#6040: Adding a type signature changes heap allocation into stack allocation
without changing the actual type
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Reporter: tibbe | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Changes (by simonpj):
* difficulty: => Unknown
Comment:
Can you give a repo case that shows a performance effect?
I'm guessing, but I think that the difference is something like this:
{{{
f1 x = g 100 f2 x = g x 0
where where
g 0 = x g x 0 = x
g n = g (n-1) g x n = g x (n-1)
}}}
* In `f1` we'll heap-allocate a function closure capturing the free
variable `x`, but the recursive calls have just one argument (plus one for
the function closure itself).
* In `f2` we'll pass `x` as an argument. Indeed `g` will be floated out
to be a top-level function. (And in the example you give I don't
undersatnd why `go` and `insert` are separate functions, but that's a
stylistic thing.)
If there were 100 free varaibles instead of 1, it might well be a good
plan to use `f1`, to save passing 100 arguments in each call. But since
there is only one it'd be better to turn the free variable into an
argument, by lambda lifting. This is the reverse of the "static argument
transformation".
The transformation should be done right at the end, because in general
turning a free varaible into an argument is a bad idea (loss of
information).
I've been meaning to do this for some time; if you have data to show it's
important I could up the priority!
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6040#comment:1>
GHC <http://www.haskell.org/ghc/>
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