[GHC] #12600: Overloaded method causes insufficient specialization

15 Sep 2016

      #12600: Overloaded method causes insufficient specialization
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           Reporter:  akio           |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The `foo` function in the following code does not get specialized
 completely by `ghc -O2`, even though all the overloaded functions in the
 module are marked `INLINE`. Specifically, it gets compiled into a call to
 a function with an `Eq Int` dictionary passed at runtime.

 {{{#!hs
 module Foo where

 class Eq1 f where
   eq1 :: (Eq a) => f a -> f a -> Bool

 data F a = F !a !a
 data G f a = G !(f a) !(f a)

 instance Eq1 F where
   eq1 = \(F a b) (F c d) ->
     -- In order to reproduce the problem, the body of this function needs
 to be
     -- large enough to prevent GHC from voluntarily inlining it.
     larger $ larger $ larger $ larger $ larger $ larger $
       a == c && b == d
   {-# INLINE eq1 #-}

 larger :: a -> a
 larger = id
 {-# NOINLINE larger #-}

 instance (Eq1 f) => Eq1 (G f) where
   eq1 = \(G a b) (G c d) -> eq1 a c && eq1 b d
   {-# INLINE eq1 #-}

 foo :: G F Int -> G F Int -> Bool
 foo a b = eq1 a b
 }}}

 Looking at the dumps, it looks like there may be a problem is the
 specializer. It creates a specialization of `eq1` with the type

 `(Eq a) => G F a -> G F a -> Bool`

 rather than the fully-specialized type:

 `G F Int -> G F Int -> Bool`

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12600
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