[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:                    |
-------------------------------------+-------------------------------------

Comment (by phadej):

 I might be pointing out the obvious, but given Simon's example

 {{{
 module Expfoo (f, foo, bar) where

 f :: Int -> Bool -> Bool
 {-# INLINE f #-}
 f y x = case x of { True -> y>0 ; False -> y<0 }

 foo :: Int -> Bool -> Bool
 foo y x = f (y+1) $
           f (y+2) $
           f (y+3) $
           f (y+4) $
           f (y+5) $
           f (y+6) $
           f (y+7) $
           f (y+8) $
           f (y+9) $
           f y x

 bar :: Bool
 bar = foo 10 True
 }}}

 The -ddump-simpl -ddump-simpl-stats say

 {{{
 2157 PostInlineUnconditionally
   2060 x_a28M

 2129 KnownBranch
   2060 wild_a28K

 1024 FillInCaseDefault
   512 wild_X1l
   256 wild_X1n
   128 wild_X1p
   64 wild_X1r
   32 wild_X1t
   16 wild_X1v
   8 wild_X1x
   4 wild_X1z
   2 wild_Xc
   2 wild_X1y

 bar = GHC.Types.True
 }}}

 I.e. simplifier is able to calculate `bar` value, which is great!
 But it does exponential job while trying to figure out what's `foo`.

 However if we only export

 {{{
 module Expfoo (f, bar) where
 }}}

 then `bar` is still `True`, but stats looks way better, all Grand total
 simplifier statistics are under 12, e.g.
 {{{
 8 CaseOfCase 8 wild_Xb
 73 KnownBranch
   11 wild1_a28w
   10 wild_a28s
   9 wild1_a28m
   9 wild_a28K
   9 wild1_a28O
   8 wild_a28i
   2 wild_Xb
   2 wild_X17
   2 wild_X19
   2 wild_X1b
   2 wild_X1d
   2 wild_X1f
   2 wild_X1h
   2 wild_X1j
   1 wild_X1m
 1 FillInCaseDefault 1 wild_Xb
 12 SimplifierDone 12
 }}}

 I'm quite sure that while fixing the exponential time issue,
 we should "break" the fact that `bar` is fully simplified to `True`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13253#comment:23>
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