[GHC] #13745: Investigate compile-time regressions in regex-tdfa-1.2.2

[GHC]

#13745: Investigate compile-time regressions in regex-tdfa-1.2.2
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  high           |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.0.1
           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:
-------------------------------------+-------------------------------------
 hvr [[https://mail.haskell.org/pipermail/ghc-
 devs/2017-May/014208.html|points out]] that GHC 8.2 takes significantly
 longer to compile `regex-tdfa-1.2.2` than 8.0.2. Interestingly, it seems
 that compiler allocations fell dramatically while compilation time rose.

 {{{
 GHC 8.0.2:

 <<ghc: 89544610040 bytes, 2301 GCs, 189183965/391103872 avg/max bytes
 residency (29 samples), 1075M in use, 0.001 INIT (0.001 elapsed),
 55.747 MUT (61.520 elapsed), 23.276 GC (23.278 elapsed) :ghc>>

 GHC 8.2.1:

 <<ghc: 115210990008 bytes, 6383 GCs, 116487828/234031864 avg/max bytes
 residency (77 samples), 670M in use, 0.001 INIT (0.000 elapsed),
 64.154 MUT (68.262 elapsed), 37.114 GC (37.077 elapsed) :ghc>>
 }}}

 == Reproducing ==

 {{{
 $ cabal unpack regex-tdfa-1.2.2
 $ cd regex-tdfa-1.2.2
 $ cabal install --only-dependencies
 $ cabal configure
 $ cabal build
 $ time ghc -idist/build/autogen -O2 Text/Regex/TDFA.hs \
    -XMagicHash -XFlexibleInstances -XMultiParamTypeClasses \
    -XRecursiveDo -XBangPatterns -XRankNTypes -XUnboxedTuples \
    -XFlexibleContexts -XUnliftedFFITypes
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13745>
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