]);
array[k] += foo(i+1, tr[k][i+1], ex[j][i+1]);
array[k] += foo(i+2, tr[k][i+2], ex[j][i+2]);
array[k] += foo(i+3, tr[k][i+3], ex[j][i+3]);
}
if (i < N) {
for (; i < N; ++i) {
if (j >= E[i]) continue;
array[k] += foo(i, tr[k][i], ex[j][i]);
}
}
}
}
I will be running this code in parallel using Intel's TBB so that it takes advantage of multiple cores. After this is finished running, another function prints out what is in array[] and right now, with my unrolling, the output isn't identical. Any help is appreciated.
Thanks,
Hristo
