Performance: Faster to define a function writing out all
simonpj at microsoft.com
Tue May 13 04:47:47 EDT 2008
| > Anyway, as I am still wondering why ghc creates different code for
| > returnP a = return a
| > returnP = return
| Ah, now I rember this coming up before.
| Simon, is this a CAF issue? Or did it trigger the -fno-state-hack case?
I'm not sure. A small example would be good.
| I've definitely run into the odd other case where point-freeing
| a bit of code messes with the complexity.
That should not happen -- except for the state-hack. Which is this:
f1 :: Char -> IO ()
f1 = \c. let i = ord c in \s. print i s
Here s::State World. Is this equivalent to
f2 = \c.\s. print (ord c) s
The latter is very much more efficient than 'f1', because it doesn't build an intermediate lambda. This matters a lot in I/O-intensive programs. But if 'f' is called thus:
forever (f 'w')
then (ord 'w') is computed once for each call of f2, but is shared among all calls to f1. And that can make a big difference too.
I have no idea whether this is playing a role in the example you are discussing -- my guess is not, and there may be another performance bug lurking. So eliciting a small demo would be great.
More information about the Glasgow-haskell-users