September 2017 - ghc-devs

GHC 8.2.1 release commit
by Harendra Kumar 13 Sep '17

13 Sep '17

Hi, In the GHC git repo how do I figure out which commit belongs to release 8.2.1. I cannot find 8.2.1 in "git tag" output. I tried "git log" and searching for 8.2.1 but there seems to be no definitive comment marking 8.2.1. -harendra

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Does span run in bounded space?
by Facundo Domínguez 13 Sep '17

13 Sep '17

Dear devs, I used to think that span is a function which doesn't run in bounded space. But ghc defied this understanding. The program copied below runs in bounded space with "ghc-8.2.1 -O0 main.hs; ./main" but it does not if run with "runghc-8.2.1 -O0 main.hs". Does someone have any insights on what's the optimization that is making a difference in the memory footprint of these two methods of running? Or is there a bug in ghci, and span is supposed to run in bounded space after all? Thanks, Facundo import Prelude hiding (span) import Data.List (foldl') main = case span (>0) (replicate 100000000 1) of (xs, ys) -> do print (foldl' (+) 0 xs) print (foldl' (+) 0 ys) span :: (a -> Bool) -> [a] -> ([a],[a]) span _ xs@[] = (xs, xs) span p xs@(x:xs') | p x = let (ys, zs) = span p xs' in (x : ys, zs) | otherwise = ([],xs)

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Feedback on applying implicit exceptions wanted
by Andreas Klebinger 13 Sep '17

13 Sep '17

I'm currently working out ideas for potential optimizations in pattern matching compilation for GHC. While I think the theory works out fine in regards to exceptions I would like feedback on the possible implications. Trac ticket with details is here: https://ghc.haskell.org/trac/ghc/ticket/14201#comment:4 As for the theory I worked based of the assumptions that error = bottom and pattern match failure = error. Otherwise we get different results from both cases even in theory. While I mostly worry about the practical implications I would also appreciate any other feedback.

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Semigroup repeat (base package)
by Harendra Kumar 12 Sep '17

12 Sep '17

Hi, I am sending this question here since base ships with ghc, let me know if this is not the right forum for this. I could not find a function that repeats a value using a semigroup append. I am looking for something like this: srepeat :: Semigroup a => a -> a srepeat x = xs where xs = x <> xs Is it already available somewhere? Does it make sense to add it to Data.Semigroup? Thanks, Harendra

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Re: Performance degradation when factoring out common code
by Harendra Kumar 09 Sep '17

09 Sep '17

Thanks Mikolaj! I have seen some surprising behavior quite a few times recently and I was wondering whether GHC should do better. In one case I had to use SPECIALIZE very aggressively, in another version of the same code it worked well without that. I have been doing a lot of trial and error with the INLINE/NOINLINE pragmas to figure out what the right combination is. Sometimes it just feels like black magic, because I cannot find a rationale to explain the behavior. I am not sure if there are any more such problems lurking in, perhaps this is an area where some improvement looks possible. -harendra On 8 September 2017 at 17:10, Mikolaj Konarski <mikolaj.konarski(a)gmail.com> wrote: > Hello, > > I've had a similar problem that's been fixed in 8.2.1: > > https://ghc.haskell.org/trac/ghc/ticket/12603 > > You can also use some extreme global flags, such as > > ghc-options: -fexpose-all-unfoldings -fspecialise-aggressively > > to get most the GHC subtlety and shyness out of the way > when experimenting. > > Good luck > Mikolaj > > > > On Fri, Sep 8, 2017 at 11:21 AM, Harendra Kumar > <harendra.kumar(a)gmail.com> wrote: > > Hi, > > > > I have this code snippet for the bind implementation of a Monad: > > > > AsyncT m >>= f = AsyncT $ \_ stp yld -> > > let run x = (runAsyncT x) Nothing stp yld > > yield a _ Nothing = run $ f a > > yield a _ (Just r) = run $ f a <> (r >>= f) > > in m Nothing stp yield > > > > I want to have multiple versions of this implementation parameterized by > a > > function, like this: > > > > bindWith k (AsyncT m) f = AsyncT $ \_ stp yld -> > > let run x = (runAsyncT x) Nothing stp yld > > yield a _ Nothing = run $ f a > > yield a _ (Just r) = run $ f a `k` (bindWith k r f) > > in m Nothing stp yield > > > > And then the bind function becomes: > > > > (>>=) = bindWith (<>) > > > > But this leads to a performance degradation of more than 10%. inlining > does > > not help, I tried INLINE pragma as well as the "inline" GHC builtin. I > > thought this should be a more or less straightforward replacement making > the > > second version equivalent to the first one. But apparently there is > > something going on here that makes it perform worse. > > > > I did not look at the core, stg or asm yet. Hoping someone can quickly > > comment on it. Any ideas why is it so? Can this be worked around somehow? > > > > Thanks, > > Harendra > > > > _______________________________________________ > > ghc-devs mailing list > > ghc-devs(a)haskell.org > > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > > >

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Re: Performance degradation when factoring out common code
by Harendra Kumar 08 Sep '17

