GUB's basic document
https://github.com/gperciva/gub/blob/master/web/basics.html
says
```
<h3>SHARING DIRECTORIES</h3>
GUB uses an ABI environment variable to work around some build
bugs in some packages. This variable is not checked by the
environment-changed function, so problems will probably occur if
you share your GUB build directory between a 32-bit and 64-bit OS.
If you change your operating system, delete the target/ directory
of GUB before trying to build it again.
I think this is quite a difficult one to answer.
My main machine was (IIRC) 64 bit originally - that's partly why I
used a VM for GUB, since GUB used not to work in a 64 bit environment.
When I upgraded from Ubuntu 10.04, via 12 to 14.04, the machine became
32 bit, since I selected that option because it seemed that the 64 bit
was not deigned for Intel CPUs. I think I may have misunderstood
that, but whatever the reason, I now run a 32 bit main machine. As
part of the move to Ubuntu 14, I needed to upgrade the Oracle VM
software, but did not change the VM spec at all. So I think the
answer is that the actual VM disk has never seen a 64 bit environment,
but has run within a 64 bit environment.