>There needs to be a dependency on the index for duff's device to work right?
For it to work "right" the work inside the loop has to be quite minor. The point being that it saves on cpu cycles by reducing the number of times it does i < loop_size, i++, and the number of times it has to branch back to the beginning of the loop. It's almost always some kind of memory copy, or a simple read, lookup, write in which the inside of the loop will complete in a few cpu cycles.
If the work inside the loop is 100x the cost of implementing the loop there is no point.
For it to work "right" the work inside the loop has to be quite minor. The point being that it saves on cpu cycles by reducing the number of times it does i < loop_size, i++, and the number of times it has to branch back to the beginning of the loop. It's almost always some kind of memory copy, or a simple read, lookup, write in which the inside of the loop will complete in a few cpu cycles.
If the work inside the loop is 100x the cost of implementing the loop there is no point.