[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:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
  https://github.com/ntc2/ghc-8.2.1  |
  -regex-lazy-text-bug               |
           Blocking:                 |   Related Tickets:  13745
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 When using `Text.RE.TDFA` from the regex-tdfa package on GHC 8.2 to match
 trivial regexes against `Data.Text.Lazy` strings, I see **exponential
 runtime**. On GHC 8.0, or using `Data.Text` strict strings or `String`
 strings, the runtime is as expected.

 Here's my repo with simple test code illustrating the regression and
 timing stats: https://github.com/ntc2/ghc-8.2.1-regex-lazy-text-bug.

 In summary, for the problematic combination of GHC version and libraries,
 the run times are 3s, 10s, 22s, and 40s
 for counting regex matches in files with 10000,
 20000, 30000, and 40000 lines, respectively. For all of the
 unproblematic combinations -- i.e. GHC 8.0.2, `String`, strict
 `Data.Text`, or building with profiling -- the run time is always about
 1s!

 And **this is a Heisenbug**, in that the performance problem goes away if
 I build with profiling support!

 There is the separate Issue #13745 about **compile-time regressions** in
 GHC 8.2 + the regex-tdfa package, that
 [https://ghc.haskell.org/trac/ghc/ticket/13745#comment:18 I commented on].

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