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Inconsistency in demand analysis

    • Closed Issue created by rl@cse.unsw.edu.au @trac-rl

    A small program:

    {-# LANGUAGE MagicHash, UnboxedTuples #-}
module U where
import GHC.Prim
import GHC.Types

idx :: Addr# -> Int -> Int
{-# INLINE idx #-}
idx a (I# i) = case readIntOffAddr# a i realWorld# of (# _, y #) -> I# y

f :: Int -> Int -> Int
{-# INLINE f #-}
f x y = y + x

foo :: Addr# -> Int -> Int
foo a n = n `seq` loop (idx a 0) 1
  where
    loop x i = case i >= n of
      False -> loop (f x (idx a i)) (i+1)
      True  -> x

    GHC infers the demand LU(L) for loop, only unboxes the second argument, ultimately generates a loop of type Int -> Int# -> Int and always allocates a thunk for the first argument:

          $wloop_si9 [Occ=LoopBreaker] :: Int -> Int# -> Int
      [LclId, Arity=2, Str=DmdType LL]
      $wloop_si9 =
        \ (w1_shU :: Int) (ww1_shX :: Int#) ->
          case >=# ww1_shX ww_si5 of _ {
            False ->
              $wloop_si9
                (case readIntOffAddr# @ RealWorld w_si2 ww1_shX realWorld#
                 of _ { (# _, y_a9S #) ->
                 case w1_shU of _ { I# y1_ahb -> I# (+# y_a9S y1_ahb) }
                 })
                (+# ww1_shX 1);
            True -> w1_shU
          }; }

    But if I change the pragma on f from INLINE to NOINLINE, loop gets the demand U(L)U(L)m and GHC manages to unbox both arguments as well as the result, producing a nice tight loop:

          $wloop_sii [Occ=LoopBreaker] :: Int# -> Int# -> Int#
      [LclId, Arity=2, Str=DmdType LL]
      $wloop_sii =
        \ (ww1_shW :: Int#) (ww2_si0 :: Int#) ->
          case >=# ww2_si0 ww_sib of _ {
            False ->
              case readIntOffAddr# @ RealWorld w_si8 ww2_si0 realWorld#
              of _ { (# _, y1_Xac #) ->
              case f (I# ww1_shW) (I# y1_Xac) of _ { I# ww3_Xin ->
              $wloop_sii ww3_Xin (+# ww2_si0 1)
              }
              };
            True -> ww1_shW
          }; }

    It would be nice if this happened in both cases.

    Edited
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