Performance degradation when factoring out common code

[Harendra Kumar]

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 at 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 at 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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> > ghc-devs at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> >
>
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