[GHC] #13253: Exponential compilation time with RWST & ReaderT stack with `-02`

[GHC]

#13253: Exponential compilation time with RWST & ReaderT stack with `-02`
-------------------------------------+-------------------------------------
        Reporter:  phadej            |                Owner:  dfeuer
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 This patch
 {{{
 modified   compiler/simplCore/SimplUtils.hs
 @@ -1255,7 +1255,8 @@ postInlineUnconditionally env top_lvl bndr occ_info
 rhs
          -- in allocation if you miss this out
        OneOcc { occ_in_lam = in_lam, occ_int_cxt = int_cxt }
                 -- OneOcc => no code-duplication issue
 -        ->     smallEnoughToInline dflags unfolding     -- Small enough
 to dup
 +        ->   not (isJoinId bndr)
 +          && smallEnoughToInline dflags unfolding     -- Small enough to
 dup
                          -- ToDo: consider discount on smallEnoughToInline
 if int_cxt is true
                          --
                          -- NB: Do NOT inline arbitrarily big things, even
 if one_br is True
 }}}
 makes a big difference.

 It makes my reproducer work in linear time.

 Nofib says
 {{{
         Program           Size    Allocs   Runtime   Elapsed  TotalMem
 --------------------------------------------------------------------------------
        fibheaps          -2.6%     +1.1%     0.033     0.033      0.0%
          gamteb          -2.3%     +4.0%     0.062     0.062      0.0%
             ida          -2.8%     +2.0%     0.107     0.107    +33.3%
            mate          -2.3%    -19.6%     -5.8%     -5.8%      0.0%
            para          -2.8%     +0.7%     -2.1%     -2.4%      0.0%
 --------------------------------------------------------------------------------
             Min          -4.9%    -19.6%     -8.6%     -8.6%      0.0%
             Max          -1.5%     +4.0%     +3.5%     +3.4%    +33.3%
  Geometric Mean          -2.5%     -0.1%     -0.5%     -0.5%     +0.5%
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13253#comment:24>
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