[GHC] #13422: INLINE CONLIKE sometimes fails to inline

13 Mar 2017

      #13422: INLINE CONLIKE sometimes fails to inline
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #7206
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I got surprisingly poor results from the `cheapBuild` experiment (see
 #7206), so I decided to try a simple example.

 {{{#!hs
 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:

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

 produces

 {{{#!hs
 $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#
       }
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13422
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