#10215: Optimizer has bugs regarding handling of -0.0
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Reporter: lerkok | Owner:
Type: bug | Status: closed
Priority: high | Milestone:
Component: Compiler | Version: 7.8.3
Resolution: duplicate | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: Incorrect result | Unknown/Multiple
at runtime | Test Case:
Blocked By: | Blocking:
Related Tickets: #9238 | Differential Rev(s):
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Comment (by Ben Gamari ):
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
}}}
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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10215#comment:2
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