RE: Performance degradation when factoring out common code

I know that this is not an easy request, but can either of you produce a small example that demonstrates your problem? If so, please open a ticket. I don’t like hearing about people having to use trial and error with INLINE or SPECIALISE pragmas. But I can’t even begin to solve the problem unless I can reproduce it. Simon From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Harendra Kumar Sent: 08 September 2017 13:50 To: Mikolaj Konarski Cc: ghc-devs@haskell.org Subject: Re: Performance degradation when factoring out common code 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 mailto:harendra.kumar@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 mailto:mikolaj.konarski@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 mailto:harendra.kumar@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

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