[GHC] #7619: Make worker-wrapper unbox data families

23 Jan 2013

      #7619: Make worker-wrapper unbox data families
-----------------------------+----------------------------------------------
Reporter:  akio              |          Owner:                  
    Type:  feature request   |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.7               |       Keywords:  type family     
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
-----------------------------+----------------------------------------------
 I noticed that the worker-wrapper optimization doesn't unbox arguments
 whose type is a data family instance. For example in this module:

 {{{
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE BangPatterns #-}
 module Foo where
 data family Foo a

 data instance Foo Int = FooInt Int Int

 foo :: Foo Int -> Int
 foo (FooInt a b) = loop a b
     where
         loop 0 y = length $ replicate y b
         loop x !y = loop (mod y x) x

 foo1 :: (Int, Int) -> Int
 foo1 (a, b) = loop a b
     where
         loop 0 y = length $ replicate y b
         loop x !y = loop (mod y x) x
 }}}

 foo and foo1 both get worker-wrapper applied, with worker functions of the
 following types:

 {{{
 $wfoo :: Foo Int -> Int#
 $wfoo1 :: Int# -> Int# -> Int#
 }}}

 It would be nice if $wfoo could get the same type as $wfoo1.

 This issue happened in real life with unboxed vectors from the vector
 package, resulting in a lot of boxing with unboxed vector constructors
 immediately followed by unboxing.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7619
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