how to write a ghc primop that acts on an unevaluated argument?

[Carter Schonwald]

Hey All,
as part of trying to get some fixups for how prefetch works into 7.10,
i'm adding a "prefetchValue" primop that prefetchs the memory location of a
lifted heap value

namely

several operations of the following form

primop PrefetchValueOp1 "prefetchValue1#" GenPrimOp
   a -> State# s -> State# s
   with strictness  = { \ _arity -> mkClosedStrictSig [botDmd, topDmd]
topRes }

I'd like some feedback on the strictness information design by someone
who's familiar with how that piece of GHC. the idea being that
prefetchValue is lazy in its polymorphic argument (it doesn't force it, it
just does a prefetch on the heap location, which may or may not be
evaluated).

https://phabricator.haskell.org/D350

is the code in question. And i *believe* i'm testing for being lazy in that
argument correctly.

thoughts?

many thanks!
-Carter
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