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 <b...@smart-cactus.org> wrote: > On September 25, 2020 6:21:23 PM EDT, Ryan Scott <ryan.gl.sc...@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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