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

I think as long as it's bounded it's ok. On Sat, Sep 26, 2020 at 8:52 PM Ben Gamari wrote:

I think it would be worth trying to add tuples up to width 64. The only real cost here is the interface file size of GHC.Tuple and if adding a 63-wide tuple really does induce a crash then that is a bug in its own right that deserves investigation.

- Ben

On September 26, 2020 8:26:32 AM EDT, 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 < 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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