[GHC] #13397: Optimise calls to tagToEnum#

[GHC]

#13397: Optimise calls to tagToEnum#
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              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 rwbarton):

 I got rather different results from nofib, attached above. Note that these
 are for a full recompilation from scratch. Judging from the Size column in
 your nofib output, I guess that you probably also did a full recompilation
 from scratch.

 In the program that regressed the most in my nofib run, tak, it looks like
 the code generator just output basic blocks in a different order. Probably
 the order of the `then` and `else` branches of a conditional got reversed.
 tak is known to be very sensitive to (poorly-understood) alignment effects
 (#8279) so I'm inclined to assume this is just noise that we can't do much
 about.

 Not really sure what to make of the larger regressions that you saw, or
 why I can't reproduce them.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13397#comment:3>
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