| > Why has the behavior changed? Because it depends whether eta-expansion
happens
| before float-out. Now it does. I could change that back. The eta expander
knows that
| (sc_sel d) is cheap, so it turns it into this
| > exp = /\a. \d.\x. let d1 = sc_sel d in ....(fromInteger d1 3)....
| > and now again the floater can do nothing.
|
| Couldn't this be handled by what you suggested for
| http://hackage.haskell.org/trac/ghc/ticket/2607?
Eh? I see no connection.
| I'm not sure it's that unusual. I remember complaining about GHC not
floating method
| selectors out of loops. I this we even had an email discussion about this a
couple of
| months ago.
We *do* now float method selections out of loops. Eg
f :: Num a => a -> a
f = /\a. \(d:Num a). \x:a.
letrec g = \v -> ....(fromInteger a d 1)..(negate a d v).....g....
in blah
Here we will float the (fromInteger a d 1) and (negate a d) out of the g-loop,
to give
f = /\a. \(d:Num a). \x:a.
let fi = fromInteger a d 1
nd = negate a d
letrec g = \v -> ....fi..(nd v).....g....
in blah
But we will *not* float here:
f =/\a. \(d:Num a). \v:a. ...(fromInteger a d a1) ...(negate a d v)....
You could imagine we could get this
f = /\a. \(d:Num a).
let fi = fromInteger a d 1
nd = negate a d
in \v. ...fi...(nd v)...
It is conceivable this could be a win, as in
map (f a d) xs
(and that is the case in the MethSharing performance test) but it seldom is.
GHC's heuristic is never to float out *between* two value lambdas.
We could change that, but it's been that way for a long time.
Simon
_______________________________________________
Cvs-ghc mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/cvs-ghc
