[GHC] #12706: Collecting type info is slow

[GHC]

#12706: Collecting type info is slow
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        Reporter:  vshabanov         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHCi              |              Version:  8.0.1
      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 vshabanov):

 I tried profiled compiler. Unfortunately I don't know GHC internals well
 enough to fix it but, anyway, here are my findings.

 Most time is spent in source:ghc/ghc/GHCi/UI/Info.hs

 Half time is spent in `typecheckModule` call and another half in
 `everythingAllSpans`.

 First problem here is a bad `cacheInvalid` function. It checks file time
 of object file, but there are no object files in GHCi. So types are always
 collected for all modules instead of new/modified ones. I don't know how
 to correctly check what files were recompiled but it's the first thing to
 do to shorten "Collecting type info" phase. Clearing cache on `:l` in GHCi
 is also necessary to free memory.

 I'm also not sure about call to `typecheckModule`. Aren't modules already
 type checked? Maybe it's possible to get type check results that were
 already calculated.

 Harder thing to do is to optimize `everythingAllSpans`. It uses SYB
 approach to extract `LHsBind`, `LHsExpr` and `LPat` from
 `TypecheckedSource = Bag (Located HsBindLR)`. It could be rewritten using
 direct recursion over `HsBindLR` without `cast` and `gmapQ`. But it looks
 like quite a big task.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12706#comment:3>
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