There's an interesting interaction with implicit parallelism. For a given reduction to occur at most once you need communication across threads so that two don'

There's an interesting interaction with implicit parallelism. For a given reduction to occur at most once you need communication across threads so that two don't do the same reduction at once. I suspect at most once per hardware thread is a faster choice - repeating some calculation for decreased communication. Or you need a static assignment of reduction to threads which means predicting how long each reduction will take.