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

[John Ky]

Hi Rahul,

I see mention of the popcount instruction in nativeGen/X86/CodeGen.hs.  In
particular it looks like it only activates if sse4_2 is activated?  Maybe
all I need to do is activate this somehow?

Cheers,

-John

On Wed, 23 Mar 2016 at 12:07 <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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