Luite is currently working on unboxed tuple support in the interpreter. This will also be limited, as getting a generic solution for arbitrary sized tuples raises a lot of complications.

Thus form a practical point of view, I’d go for (1) ;-)

We’ll need to rethink and get SIMD proper support at some point though, the lack of such is rather sad.

On Sat, 26 Sep 2020 at 8:27 PM, Ryan Scott <ryan.gl.scott@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@smart-cactus.org> wrote:
On September 25, 2020 6:21:23 PM EDT, Ryan Scott <ryan.gl.scott@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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