[GHC] #15938: Hadrian's recompilation check is extremely slow

[GHC]

#15938: Hadrian's recompilation check is extremely slow
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        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.8.1
       Component:  Build System      |              Version:  8.6.2
  (Hadrian)                          |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 I quickly rebuilt Hadrian with profiling and reproduced this. I used
 `hadrian/build.cabal.sh +RTS -p` to start Hadrian in a half-built tree and
 Ctrl-C'd it as soon as it started producing output (which took roughly 30
 seconds of wall clock time):
 {{{
 $ hadrian/build.cabal.sh +RTS -p
 Up to date
 Up to date
 | Run Ghc FindHsDependencies Stage0: libraries/ghc-heap/GHC/Exts/Heap.hs
 (and 7 more) => _build/stage0/libraries/ghc-heap/.dependencies.mk
 | Remove file _build/stage0/libraries/ghc-heap/.dependencies.mk.bak
 | Run Ghc CompileHs Stage0: _build/stage0/libraries/ghc-
 heap/build/GHC/Exts/Heap/Constants.hs => _build/stage0/libraries/ghc-
 heap/build/GHC/Exts/Heap/Constants.o
 | Run Ghc CompileHs Stage0: libraries/ghc-
 heap/GHC/Exts/Heap/ClosureTypes.hs => _build/stage0/libraries/ghc-
 heap/build/GHC/Exts/Heap/ClosureTypes.o
 ^CshakeArgsWith   0.000s    0%
 Function shake  0.465s    1%
 Database read   0.072s    0%
 With database   0.001s    0%
 Running rules  31.903s   98%  =========================
 Total          32.442s  100%
 user interrupt
 }}}

 I will attach the profile in a moment.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15938#comment:11>
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