Containers INLINE issue continues

Don Stewart dons at
Sat Sep 25 01:57:07 EDT 2010

> Hi,
> we still need to find consensus on containers INLINE issue
> (sorry about me already deciding and pushing :().
> To recapitulate, one part of the containers performance improvements was
> to put INLINE on nearly all Data.Map methods. That has positive impact
> on performance (especially for Map Int, Map Word, Map Char, etc, where
> the comparison is just an instruction; for fold method the speedup works
> for almost any type). The reason for this is that inlining allows
> specialization to happen.
> The negative effect is that the code for insert, lookup, etc. is
> repeated on every call site. That is not very reasonable (code bloat,
> I-cache misses).
> In the long run we could have something like 'SPECIALISABLE' pragma,
> which would work like c++ template specialization -- every specialization
> of a method would be only once in the resulting binary. That would have
> both speed and reasonably small code bloat.
> GHC 7.0 supports INLINABLE -- the method unfolding is present in the .hi
> file and GHC decides whether to inline or not.
> In the short run we need to choose some kind of compromise. What are the
> possibilities:
> a) we remove all INLINE and will mark every method INLINABLE. This keeps
>   smallest code, but if someone needs great performance, they can
>   persuade GHC-7.0 inliner to inline the methods from containers.
> b) we leave INLINE on small amount of small methods where there is a big
>    gain, and mark the rest as INLINABLE.
>    This is what is currently present in the repo (what I pushed without
>    consensus).
>    This is a compromise, some code bloat, small performance losses. The
>    performance gains are present also on ghc-6.* family (not true with
>    a).
> c) leave all methods marked INLINE. Biggest code bloat (numbers in some
>    of my previous mails), theoretically biggest performance
>    (theoretically I-cache misses could be larger).
> I think b) is good in the short run, but it is only my opinion. But a)
> is good idea too and I would also support it.
> I am flying to ICFP in an hour. I will investigate a) [what switches to
> give to GHC-7.0 to inline] and post the results, but it will take some
> time.

There's no rush, and we can discuss it all at ICFP :-)

-- Don

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