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

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 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/a1f34d37b47826e86343e368a5c00f1a4b...

On Sat, Sep 26, 2020 at 7:54 AM Ben Gamari wrote:

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

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...

However, I discovered recently that there are places where GHC *does*

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use

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unboxed tuples with arity greater than 62. For example, the

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GHC.Prim.unpackInt8X64# [2] function returns an unboxed tuple of size

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64. I

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was confused for a while about how this was even possible, but I

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realized

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later than GHC only enforces the tuple size limit in expressions and

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patterns [3]. Simply having a type signature with a large unboxed tuple

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is

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fine in and of itself, and since unpackInt8X64# is implemented as a

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primop,

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no large unboxed tuples are ever used in the "body" of the function.

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(Indeed, primops don't have function bodies in the conventional sense.)

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

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unpackWord8X64# [4], packInt8X64# [5], and packWord8X64# [6].

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But this makes me wonder: how on earth is it even possible to *use*

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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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