[GHC] #15151: Better Interaction Between Specialization and GND

[GHC]

#15151: Better Interaction Between Specialization and GND
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        Reporter:  andrewthad        |                Owner:  (none)
            Type:  feature request   |               Status:  infoneeded
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 What Andrew means is that the desired machine-code for `sortPeople` is
 bit-for-bit identical with that of `sortInt`; and the SPECIALISE pragma in
 `Sort.hs` has already made a nice specialised copy of the latter. He just
 wants to reuse it.

 Of course, the reason for creating the `PersonId` newtype might be
 precisely the have a ''different'' `Ord` instance that the underlying
 `Int` type.  Then it would be wrong to use `sortInt`.   But if the `Ord`
 instance is derived, esp via GND, then it ''is'' sound to use `sortInt`.

 I don't see an easy way to achieve exactly what you want. But there are
 two workarounds.

 * As you say, use INLINABLE on `sort`; and specialise (even with an
 explicit SPECIALISE pragma) in the module where `PersonId` is defined.

 * Define `sortPeople` like this
 {{{
 sortPeople :: MutablePrimArray s PersonId -> MutablePrimArray s PersonId
 sortPeople = coerce (sort @Int)
 }}}
   The `sort @Int` will get the efficient specialised version you want; and
 the `coerce` will lift it to the type you want.  And if you want GHC to
 use `sortPeople` you'd have to add
 {{{
 {-# RULES "sort/PersonId" sort @PersonId = sortPeople #-}
 }}}
   Interestingly, this would ''completely ignore'' any `Ord` instance for
 `PersonId`; indeed it doesn't need to have one.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15151#comment:4>
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