[GHC] #12990: Partially applied constructors with unpacked fields simplified badly

[GHC]

#12990: Partially applied constructors with unpacked fields simplified badly
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        Reporter:  dfeuer            |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  Inlining
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  D2891
       Wiki Page:                    |
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Description changed by bgamari:

@@ -13,1 +13,1 @@
- {{{
+ {{{#!hs

New description:

 When a constructor is partially applied to at least one unpacked field,
 the simplifier produces pretty lousy results.

 {{{#!hs
 data Foo a = Foo !Int a

 silly :: ((a -> Foo a) -> r) -> r
 silly f = f (Foo 3)
 }}}

 compiles to

 {{{#!hs
 -- RHS size: {terms: 9, types: 7, coercions: 0}
 $WFoo :: forall a. Int -> a -> Foo a
 $WFoo =
   \ (@ a_aqI) (dt_ayl :: Int) (dt_aym :: a_aqI[sk:1]) ->
     case dt_ayl of { I# dt_ayn -> Foo dt_ayn dt_aym }

 -- RHS size: {terms: 2, types: 0, coercions: 0}
 silly2 :: Int
 silly2 = I# 3#

 -- RHS size: {terms: 3, types: 3, coercions: 0}
 silly1 :: forall a. a -> Foo a
 silly1 = \ (@ a_ayG) -> $WFoo silly2

 -- RHS size: {terms: 5, types: 9, coercions: 0}
 silly :: forall a r. ((a -> Foo a) -> r) -> r
 silly =
   \ (@ a_ayG) (@ r_ayH) (f_axY :: (a -> Foo a) -> r) -> f_axY silly1
 }}}

 That is, GHC represents `Foo 3` as a closure containing a ''boxed'' `Int`.
 Manually eta-expanding would fix it.

 {{{#!hs
 silly :: ((a -> Foo a) -> r) -> r
 silly f = f (\x -> Foo 3 x)
 }}}

 compiles to

 {{{#!hs
 -- RHS size: {terms: 5, types: 4, coercions: 0}
 silly1 :: forall a. a -> Foo a
 silly1 = \ (@ a_ayO) (x_ay9 :: a) -> Foo 3# x_ay9

 -- RHS size: {terms: 5, types: 9, coercions: 0}
 silly :: forall a r. ((a -> Foo a) -> r) -> r
 silly =
   \ (@ a_ayO) (@ r_ayP) (f_ay8 :: (a -> Foo a) -> r) -> f_ay8 silly1
 }}}

 in which the `Int` is unpacked into the closure. This transformation is
 valid whenever the value to be stored unpacked is known not to be bottom,
 and is certainly a good idea if it's known to have been forced.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12990#comment:9>
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