[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:  8.4.1
       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):

 Quick data point: in a test case using a modified `Lazy.hs`, the bad
 behavior can be reproduced when running certain regular expressions
 against a large test data set, but not others:

 - `^def `: bad
 - `^def[^.]`: bad
 - `^[^.]`: good
 - `^d[^.]`: good
 - `^de[^.]`: good
 - `^def[^.]`: bad
 - `^[^.]def`: good

 (I picked `[^.]` as a subexpression that can never match).

 Particularly interesting is `^de[^.]` vs. `^def[^.]`: it seems that having
 to consume 4 or more tokens from the start of the input triggers the bad
 behavior.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14519#comment:8>
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