Ben Gamari authored Oct 02, 2015 and Ben Gamari committed Oct 02, 2015

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