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FoatOut is not floating bottoming expressions properly

    • Closed Issue created by Simon Peyton Jones @simonpj

    Consider part of Eric's program in #13143 (closed), and pretty much exactly what is in GHC.Arr.

    {-# INLINE index #-}
index :: (Int, Int) -> Int -> Int
index b@(l,h) i
  | l <= i && i < h = 0
  | otherwise       = indexError b i 0

{-# NOINLINE indexError #-}
indexError :: (Int, Int) -> Int -> Int -> b
indexError rng i tp = error (show rng)

    Before float-out we have

    index =
  \ (b_ay8 :: (Int, Int)) (i_ayb :: Int) ->
    case b_ay8 of wild_Xe { (l_ay9, h_aya) ->
    case &&
           (leInt l_ay9 i_ayb) (ltInt i_ayb h_aya)
    of {
      False -> indexError @ Int wild_Xe i_ayb (I# 0#);
      True -> GHC.Types.I# 0#
    } }

    and after float-out we see

    index =
  \ (b_ay8 :: (Int, Int)) (i_ayb :: Int) ->
    case b_ay8 of wild_Xe { (l_ay9, h_aya) ->
    case &&
           (leInt l_ay9 i_ayb) (ltInt i_ayb h_aya)
    of {
      False -> indexError @ Int b_ay8 i_ayb lvl_s2cd;
      True -> GHC.Types.I# 0#
    } }

    We've floated the (I# 0#). But we should have floated the entire call to indexError thus:

          False -> lvl_xxx b_ay8 i_ayb

    with

    lvl_xxx b i = indexError @Int b i (I# 0#)

    Why? Because it makes the expression smaller and moves the error handling code out of the way.

    Float-out makes some attempt to to this, but it's not right yet.

    Trac metadata
    Trac field Value
    Version 8.0.1
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
    Test case
    Differential revisions
    BlockedBy
    Related
    Blocking
    CC
    Operating system
    Architecture
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