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

[Ben Gamari]

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 <ryan.gl.scott at 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 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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