[GHC] #8793: Improve GHC.Event.IntTable performance

[GHC]

#8793: Improve GHC.Event.IntTable performance
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        Reporter:  cdk               |                Owner:
            Type:  task              |               Status:  patch
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Core Libraries    |              Version:  7.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Thank you for taking the time to get more insight.  Insight is what we
 need to take sensible action!


 > The cmm of both versions sheds more light on the reason for the speedup:
 GHC compiles the pure version to a nice loop which does not jump back to
 the beginning of the function, but behind the stack check (the stack is
 needed to evaluate unevaluated buckets), while the IO version just calls
 itself recursively (i.e. jumps before the stack check).

 Interesting, though I don't yet understand the details.  Could you boil
 out a standalone example that demonstrates just this single issue?  I.e.
 two versions of a function, one of which repeats the stack check and one
 of which doesn't, and show the code side by side?


 > but it seems hard or impossible to correctly identity such unused
 arguments (I mean, it is used, but only in a function which does not use
 it...).

 Well GHC's strictness analyser should find exactly this case. I'm puzzled
 why it does not.  Again, could you spare a moment to make a standalone
 reproducer for just this issue?  Or at least a smallish function I can
 compile in isolation to see this argument not disappearing.

 Thanks

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8793#comment:14>
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