Performance degradation when factoring out common code

[Harendra Kumar]

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