Strict constructor fields inspected in loop

Here is a small example to illustrate the problem:

module T where

data S a b = S !a !b

class C a where
 make :: a -> S a a

instance C Int where
 {-# NOINLINE make #-}
 make n = S n n

foo :: (C a, Num a) => a -> Int -> a
{-# INLINE foo #-}
foo x k = k `seq` m `seq` go k 0
 where
   S m n = make x

   go 0 i = i
   go k i = go (k-1) (i + m)

module U where

import T

bar :: Int -> Int -> Int
bar s k = foo s k + 1

Relying on !LiberateCase seems to be the only way to unbox m outside of the loop in bar. The seq in foo doesn't help because it gets eliminated immediately.

GHC does have enough information to do this:

U.bar =
 \ (s_aaw [Dmd=Just S(A)] :: GHC.Types.Int)
   (k_aax [Dmd=Just U(L)] :: GHC.Types.Int) ->
   case k_aax
   of k1_ajh [Dmd=Just U(L)] { GHC.Types.I# ipv_ajj [Dmd=Just A] ->
   case T.$fCInt_$cmake s_aaw of _ { T.S m_ajy [Dmd=Just U(T)] _ ->
   ...

Note the demand on m. If it was an argument instead of a local binding, it would be unboxed by w/w.

Also, the seq does help if we use lazy pairs instead of strict ones.

Trac metadata

Trac field	Value
Version	6.13
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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