[Haskell-beginners] let x = x in x (GHC.Prim)

[Takenobu Tani]

Hi John,

> popCnt64# = let x = x in x

It represents "bottom, or undefined, or infinite loop" [1].
(Almost primitives can't be represented with Haskell language.)


In your case, It's good to use FFI call [2] like as Sylvain's nice code.

If you dig GHC compiler's primitives, followings [3][4] may be useful.


[1] https://www.fpcomplete.com/blog/2015/02/primitive-haskell
[2]
https://downloads.haskell.org/~ghc/master/users-guide/ffi-chap.html#primitive-imports
[3] https://ghc.haskell.org/trac/ghc/wiki/Commentary/PrimOps
[4] http://www.well-typed.com/blog/2014/06/understanding-the-realworld/

Cheers,
Takenobu


2016-03-23 10:10 GMT+09:00 Sylvain Henry <sylvain at haskus.fr>:

> Hi,
>
> You can also test your primop with a foreign primop.
>
> See here for an example of assembly code (calling cpuid):
>
> https://github.com/hsyl20/ViperVM/blob/master/src/lib/ViperVM/Arch/X86_64/cpuid.c
>
> And here for the Haskell part with the foreign primop that calls the
> assembly code:
>
> https://github.com/hsyl20/ViperVM/blob/master/src/lib/ViperVM/Arch/X86_64/Cpuid.hs#L56
>
> Cheers,
> Sylvain
>
>
> On 23/03/2016 02:07, rahulmutt at gmail.com wrote:
>
> Hi John,
>
> ghc-prim is just a stub package generated for the purpose of
> documentation. All primops are defined in a low level language called Cmm
> in GHC. If you want to make it even faster, you'll need to learn Cmm and
> update the definition in GHC. If you want to a specialized implementation
> for x86 systems, you may need to modify the NCG (Native Code Generator)
> which requires a knowledge of assembly language.
>
> Hope that helps!
> Rahul Muttineni
>
> Sent from my BlackBerry 10 smartphone.
> *From: *John Ky
> *Sent: *Wednesday 23 March 2016 4:40 AM
> *To: *The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell
> *Reply To: *The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell
> *Subject: *[Haskell-beginners] let x = x in x (GHC.Prim)
>
> Hello Haskellers,
>
> I'm trying to write a faster popCount function for x86 systems.
>
> I tried cloning the ghc-prim package and repurposing it for my own needs,
> but it isn't working as hoped.
>
> In particular, popCnt64# was implemented in GHC.Prim as:
>
> popCnt64# = let x = x in x
>
> Which shouldn't terminate.  Yet when I call it, it magically finds the C
> implementation in hs_popcnt64 and returns the correct value.
>
> My cloned project doesn't behave that way.  Instead it doesn't terminate
> as I would expect.
>
> Anyone know what's happening here, if there is a way to make this work or
> tell me if I'm going about this completely the wrong way?
>
> Cheers,
>
> -John
>
>
>
>
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