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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
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