[GHC] #8256: adding locality levels to prefetch# and friends

[GHC]

#8256: adding locality levels to prefetch# and friends
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        Reporter:  carter            |            Owner:  carter
            Type:  task              |           Status:  patch
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #8252 ,#8107
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Comment (by thoughtpolice):

 This patch looks to be mostly in pretty good shape, so I think it can go
 in with a little more work.

 In regards to point 1 from @tibbe, IMO it's perfectly reasonable to just
 have the code generator emit an empty piece of code for the other
 backends. prefetch can't change the semantics, only the performance - so
 simply emitting a no-op in the other backends is a perfectly legitimate
 thing to do, and easy. I'm not aware of any hardware we support where
 prefetching isn't strictly an optimization.

 As for names? I don't really mind that much. The usage of the 0-3
 constants for locality argument comes from GCC, where `__builtin_prefetch`
 has a locality argument ranging from 0 to 3. There is no real mnemonic
 here: 3 means it should be left in cache if possible as it has high
 locality, 0 means it can be evicted after access (low locality,) and 1-2
 are just "somewhere inbetween 0 and 3." I imagine LLVM inherited this
 convention from GCC, and I'm not sure what else we could sensibly call
 them.

 That said, we do need a test, and the primops need docs. There are also a
 lot of typos and random comments laying around which would be nice to get
 cleaned up (mostly punctuation, stuff like that.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8256#comment:11>
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