Improve optimisation of pattern synonym matching
Currently GHC does a very poor job of optimising pattern matches which include pattern synonyms.
It would be good to modify sameGroup in compiler/deSugar/Match.hs to be a lot smarter about when it is safe to group two pattern matches together.
Grouping was originally disabled as it is a bit trickier than originally thought. See #11224 (closed) for details.
The rule was originally to group two pattern matches together PgSyn p1 and PgSyn p2 when p1 == p2. This rule wasn't safe when p1 had a polymorphic return type and thus could have a different type on each branch even though syntactically identical. A good solution to this ticket would come up with a smarter rule about when it is and isn't safe to group two together in the same spirit as the more complicated rules for view patterns.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.10.3 |
| Type | Task |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
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