[GHC] #10034: Regression in mapM_ performance

[GHC]

#10034: Regression in mapM_ performance
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                   Owner:  ekmett
            Type:  bug               |                  Status:  new
        Priority:  highest           |               Milestone:  7.10.1
       Component:  Core Libraries    |                 Version:  7.10.1-rc2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  Phab:D632
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Changes (by simonpj):

 * priority:  normal => highest


Comment:

 I'm very unhappy with applying this patch without any comments, and
 without any understanding of what is going on.  Now `mapM_` has an INLINE
 pragma that may or may not be necessary, and an eta-expanded argument, and
 no one understands which of the two is important, if either.  Maybe there
 is a simplifier bug underlying this, which is simply being covered up.

 Please please can we have a reproducible test case that shows the
 regression.  Ideally one that just showed it, but if it's necessary to
 switch Call Arity off (which you hypothesise is making the test case
 harder to create) then do that.

 But please let's NOT just cover up the problem.  I'm quite inclined to
 revert the patch, just to serve as a forcing function to try to elicit
 this information.  For now I'll just increase the priority to try to make
 sure we notice it.

 I know it's a nuisance, but this is an example of what people mean by
 incurring technical debt.

 Thanks!

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10034#comment:5>
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