just for reference, the current ops are

primop PrefetchByteArrayOp2 "prefetchByteArray2#" GenPrimOp

ByteArray# -> Int# -> ByteArray#

with llvm_only = True

primop PrefetchMutableByteArrayOp2 "prefetchMutableByteArray2#" GenPrimOp

MutableByteArray# s -> Int# -> State# s -> State# s

with has_side_effects = True

llvm_only = True

primop PrefetchAddrOp2 "prefetchAddr2#" GenPrimOp

Addr# -> Int# -> Addr#

with llvm_only = True

one of the things my patch (for 7.8) will do is add preliminary native code gen support for prefetch, admittedly in the form of Nops. (later work will fix that to be a bit more actionable)

any opinions that help us suss this out would be appreciated

On Mon, Sep 16, 2013 at 1:08 AM, Carter Schonwald <carter.schonwald@gmail.com> wrote:

Hey all,
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