08 Sep '17

I tested my code on GHC 8.2.1, there is a good news and a bad news. The good news is that with 8.2.1 performance of my code has improved by a huge margin of around 20%, I had reported a similar perf improvement earlier in another one of my package with 8.2. The bad news is that the problem that I reported today is not fixed. I am still seeing a 15% difference between automatic and manual inlining. I guess I should raise a ticket. -harendra On 8 September 2017 at 17:10, Mikolaj Konarski <mikolaj.konarski(a)gmail.com> wrote: > Hello, > > I've had a similar problem that's been fixed in 8.2.1: > > https://ghc.haskell.org/trac/ghc/ticket/12603 > > You can also use some extreme global flags, such as > > ghc-options: -fexpose-all-unfoldings -fspecialise-aggressively > > to get most the GHC subtlety and shyness out of the way > when experimenting. > > Good luck > Mikolaj > > > > On Fri, Sep 8, 2017 at 11:21 AM, Harendra Kumar > <harendra.kumar(a)gmail.com> wrote: > > Hi, > > > > I have this code snippet for the bind implementation of a Monad: > > > > AsyncT m >>= f = AsyncT $ \_ stp yld -> > > let run x = (runAsyncT x) Nothing stp yld > > yield a _ Nothing = run $ f a > > yield a _ (Just r) = run $ f a <> (r >>= f) > > in m Nothing stp yield > > > > I want to have multiple versions of this implementation parameterized by > a > > function, like this: > > > > bindWith k (AsyncT m) f = AsyncT $ \_ stp yld -> > > let run x = (runAsyncT x) Nothing stp yld > > yield a _ Nothing = run $ f a > > yield a _ (Just r) = run $ f a `k` (bindWith k r f) > > in m Nothing stp yield > > > > And then the bind function becomes: > > > > (>>=) = bindWith (<>) > > > > But this leads to a performance degradation of more than 10%. inlining > does > > not help, I tried INLINE pragma as well as the "inline" GHC builtin. I > > thought this should be a more or less straightforward replacement making > the > > second version equivalent to the first one. But apparently there is > > something going on here that makes it perform worse. > > > > I did not look at the core, stg or asm yet. Hoping someone can quickly > > comment on it. Any ideas why is it so? Can this be worked around somehow? > > > > Thanks, > > Harendra > > > > _______________________________________________ > > ghc-devs mailing list > > ghc-devs(a)haskell.org > > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > > >

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Performance degradation when factoring out common code
by Harendra Kumar 08 Sep '17

08 Sep '17

Hi, I have this code snippet for the bind implementation of a Monad: AsyncT m >>= f = AsyncT $ \_ stp yld -> let run x = (runAsyncT x) Nothing stp yld yield a _ Nothing = run $ f a yield a _ (Just r) = run $ f a <> (r >>= f) in m Nothing stp yield I want to have multiple versions of this implementation parameterized by a function, like this: bindWith k (AsyncT m) f = AsyncT $ \_ stp yld -> let run x = (runAsyncT x) Nothing stp yld yield a _ Nothing = run $ f a yield a _ (Just r) = run $ f a `k` (bindWith k r f) in m Nothing stp yield And then the bind function becomes: (>>=) = bindWith (<>) But this leads to a performance degradation of more than 10%. inlining does not help, I tried INLINE pragma as well as the "inline" GHC builtin. I thought this should be a more or less straightforward replacement making the second version equivalent to the first one. But apparently there is something going on here that makes it perform worse. I did not look at the core, stg or asm yet. Hoping someone can quickly comment on it. Any ideas why is it so? Can this be worked around somehow? Thanks, Harendra

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Help with #14140
by David Feuer 07 Sep '17

07 Sep '17

Could you maybe point me toward where the constant folding is happening in this context? I'd like to take a glance and see if I can guess how to upgrade it to deal with what we know things *aren't*. Thanks in advance. David FeuerWell-Typed, LLP

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A newcomer: how to start
by Sergey Bykov 07 Sep '17

07 Sep '17

Hi, my name is Sergey and I'm recently joined to this mailing list. I would like to help GHC's community and start contributing to the GHC project. Could someone recommend a few tasks, which seems to be suitable for a beginner? About myself: I'm pretty experienced Research Scientist/Software Developer in such areas like discrete optimization and machine learning. Thanks in advance!

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Re: [GHC] #13716: Move CI to Jenkins
by davean 07 Sep '17

07 Sep '17

Thats actually how this project started. On Wed, Sep 6, 2017 at 7:05 PM, GHC <ghc-devs(a)haskell.org> wrote: > #13716: Move CI to Jenkins > -------------------------------------+---------------------- > --------------- > Reporter: bgamari | Owner: (none) > Type: task | Status: new > Priority: normal | Milestone: > Component: None | Version: 8.0.1 > Resolution: | Keywords: > Operating System: Unknown/Multiple | Architecture: > | Unknown/Multiple > Type of failure: None/Unknown | Test Case: > Blocked By: 13897 | Blocking: > Related Tickets: #11958 | Differential Rev(s): > Wiki Page: | > -------------------------------------+---------------------- > --------------- > > Comment (by refold): > > Is there any chance Cabal/cabal-install could piggyback on this effort? > We've outgrown free services like Travis and AppVeyor and could use > support for additional platforms (OpenBSD and maybe ARM) too. > > -- > Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13716#comment:6> > GHC <http://www.haskell.org/ghc/> > The Glasgow Haskell Compiler >

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