[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 carter):

 Simon, good points.

 No i don't have a benchmark showing that adding the 0-3 levels is
 justified, though I could probably cook up one that shows a measurable
 (albeit small) difference when I have time. The prior LLVM only prefetch
 operation was equivalent to the {{{ prefetch*3# }}} family of operations.

 The 0-3 stuff also in part motivated as having a simple concrete thorn to
 motivate the "Static literal data" extension I think we need so as to
 properly support some other interesting primops in a somewhat type safe
 way, and prefetch is the simplest example, where exposing only 4 variants
 would still be useful.  So assuming I work out that Static data extension
 right, We could have something like  {{{ prefetchByteArray :: Static Int#
 -> ByteArray# -> Int# ->ByteArray# }}}  rather than the 0-3 variants for
 7.10. So yes, I agree that the number suffix stuff is a bit of a hack, and
 its one that motivates some more systemic solutions going forward.

 On the can_fail, explained that way, I think you're absolutely right (or
 at least more likely right than I).
 I'll adjust that branch accordingly later today/tomorrow and link to the
 diff again.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8256#comment:43>
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