[GHC] #14644: Improve cmm/assembly for pattern matches with two constants.

[GHC]

#14644: Improve cmm/assembly for pattern matches with two constants.
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        Reporter:  AndreasK          |                Owner:  AndreasK
            Type:  task              |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
  (CodeGen)                          |             Keywords:  Codegen, CMM,
      Resolution:                    |  Patterns, Pattern Matching
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4294
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"7ff6023537fdef32bbe9b4c357012d705d9b931f/ghc" 7ff6023/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="7ff6023537fdef32bbe9b4c357012d705d9b931f"
 cmm: Use two equality checks for two alt switch with default

 For code like:
 f 1 = e1
 f 7 = e2
 f _ = e3

 We can treat it as a sparse jump table, check if we are outside of the
 range in one direction first and then start checking the values.

 GHC currently does this by checking for x>7, then x <= 7 and at last x
 == 1.

 This patch changes this such that we only compare for equality against
 the two values and jump to the default if non are equal.

 The resulting code is both faster and smaller.
 wheel-sieve1 improves by 4-8% depending on problem size.

 This implements the idea from #14644

 Reviewers: bgamari, simonmar, simonpj, nomeata

 Reviewed By: simonpj, nomeata

 Subscribers: nomeata, simonpj, rwbarton, thomie, carter

 Differential Revision: https://phabricator.haskell.org/D4294
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14644#comment:15>
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