[GHC] #8905: Function arguments are always spilled/reloaded if scrutinee is already in WHNF

[GHC]

#8905: Function arguments are always spilled/reloaded if scrutinee is already in
WHNF
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        Reporter:  tibbe                    |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler                 |          Version:  7.9
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  Runtime performance bug  |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by tibbe):

 I've attached the Cmm for the biggest loss in the nofib run above,
 `cryptarithm1`. There's definitely more code generated with simonmar's
 patch (which nofib strangely doesn't report).

 Just to validate nofib's result, I reran this benchmark manually and
 grabbed the results for the best out of five runs:

 master:
 {{{
 real    0m0.328s
 user    0m0.320s
 sys     0m0.007s
 }}}

 no-spill:
 {{{
 real    0m0.333s
 user    0m0.325s
 sys     0m0.007s
 }}}

 That's a 1.5% runtime difference. Looking at the mutator time with `+RTS
 -s` (using five new runs), I can see no difference (`+RTS -s` has lower
 resolution).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8905#comment:13>
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