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

[Ben Gamari]

On September 25, 2020 6:21:23 PM EDT, Ryan Scott <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