[Haskell-cafe] INLINABLE and SPECIALIZE, GHC 8.0.1

[Jyotirmoy Bhattacharya]

Sorry, realized that attachments cannot be sent to the list. I have put the
code on GitHub here https://github.com/jmoy/testing-specialize

On Fri, Jun 17, 2016 at 4:31 PM, Jyotirmoy Bhattacharya <
jyotirmoy at jyotirmoy.net> wrote:

> I'm trying to write a mutable hash table library that would work both in
> the IO and ST monads, so I'm using the PrimMonad typeclass [see
> JmoyHash.hs  in the attached archive].
>
> For efficiency, I would like the functions to be specialized to the
> concrete monad at the call site. According to Section 9.31.9.2 of the GHC
> User's Guide
> <https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#specialize-for-imported-functions>
>
> The optimiser *also* considers each *imported* INLINABLE overloaded
> function, and specialises it for the different types at which it is called
> in M.
>
> So I marked all the functions in my library as INLINABLE. Yet adding a
> SPECIALIZE pragma in Main.hs (currently commented out) for an imported
> function improves runtime performance by 3x, which should not be happening
> since, if I understand the manual right, the function should have been
> specialized anyway since it is marked INLINABLE.
>
> I am writing to this list rather than filing a bug since I'm not sure if
> I'm reading the manual right and I have not explicitly verified that the
> specialization is not happening. I would greatly appreciate any help on
> both counts.
>
> I'm using GHC 8.0.1 with the -O2 flag.
>
> Marking the function as INLINE might solve the problem but that's
> something I don't want to do as it seems to me that specialization and not
> inlining is what's appropriate here.
>
> Regards,
> Jyotirmoy Bhattacharya
>
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