[GHC] #13395: 3x slowdown on GHC HEAD with file containing lots of overloaded string literals

[GHC]

#13395: 3x slowdown on GHC HEAD with file containing lots of overloaded string
literals
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
           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:
-------------------------------------+-------------------------------------
 I just noticed that compiling `xmlhtml-0.2.3.5` from Hackage takes an
 extraordinarily long time on GHC HEAD. The slowdown can be attributed to
 the `Text.XmlHtml.HTML.Meta` module. I've extracted it and tweaked it so
 that it requires no dependencies to build (I now use `ByteString` instead
 of `Text`, but the slowdowns for both types were about the same.)

 On GHC 8.0.2, it takes about 3 seconds to compile the attached module:

 {{{
 $ /usr/bin/time /opt/ghc/8.0.2/bin/ghc -fforce-recomp
 TextXmlHtmlHTMLMeta.hs
 [1 of 1] Compiling Text.XmlHtml.HTML.Meta ( TextXmlHtmlHTMLMeta.hs,
 TextXmlHtmlHTMLMeta.o )
 3.44user 0.07system 0:03.51elapsed 99%CPU (0avgtext+0avgdata
 208480maxresident)k
 0inputs+21688outputs (0major+43934minor)pagefaults 0swaps
 }}}

 But on GHC HEAD, it takes about 10 seconds:

 {{{
 $ /usr/bin/time ~/Software/ghc2/inplace/bin/ghc-stage2 -fforce-recomp
 TextXmlHtmlHTMLMeta.hs
 [1 of 1] Compiling Text.XmlHtml.HTML.Meta ( TextXmlHtmlHTMLMeta.hs,
 TextXmlHtmlHTMLMeta.o )
 9.93user 0.11system 0:10.03elapsed 100%CPU (0avgtext+0avgdata
 233656maxresident)k
 0inputs+21464outputs (0major+54853minor)pagefaults 0swaps
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13395>
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