[GHC] #9279: Optimisation bug

7 Jul 2014

      #9279: Optimisation bug
------------------------------+--------------------------------------------
       Reporter:  simonmar    |             Owner:
           Type:  bug         |            Status:  new
       Priority:  normal      |         Milestone:
      Component:  Compiler    |           Version:  7.8.2
       Keywords:              |  Operating System:  Unknown/Multiple
   Architecture:              |   Type of failure:  Runtime performance bug
  Unknown/Multiple            |         Test Case:
     Difficulty:  Unknown     |          Blocking:
     Blocked By:              |
Related Tickets:              |
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 I have a strange problem with a local function binding that is not being
 inlined.  I've attached the repro code, build it like this:

 {{{
 $ ghc -O2 Haxl/Core/Monad.hs
 }}}

 There are several modules, but the one with the problem is
 `Haxl/Core/Monad.hs`.  In particular
 `Haxl.Core.Monad.$fApplicativeGenHaxl2`, which contains this fragment:

 {{{
             let {
               $wa4_s6YQ
               $wa4_s6YQ =
                 \ w_s6YF _ ww1_s6YM _ _ w1_s6YI ->
                   case GHC.Prim.readMutVar# ipv7_X5Ne w1_s6YI
                   of _ { (# ipv10_X5QO, ipv11_X5QQ #) ->
                   case ipv11_X5QQ of _ {
                     Haxl.Core.Monad.IVarFull a2_a3kK -> case lvl6_r7qi of
 wild3_00 { };
                     Haxl.Core.Monad.IVarEmpty dt2_d4Vs ->
                       case GHC.Prim.writeMutVar#
                              ipv7_X5Ne (Haxl.Core.Monad.IVarFull w_s6YF)
 ipv10_X5QO
                       of s2#_a5OR { __DEFAULT ->
                       case GHC.Prim.readMutVar# dt2_d4Vs s2#_a5OR
                       of _ { (# ipv12_X5R0, ipv13_X5R2 #) ->
                       case GHC.Prim.readMutVar# ww1_s6YM ipv12_X5R0
                       of _ { (# ipv14_X5SS, ipv15_X5SU #) ->
                       letrec {
                         go_a5ti
                         go_a5ti =
                           \ ds10_a5tj ->
                             case ds10_a5tj of _ {
                               [] -> ipv15_X5SU;
                               : y_a5to ys_a5tp -> GHC.Types.: (y_a5to
 w_s6YF) (go_a5ti ys_a5tp)
                             }; } in
                       case go_a5ti ipv13_X5R2 of x'_a5P6 { __DEFAULT ->
                       case GHC.Prim.writeMutVar# ww1_s6YM x'_a5P6
 ipv14_X5SS
                       of s2#1_a5P7 { __DEFAULT ->
                       (# s2#1_a5P7, Haxl.Core.Monad.cacheRequest6 #)
                       }
                       }
                       }
                       }
                       }
                   }
                   } } in
             let {
               a2_s6lH
               a2_s6lH =
                 \ w_s6YF _ w2_s6YH w3_s6YI ->
                   case w2_s6YH
                   of _
                   { Haxl.Core.Monad.SchedState ww1_s6YL ww2_s6YM ww3_s6YN
 ww4_s6YO ->
                   $wa4_s6YQ w_s6YF ww1_s6YL ww2_s6YM ww3_s6YN ww4_s6YO
 w3_s6YI
                   } } in
 }}}

 I want `$wa4` to be inlined at its single occurrence in `a2`.  I believe
 the reason it is not being inlined is that `a2` is a wrapper, but the
 situation seems silly because the wrapper isn't going away either, so we
 have a redundant closure being built (this is in my inner loop).

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9279
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