#1547: Arity can decrease with -prof
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Reporter: igloo | Owner:
Type: bug | Status: new
Priority: high | Milestone: 6.10 branch
Component: Profiling | Version: 6.6.1
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: stm package/tests/conc052 | Architecture: Unknown
Os: Unknown |
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Comment (by simonpj):
I had a half-done change in `SimplUtils` which I'm just going to dump here
for now. The actual code change (which I am not sure is right) is this:
{{{
hunk ./compiler/simplCore/SimplUtils.lhs 827
- any isRuntimeVar bndrs
+ any isRuntimeVar bndrs || not (exprIsTrivial body)
+ -- Note [RHS eta expansion]
}}}
and the note is this:
{{{
Note [RHS eta expansion]
~~~~~~~~~~~~~~~~~~~~~~~~
The basic idea is to transform
f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
(n >= 0)
where (in both cases)
* The xi can include type variables
* The yi are all value variables
* N is a NORMAL FORM (i.e. no redexes anywhere)
wanting a suitable number of extra args.
This is OK even if n=0; for example:
let g=\xs. x:xs in (\ys. map g ys)
Here we can eta expand to
\ys. let g=\xs. x:xs in map g ys
You might think the f-binding woudl have floated, but it may not when
we are profiling:
f = scc "foo" (let g=\xs. x:xs in (\xs. map g ys))
Furthermore, f's arity might have been 1 before, if it originally looked
like
h g ys = map g ys
f = scc "foo" (h (\xs. x:xs))
and we do not expect like the arity to decrease so that it now looks
like zero (to the cheap-and-cheerful exprArity).
However, we must be careful not to undo the effect of eta-reduction, hence
the check for `(not (exprIsTrivial body))`.
}}}
I just want to capture the state of play because I can't finish this
today.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1547#comment:2>
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