[GHC] #14338: Simplifier fails with "Simplifier ticks exhausted"

10 Oct 2017

      #14338: Simplifier fails with "Simplifier ticks exhausted"
-------------------------------------+-------------------------------------
           Reporter:  dredozubov     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 While upgrading a codebase to GHC 8.2.1(it compiles reliably with 7.10.3)
 we encountered an issue with core simplifier(and rewrite rules if I
 correctly understood the meaning of RuleFired):

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.1 for x86_64-apple-darwin):
         Simplifier ticks exhausted
   When trying RuleFired Class op $p2HModify
   To increase the limit, use -fsimpl-tick-factor=N (default 100)
   If you need to do this, let GHC HQ know, and what factor you needed
   Total ticks:     10450410

   1494659 PreInlineUnconditionally
     149100 w_ipsJ
     149095 w_ipps
     149084 w_iplQ
     149084 w1_iplR
     149084 w2_iplS
     149084 w3_iplT
     149078 w_ipm7
     149078 w1_ipm8
     149078 w2_ipm9
     149078 w3_ipma
     120 $d~_iplr
     120 $d~1_ipls
     120 irred_iplu
     120 eta_iplv
     39 v_spsR
     38 v_sp9c
     38 v1_sp9d
     38 v_spii
     38 v1_spij
     38 v2_spik
     38 v3_spil
     37 v_sp99
     37 v1_sp9a
     37 v_spin
     37 v1_spio
     37 v2_spip
     37 v3_spiq
     36 v_sp9f
     36 v1_sp9g
     36 v_spis
 ...skipping...
     1 cobox1_apuk
     1 cobox_apuq
     1 cobox1_apur
     1 cobox_apux
     1 cobox1_apuy
   2 CaseIdentity 2 ds1_iqAB
   4 FillInCaseDefault
     1 nt_sqyY
     1 nt_sqz9
     1 nt_sqze
     1 nt_sqzj
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/simplCore/SimplMonad.hs:199:31 in
 ghc:SimplMonad

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 We tried to shrink reproducible example to something reasonable and this
 is what we got: https://github.com/4e6/webapp-template-hs/tree/simpl-tick-
 factor

 Compiling it with stack build --ghc-options='-fsimpl-tick-factor=1000'(ten
 times the default) will demonstrate the issue.

 It fails reliably with a combination of servant and hset libraries.
 Removing one route from a servant API or moving the PayloadX to the head
 of type-level list makes it compilable again.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14338
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