#2255: Improve SpecConstr for free variables
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Reporter: simonpj | Owner:
Type: run-time performance bug | Status: new
Priority: normal | Milestone: _|_
Component: Compiler | Version: 6.8.2
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Architecture: Unknown | Os: Unknown
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This ticket records a suggestion for improving `SpecConstr`, so we don't
lose it. The original `SpecConstr` transformation is described in
"[http://research.microsoft.com/%7Esimonpj/papers/spec-constr Call pattern
specialisation for Haskell]". Consider this program
{{{
f x = let g y = ...case x of { z:zs -> e1; [] -> e2 } ...
in
...case x of
z:zs -> if ... then g 3 else g 4
[] -> ...
}}}
Here 'x' is free in 'g', but x's value is known at g's call sites. It's
not enough just to know "x is a cons" inside g; we must also have access
to z,zs. So perhaps the thing to do is to imagine lambda-lifting 'g' to
become 'gl' thus:
{{{
gl x y = ...case x of { z:zs -> e1; [] -> e2 } ...
f x = ...case x of
z:zs -> if ... then gl x 3 else gl x 4
[] -> ...
}}}
Now the `SpecConstr` transformation will apply nicely. And it's arguably
a bad shortcoming that currently the mere act of lambda lifting can affect
how effective `SpecConstr` is.
Of course, we only want to lambda-lift wrt the parameters that'll be
specialised, so this transformation probably wants to be done at the same
time as the rest of `SpecConstr`. I don't have much idea of how hard
that'd be, but it's a nice idea.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2255>
GHC <http://www.haskell.org/ghc/>
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