[GHC] #14519: Exponential runtime performance regression in GHC 8.2 + Data.Text.Lazy + Text.RE.TDFA

[GHC]

#14519: Exponential runtime performance regression in GHC 8.2 + Data.Text.Lazy +
Text.RE.TDFA
-------------------------------------+-------------------------------------
        Reporter:  ntc2              |                Owner:  tdammers
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime           |            Test Case:
  performance bug                    |  https://github.com/ntc2/ghc-8.2.1
                                     |  -regex-lazy-text-
                                     |  bug/tree/07b7bb32c6e90e8f2d2eada4b59943f37e632d53
      Blocked By:                    |             Blocking:
 Related Tickets:  #13745, #14564    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by tdammers):

 Specifically, running the minimal test code over defs-30000, with various
 flags, and an execution time limit of 1 second (enough to detect the "bad"
 behavior), gives us this list:

 {{{
 unoptimized (-O0): Time: 101 ms
 no-float-in (-O -fno-float-in): Timeout exceeded
 no-strictness (-O -fno-strictness): Timeout exceeded
 no-full-laziness (-O -fno-full-laziness): Timeout exceeded
 no-specialise (-O -fno-specialise): Timeout exceeded
 no-do-eta-reduction (-O -fno-do-eta-reduction): Timeout exceeded
 no-cse (-O -fno-cse): Timeout exceeded
 no-case-merge (-O -fno-case-merge): Timeout exceeded
 no-enable-rewrite-rules (-O -fno-enable-rewrite-rules): Time: 88 ms
 no-worker-wrapper (-O -fno-worker-wrapper): Timeout exceeded
 }}}

 In other words, among these optimizations, `enable-rewrite-rules` seems to
 be the one that triggers the bad behavior.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14519#comment:34>
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