INLINE CONLIKE sometimes fails to inline

I got surprisingly poor results from the cheapBuild experiment (see #7206), so I decided to try a simple example.

foo :: Int -> Int
foo n = s + p
  where
    nums = [1..n]
    s = sum nums
    p = product nums

The idea is that we want nums to inline into s and p; the lost sharing is unimportant. Unfortunately, this didn't fuse. I got

-- RHS size: {terms: 44, types: 31, coercions: 0, joins: 2/3}
$wfoo :: Int# -> Int#
$wfoo
  = \ (ww_s2HT :: Int#) ->
      let {
        nums_s2Gq :: [Int]
        nums_s2Gq = eftInt 1# ww_s2HT } in
      joinrec {
        $wgo_s2HP :: [Int] -> Int# -> Int#
        $wgo_s2HP (w_s2HJ :: [Int]) (ww1_s2HN :: Int#)
          = case w_s2HJ of {
              [] ->
                joinrec {
                  $wgo1_s2HI :: [Int] -> Int# -> Int#
                  $wgo1_s2HI (w1_s2HC :: [Int]) (ww2_s2HG :: Int#)
                    = case w1_s2HC of {
                        [] -> +# ww1_s2HN ww2_s2HG;
                        : y_a2F9 ys_a2Fa ->
                          case y_a2F9 of { I# y1_a2Gf ->
                          jump $wgo1_s2HI ys_a2Fa (*# ww2_s2HG y1_a2Gf)
                          }
                      }; } in
                jump $wgo1_s2HI nums_s2Gq 1#;
              : y_a2F9 ys_a2Fa ->
                case y_a2F9 of { I# y1_a2EX ->
                jump $wgo_s2HP ys_a2Fa (+# ww1_s2HN y1_a2EX)
                }
            }; } in
      jump $wgo_s2HP nums_s2Gq 0#

-- RHS size: {terms: 10, types: 4, coercions: 0, joins: 0/0}
foo :: Int -> Int
foo
  = \ (w_s2HQ :: Int) ->
      case w_s2HQ of { I# ww1_s2HT ->
      case $wfoo ww1_s2HT of ww2_s2HX { __DEFAULT -> I# ww2_s2HX }
      }

I verified that cheapBuild actually appeared (in -dverbose-core2core), but it didn't fuse, and ultimately inlined.

Interestingly, forcing the argument manually fixed the problem:

foo :: Int -> Int
foo !n = s + p
  where
    nums = [1..n]
    s = sum nums
    p = product nums

produces

$wfoo :: Int# -> Int#
$wfoo
  = \ (ww_s2Ia :: Int#) ->
      case tagToEnum# (># 1# ww_s2Ia) of {
        False ->
          joinrec {
            $wgo_s2I6 :: Int# -> Int# -> Int#
            $wgo_s2I6 (w_s2I0 :: Int#) (ww1_s2I4 :: Int#)
              = case tagToEnum# (==# w_s2I0 ww_s2Ia) of {
                  False -> jump $wgo_s2I6 (+# w_s2I0 1#) (+# ww1_s2I4 w_s2I0);
                  True ->
                    let {
                      x_X2Fz :: Int#
                      x_X2Fz = +# ww1_s2I4 w_s2I0 } in
                    joinrec {
                      $wgo1_s2HY :: Int# -> Int# -> Int#
                      $wgo1_s2HY (w1_s2HS :: Int#) (ww2_s2HW :: Int#)
                        = case tagToEnum# (==# w1_s2HS ww_s2Ia) of {
                            False -> jump $wgo1_s2HY (+# w1_s2HS 1#) (*# ww2_s2HW w1_s2HS);
                            True -> +# x_X2Fz (*# ww2_s2HW w1_s2HS)
                          }; } in
                    jump $wgo1_s2HY 1# 1#
                }; } in
          jump $wgo_s2I6 1# 0#;
        True -> 1#
      }

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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