I"ve been doing some work to finish up my prefetch patch that  I hope to land in 7.8, and one design matter thats come up is what the optimization semantics should be!

is it appropriate to duplicate prefetches? (probably yes)

is it appropriate to float OUT prefetched? (probably not?)

operations that are dupable which shouldn't float out are marked with the can_fail= true

property,

operations which cannot be safely floated out and can't be duped are marked as having side effects.

on the GHC irc channel, Duncan was patient enough to walk my through understanding this, and this makes me lean towards the conclusion that duplication is ok, but floating out isnt. Thus all the prefetch ops (pure or not) should have the attribute "can fail", rather than no attribut or "has effects".

this is exploiting that "can fail"  really just means "don't float out".

thoughts and counter points?

thanks

-Carter