I should also point out that I saw performance improvements by manually factoring out and propagating some common expressions to outer loops in performance sensitive paths. Now I have made this a habit to do this manually. Not sure if something like this has also been fixed with that ticket or some other ticket.
-harendra On 8 September 2017 at 17:34, Harendra Kumar <harendra.ku...@gmail.com> wrote: > 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.konar...@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.ku...@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@haskell.org >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >> > >> > >
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