> For maximum efficiency you'd avoid addc like hell, because it blocks internal precomputation, and use guarantees like he did, avoiding overflow better.

Whether that is the case depends on the CPU. I hit memory bandwidth limits before adc becomes a problem. But I concede that you have a valid point and that there are definitely cases where leaving sufficient space for carries is the better strategy.

Anyway, we can probably both agree that "% 1000000000" is not a great idea.