Re: Unwanted eta-expansion

The only difference in generated code is in the showsPrec function for a newtype-wrapped Int (ThreadId). The code actually never (explicitly) uses Show instance -- it's only needed as a dependency for Num instance. Could it be some cache effect? I'll see if I can reduce that code and then will file a bug. * Simon Peyton-Jones [2011-10-12 10:56:04+0000]

Roman, Jan-Willem

I'm maxed out at the moment, and will be so for at least a week.

If you think there is something mysterious and J-W agrees, could you create a ticket, with the smallest example you can, and instructions to reproduce? That'd be brilliant.

Of course, Jan-Willem, if you have a moment to diagnose a bit more about WHY it's going slower that would be even better!

Thanks

Simon

| -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Jan-Willem Maessen | Sent: 10 October 2011 02:51 | To: Roman Cheplyaka | Cc: glasgow-haskell-users@haskell.org | Subject: Re: Unwanted eta-expansion | | On Sun, Oct 9, 2011 at 10:54 AM, Roman Cheplyaka wrote: | > * Jan-Willem Maessen [2011-10-08 12:32:18-0400] | >> It seems to be a common misconception that eta-abstracting your | >> functions in this way will speed up or otherwise improve your code. | >> | >> Simon PJ has already provided a good explanation of why GHC eta | >> expands.  Let me take another tack and describe why the code you wrote | >> without eta expansion probably doesn't provide you with any actual | >> benefit.  Roughly speaking, you're creating a chain of closures whose | >> contents exactly describe the contents of your list (ie you've created | >> something that's isomorphic to your original list structure), and so | >> you should expect no benefit at all. | > | > Thanks for the analysis. | > | > I used myFoldl as a minimal example to ask the question. | > | > Here's an example of real code where this does make a difference: | > https://github.com/feuerbach/regex- | applicative/tree/03ca9c852f06bf9a9d51505640b8b72f07291c7d | | Ah, now things get more complicated! :-) I suspect here you're | actually entering the regexp closures, and compiling it down is | actually saving you a teensy bit of interpretive overhead. | | > ... | > What's even more interesting (and puzzling!), if remove | > -fno-do-lambda-eta-expansion for Text/Regex/Applicative/Types.hs, | > the benchmark takes 2.82 seconds. | | That *Is* odd. The only obvious code generated here would be the | newtype instances, for which this should surely be irrelevant? Can | someone at GHC HQ explain this one? | | -Jan | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

-- Roman I. Cheplyaka :: http://ro-che.info/