How is GHC.Prim.unpackInt8X64# meant to be used?

[Vilem Liepelt]

Ryan, the comment in GHC.Tuple seems to be misleading today, as suggested by this SO post: https://stackoverflow.com/questions/46412823/why-are-ghc-tuples-limited-to-size-62

I was going to add it to the comment at some point but was hesitant as I hadn’t verified the information myself.

V

On 26 Sep 2020, at 14:26, Ryan Scott <ryan.gl.scott at gmail.com <mailto:ryan.gl.scott at gmail.com> > wrote:

I had a feeling that this might be the case. Unfortunately, this technology preview is actively blocking progress on !4097, which leaves me at a loss for what to do. I can see two ways forward:

1. Remove unpackInt8X64# and friends.
2. Reconsider whether the tuple size limit should apply to unboxed tuples. Perhaps this size limit only makes sense for boxed tuples? This comment [1] suggests that defining a boxed tuple of size greater than 62 induces a segfault, but it's unclear to me if the same thing happens for unboxed tuples.

Ryan S.
-----
[1] https://gitlab.haskell.org/ghc/ghc/-/blob/a1f34d37b47826e86343e368a5c00f1a4b1f2bce/libraries/ghc-prim/GHC/Tuple.hs#L170

On Sat, Sep 26, 2020 at 7:54 AM Ben Gamari <ben at smart-cactus.org <mailto:ben at smart-cactus.org> > wrote:
On September 25, 2020 6:21:23 PM EDT, Ryan Scott <ryan.gl.scott at gmail.com <mailto:ryan.gl.scott at gmail.com> > wrote:
...
>However, I discovered recently that there are places where GHC *does*
>use
>unboxed tuples with arity greater than 62. For example, the
>GHC.Prim.unpackInt8X64# [2] function returns an unboxed tuple of size
>64. I
>was confused for a while about how this was even possible, but I
>realized
>later than GHC only enforces the tuple size limit in expressions and
>patterns [3]. Simply having a type signature with a large unboxed tuple

>is
>fine in and of itself, and since unpackInt8X64# is implemented as a
>primop,
>no large unboxed tuples are ever used in the "body" of the function.
>(Indeed, primops don't have function bodies in the conventional sense.)

>Other functions in GHC.Prim that use unboxed tuples of arity 64 include

>unpackWord8X64# [4], packInt8X64# [5], and packWord8X64# [6].
>
>But this makes me wonder: how on earth is it even possible to *use*
>unpackInt8X64#? 


I strongly suspect that the answer here is "you can't yet no one has noticed until now." The SIMD operations were essentially introduced as a technology preview and therefore never had proper tests added. Only a subset of these operations have any tests at all and I doubt anyone has attempted to use the 64-wide operations, which are rather specialized.

Cheers,

- Ben 
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