[GHC] #8731: long compilation time for module with large data type and partial record selectors

[GHC]

#8731: long compilation time for module with large data type and partial record
selectors
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       Reporter:         |             Owner:
  carter                 |            Status:  new
           Type:  bug    |         Milestone:
       Priority:         |           Version:  7.8.1-rc1
  normal                 |  Operating System:  Unknown/Multiple
      Component:         |   Type of failure:  Compile-time performance bug
  Compiler               |         Test Case:
       Keywords:         |          Blocking:
   Architecture:         |
  Unknown/Multiple       |
     Difficulty:         |
  Unknown                |
     Blocked By:         |
Related Tickets:         |
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 in both ghc 7.6 and 7.8rc,  the Constants module in LLVM-General-Pure (and
 a few other similar modules) have surprisingly long (and noticable!)
 compilation times.

 This seems to related to how GHC handles a many constructor data type with
 overlapping field names.

 the time is nearly the same for building a static module with O1 and O2

 {{{
 carter repoScratcher/bad-ast » time ghc Constant.hs -O2
 1 ↵
 [1 of 1] Compiling Constant         ( Constant.hs, Constant.o )
 ghc Constant.hs -O2  7.52s user 0.23s system 99% cpu 7.755 total
 }}}

 {{{
 carter repoScratcher/bad-ast » time ghc Constant.hs -O1
 [1 of 1] Compiling Constant         ( Constant.hs, Constant.o )
 ghc Constant.hs -O1  6.62s user 0.20s system 100% cpu 6.768 total
 }}}

 heck even with -O0 its slow (though not terrible)
 {{{
 carter repoScratcher/bad-ast » time ghc Constant.hs -O0
 [1 of 1] Compiling Constant         ( Constant.hs, Constant.o )
 ghc Constant.hs -O0  2.76s user 0.14s system 101% cpu 2.873 total
 }}}

 Seems like there might something wrong in how we handle data types like
 this, the complexity need not

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8731>
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