#1370: Obscure loop in interaction between arity and case-of-bottom
-----------------------+----------------------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.8
Component: Compiler | Version: 6.6.1
Severity: normal | Keywords:
Difficulty: Unknown | Os: Unknown
Testcase: | Architecture: Unknown
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John Meacham discovered that this program makes the simplifier loop:
{{{
newtype T = V T deriving(Eq,Ord)
}}}
It's a silly program, but it shouldn't kill GHC! I investigated and the
reason is because we get this:
{{{
rec { compare :: T -> T -> Ordering
compare [arity 2] = compare }
max :: T -> T -> T
max x y = case compare x y of { GT -> x; other -> y }
}}}
The real bug is that `compare` has been eta-reduced, but still has arity
2. That's wrong. Anyway, since `compare` diverges, we get
{{{
max x y = compare `cast` UnsafeCoerce (T->T->T) T
}}}
and now we try to eta-expand the RHS of `max`, and that is a stupid thing
to do (and loops in `CoreUtils.etaExpand`.
The real bug is the unsound eta-reducion for `compare`, which I knew about
(comments in `Note [Add IdInfo back onto a let-bound Id]` in `SimplEnv`).
It's a dark corner case, and I'm not going to fix it today.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1370>
GHC <http://www.haskell.org/ghc/>
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