[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):

 Thank you for your attention, Simon.

 I've confirmed your example: these modules `A` and `B` give the same
 behavior: the specialization in `A` is a loopbreaker and the rule only
 fires once in `B`.

 {{{#!hs
 {-# Language MagicHash #-}
 {-# OPTIONS_GHC -fspec-constr #-}
 module A where
 import GHC.Prim
 import GHC.Types

 f :: [a] -> Int#
 f [] = 0#
 f (x:xs) = f xs

 f' a b = f (a:b)   -- a call pattern to specialize
 }}}

 {{{#!hs
 {-# Language MagicHash #-}
 module B where
 import GHC.Types
 import A

 boo = I# (f [1,2,3,4,5,6,6])
 }}}

 And your explanation makes total sense: unexpected supercompilation could
 have terrible consequences. Also, that's something I'm usually aware of,
 when I'm not wearing my blinders :).

 ----

 The `SPECIALIZE INLINE` alternative I mentioned in comment:4 is
 interesting. It's possible as a "workaround" for `lengthVL` precisely
 because the type constructors (of the spine) are 1-to-1 with the data
 constructors; thus `SPECIALIZE` can be used to emulate `-fspec-constr`.

 ----

 I opened #13016 regarding the `SPECIALIZE INLINE` specialization being a
 loopbreaker -- that seems like a bug.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13014#comment:7>
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