[GHC] #13014: Seemingly unnecessary marking of a SpecConstr specialization as a loopbreaker

[GHC]

#13014: Seemingly unnecessary marking of a SpecConstr specialization as a
loopbreaker
-------------------------------------+-------------------------------------
        Reporter:  nfrisby           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  SpecConstr
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nfrisby):

 I'm seeing the same behavior if I disable `-fspec-constr` and instead
 declare.

 {{{#!hs
 {-# SPECIALIZE INLINE lengthVL :: SPEC -> VL (a ': as) -> Int# #-}
 }}}

 I get the expected rule.

 {{{#!hs
 ==================== Tidy Core rules ====================
 "SPEC lengthVL" [ALWAYS]
     forall (@ k) (@ (a :: k)) (@ (as :: [k])).
       lengthVL @ k @ (a : as)
       = lengthVL_$slengthVL @ k @ a @ as
 }}}

 but the specialization `lengthVL_$slengthVL` is again marked as a
 loopbreaker, and so it only fires once in the definition of `test`, as
 with the `-fspec-constr` route.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13014#comment:4>
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