#4941: SpecConstr generates functions that do not use their arguments
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Reporter: simonpj | Owner:
Type: task | Status: new
Priority: normal | Milestone: _|_
Component: Compiler | Version: 7.0.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Consider this function:
{{{
f :: Int -> (Bool,Bool) -> Bool -> Bool
f 0 x y = y
f n (p,q) y = f (n-1) (p,q) q
}}}
`SpecConstr` does a reasonable job, but ends up with a function like this:
{{{
T4908a.f_$s$wf =
\ (sc_sp4 :: GHC.Prim.Int#)
(sc1_sp5 :: GHC.Types.Bool)
(sc2_sp6 :: GHC.Types.Bool)
(sc3_sp7 :: GHC.Types.Bool) ->
case sc_sp4 of ds_Xom {
__DEFAULT ->
T4908a.f_$s$wf (GHC.Prim.-# ds_Xom 1) sc1_sp5 sc2_sp6 sc2_sp6;
0 -> sc3_sp7
}
}}}
Note that `sc1_sp5` is passed around the loop but never used.
I had a quick go at trying to make `SpecConstr` cleverer, but absence info
requires a fixpoint analysis, which the existing `ArgOcc` stuff doesn't
do. Nor can we rely on absence analysis from earlier in the compiler,
because CSE invalidates it.
A possibility would be to run strictness/absence analysis again after
`SpecConstr`, which would pick this up. I'm not sure what other
consequences this would have.
So there's an opportunity here, but I'm not sure how much it matters in
practice.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4941>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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