[Haskell-cafe] INLINABLE and SPECIALIZE, GHC 8.0.1

Jyotirmoy Bhattacharya jyotirmoy at jyotirmoy.net
Fri Jun 17 16:50:07 UTC 2016

Thanks! I could not find the SPECIALIZABLE pragma in the docs. Can you
please give me a pointer. As to inline, while it would work, there might be
a large cost in code size, if say the `insert` function of a hash table
were to be inlined everywhere it is used in a program.


On Fri, Jun 17, 2016 at 9:15 PM, Carter Schonwald <
carter.schonwald at gmail.com> wrote:

> You probably want specializable and specialize inline pragmas.
> On Friday, June 17, 2016, Jyotirmoy Bhattacharya <jyotirmoy at jyotirmoy.net>
> wrote:
>> 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 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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