[GHC] #10788: performance regression involving minimum

[GHC]

#10788: performance regression involving minimum
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        Reporter:  rwbarton          |                   Owner:  ekmett
            Type:  bug               |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Core Libraries    |                 Version:  7.10.1
      Resolution:  fixed             |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by simonpj):

 >  Update: I looked into this, and it seems that even if -ddump-simpl
 looks like this, with a space-leaky way of accumulating the argument, that
 in the CorePrep stage the evaluation of cmax is pulled before the call to
 go and alls is well. I did not know that there are still such things going
 on in that stage.

 Suppose we have a function call `f (g x)` and `f` is strict.  Then GHC
 keeps it looking like that so that rewrite rules apply easily.  In
 `CorePrep` GHC just makes the order of evaluation explicit, by moving to
 {{{
  case (g x) of y -> f y
 }}}
 There is no new analysis; the call always was call-by-value; but after
 `CorePrep` that fact is 100% explicit.

 I believe that the conclusion here is that GHC is behaving perfectly
 predictably; but library authors need to take a little care with pragmas
 and rules if they want to get predictable fusion.  That's what your email
 in comment:9 is about, if I read it right.  That is, you are not seeking
 any change to GHC.  Correct?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10788#comment:15>
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