#4301: Optimisations give bad core for foldl' (flip seq) ()
----------------------------------+-----------------------------------------
Reporter: daniel.is.fischer | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.12.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Runtime performance bug
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I'm not sure whether it's one freak case or a symptom of a problem that
occurs more often. Compiled with optimisations, the code
{{{
foo :: [a] -> ()
foo = foldl' (flip seq) ()
}}}
produces the core
{{{
Rec {
FSeq.foo1 :: forall a_af0. [a_af0] -> (##)
GblId
[Arity 1
NoCafRefs
Str: DmdType S]
FSeq.foo1 =
\ (@ a_af0) (w_sg9 :: [a_af0]) ->
case case w_sg9 of _ {
[] -> GHC.Unit.();
: x_afz xs_afA ->
case x_afz of _ { __DEFAULT ->
case FSeq.foo1 @ a_af0 xs_afA of _ { (# #) -> GHC.Unit.() }
}
}
of _ { () ->
GHC.Prim.(##)
}
end Rec }
}}}
for the worker (ghc-6.12.3, similar core from HEAD). Due to the boxing and
unboxing between (##) and (), there's a case on the recursive call, hence
it produces a stack overflow on long enough lists.
The problem was reported in
http://www.haskell.org/pipermail/beginners/2010-September/005287.html, my
attempt at understanding it here:
http://www.haskell.org/pipermail/beginners/2010-September/005293.html.
So far, I've only managed to produce it for the very special combination
of foldl' (flip seq) and a single (non-bottom) value datatype; any other
function to be folded or a multiple value type produce a tail-recursive
two-argument worker.
Nevertheless, on the off chance that it's a symptom of a real problem, I'm
reporting it.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4301>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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