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

[Ryan Scott]

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> wrote:

> 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
>
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