[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 dfeuer):

 Simon, I rebased your branch and fixed a comment, producing
 `wip/dfeuer-T13397`. The
 [https://perf.haskell.org/ghc/#compare/583fa9e3687b49d8c779e6d53a75af9276e4f5cf/a2df49f25cd8c332d0e9bb409428db566378eee2
 perf results] generally look fine, with a few very small regressions:

 {{{
 Nofib allocations
 -----------------
 knights    +1.35%
 pic        +1.14%
 scs        +0.81% (this ran slightly *faster*)
 last-piece +0.29%

 Nofib runtimes
 --------------
 cacheprof    +1.3%
 last-piece   +1.23%
 hidden       +1.08%
 wheel-sieve1 +0.95%
 digits-of-e1 +0.87%
 lambda       +0.47%


 Test suite allocations
 ----------------------
 T783    +0.59%
 T9675   +0.33%
 T12707  +0.23%
 }}}

 The wins seem modest, but bigger than the regressions:

 {{{
 Nofib allocations
 -----------------
 anna    -0.7%
 fft     -0.41%

 Nofib runtimes
 --------------
 cryptarithm1   -4.29%
 fannkuch-redux -3.75%
 binary-trees   -3.7%
 scs            -1.28%
 integer        -1%
 fasta          -0.98%
 mate           -0.95%
 digits-of-e2   -0.85%
 }}}

 As far as I'm concerned, this should probably be ready to merge. Should I
 do so?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13397#comment:6>
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