Performance degradation when factoring out common code

[Harendra Kumar]

I will try creating a minimal example and open a ticket for the inlining
problem, the one I am sure about.

-harendra

On 8 September 2017 at 18:35, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> *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 at haskell.org] *On Behalf Of *Harendra
> Kumar
> *Sent:* 08 September 2017 13:50
> *To:* Mikolaj Konarski <mikolaj.konarski at gmail.com>
> *Cc:* ghc-devs at 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 <harendra.kumar at 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.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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