[GHC] #9238: Negative zero broken

[GHC]

#9238: Negative zero broken
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        Reporter:  augustss          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  7.10.3
       Component:  Compiler          |              Version:  7.8.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #7858, #9451,     |  Differential Rev(s):  Phab:D1061
  #10215                             |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"eb975d2eec349429e735c272d46a7becccf393c6/ghc" eb975d2/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="eb975d2eec349429e735c272d46a7becccf393c6"
 Fix treatment of -0.0

 Here we fix a few mis-optimizations that could occur in code with
 floating point comparisons with -0.0. These issues arose from our
 insistence on rewriting equalities into case analyses and the
 simplifier's ignorance of floating-point semantics.

 For instance, in Trac #10215 (and the similar issue Trac #9238) we
 turned `ds == 0.0` into a case analysis,

 ```
 case ds of
     __DEFAULT -> ...
     0.0 -> ...
 ```

 Where the second alternative matches where `ds` is +0.0 and *also* -0.0.
 However, the simplifier doesn't realize this and will introduce a local
 inlining of `ds = -- +0.0` as it believes this is the only
 value that matches this pattern.

 Instead of teaching the simplifier about floating-point semantics
 we simply prohibit case analysis on floating-point scrutinees and keep
 this logic in the comparison primops, where it belongs.

 We do several things here,

  - Add test cases from relevant tickets
  - Clean up a bit of documentation
  - Desugar literal matches against floats into applications of the
    appropriate equality primitive instead of case analysis
  - Add a CoreLint to ensure we don't pattern match on floats in Core

 Test Plan: validate with included testcases

 Reviewers: goldfire, simonpj, austin

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D1061

 GHC Trac Issues: #10215, #9238
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9238#comment:18>
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