Hi,

I tried to get J ready for packaging, but thanks to my CPU not
supporting AVX2 and some helpful discussion in IRC, I was able to avoid
an error here.

The (guix cpu) code is not meant to be used at runtime as the comment
to ijconsole implies it wants it to be used.  Rather, you should use it
at compile time to bake in the correct tuning parameters, and it should
ideally also support "cross-tuning" (though in that case you probably
need to disable the tests because your CPU won't make sense of the
code).  ijconsole not only fails that, but it also fails at an even
more basic task in Guix' launcher scripts, which is actually knowing
the thing you launch.  (We hardcode everything, period.)

Now I could just disable everything AVX-related in J and push something
that works on x86 and amd64, but since you do claim that AVX is
important to J, there are also other options one could take here.  One
is to implement tuning for this package the correct way, though since
it reinvents build systems, that's probably going to be a hard one. 
The other would be to define package variants (e.g. jsoftware-j-with-
avx) and use those -- there ought to be an old blog post on Guix HPC
detailing the rationale behind doing that.

WDYT?  Is AVX worth the trouble?