[GHC] #10293: CallArity taking 20% of compile time

[GHC]

#10293: CallArity taking 20% of compile time
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        Reporter:  simonpj           |                   Owner:  nomeata
            Type:  bug               |                  Status:  merge
        Priority:  high              |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:  fixed             |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by nomeata):

 >  Has there been any testing to determine the effect on the speed of
 compiled code whose compilation was significantly speeded up? The Phab
 only mentions nofib times, whose compile times aren't effected anyway.

 No, unfortunately we lack good benchmarks for this. `haskell-src-exts`
 does not define any, and even if it would, there is little infrastructure
 to run and compare such benchmarks easily.

 Again intuition, but: Those cases that blew up tend to be large and
 complex, so the chances that everything fits together in a way for Call
 Arity to succeed are small.

 > Hard-coding the limit of 25 seems rash when there's only one data point
 (haskell-src-exts). This should probably be configurable at run-time until
 its had a lot more use out in the wild to see if this really is the best
 value in practice.

 People were asking for a fix that can go into 7.10.2, so this is a rash
 move. I believe the limit is still higher than the lowest reasonable
 limit, so we should be fine – but again without evidence.

 Also, it’s always annoying to add flags-dependent values to pure code with
 no `DynFlags` around, as it is the case here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10293#comment:12>
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