#609: Useful optimisation for set-cost-centre
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Reporter: simonpj | Owner:
Type: task | Status: new
Priority: normal | Milestone: _|_
Component: Compiler | Version: None
Severity: normal | Resolution: None
Keywords: | Difficulty: Moderate (1 day)
Testcase: | Architecture: Unknown
Os: Unknown |
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Changes (by simonpj):
* milestone: 6.8 branch => _|_
Old description:
> {{{
> If you compile, for example, drvrun014 with -prof -auto-all, you'll see
> stuff like
>
> (scc "c" (dataToTag#)) y
>
> This generates bad code, because we end up eta-expaning dataToTag, which
> allocates an extra function closure.
>
> We think that in general
>
> (scc "c" e) y = scc "c" (e y)
>
> to within a small constant factor. So maybe the simplifier, or CorePrep,
> or both, should do this transformation.
> }}}
New description:
If you compile, for example, drvrun014 with -prof -auto-all, you'll see
stuff like
{{{
(scc "c" (dataToTag#)) y
}}}
This generates bad code, because we end up eta-expaning dataToTag, which
allocates an extra function closure.
We think that in general
{{{
(scc "c" e) y = scc "c" (e y)
}}}
to within a small constant factor. So maybe the simplifier, or
`CorePrep`, or both, should do this transformation.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/609#comment:5>
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