Re: [GHC] #10830: maximumBy has a space leak

2 Sep 2015

      #10830: maximumBy has a space leak
-------------------------------------+-------------------------------------
        Reporter:  NeilMitchell      |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  libraries/base    |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Windows           |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by nomeata):

 In 7.8, the core is nice and fully inlined:

 {{{#!hs
 Rec {
 main_lgo
 main_lgo =
   \ z_a1k8 ds2_a1k9 ->
     case ds2_a1k9 of _ {
       [] -> z_a1k8;
       : x_a1kh xs_a1ki ->
         case compareInteger z_a1k8 x_a1kh of _ {
           __DEFAULT -> main_lgo x_a1kh xs_a1ki;
           GT -> main_lgo z_a1k8 xs_a1ki
         }
     }
 end Rec }
 }}}

 In HEAD, we have a call to `foldr1`:
 {{{#!hs
 main5 =
   \ x_a2pT y_a2pU ->
     case compareInteger x_a2pT y_a2pU of _ {
       __DEFAULT -> y_a2pU;
       GT -> x_a2pT
     }
 main2 = enumDeltaToInteger1 main4 main3
 main1 =
   \ eta_B1 ->
     case foldr1 main5 main2 of _ { __DEFAULT -> (# eta_B1, () #) }
 }}}

 I think you are right that this is some BBP-fallout. Note that `maximumBy`
 is implemented in `Data.Foldable` using `foldr1`.

 If I make `foldr1` inlinable (by using a local worker), I get
 {{{#!hs
 Rec {
 -- RHS size: {terms: 19, types: 9, coercions: 0}
 main_go
 main_go =
   \ ds2_a1Fi eta_a1Fj ->
     case ds2_a1Fi of _ {
       [] -> eta_a1Fj;
       : y_a1Fr ys_a1Fs ->
         case compareInteger eta_a1Fj y_a1Fr of _ {
           __DEFAULT -> main_go ys_a1Fs y_a1Fr;
           GT -> main_go ys_a1Fs eta_a1Fj
         }
     }
 end Rec }
 }}}
 which is equivalent to the 7.8 code.

 Given that we inline `foldr`, I do not see a reason not to inline
 `foldr1`. DR coming soon...

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10830#comment:3
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