vector alignment

[Martin Sebor <msebor@gmail.com>]

GCC tries to align a vector on its natural boundary, i.e., that
given by its size, up to MAX_OBJECT_ALIGNMENT.  Vectors that are
bigger than that are either silently [mis]aligned on that same
maximum boundary (PR 89798), silently truncated (and misaligned),
or cause an ICE (PR 89797).  Compiling the following:

   __attribute__ ((vector_size (N))) char v;

   _Static_assert (sizeof (v) == N, "size");
   _Static_assert (__alignof__ (v) == N, "alignment");

with N set to 1LLU << I shows these failures:

   I < 29   succeeds
   I < 31   fails alignment
   I < 32   ICE
   I >= 32  fails alignment and size

Attribute aligned doesn't seem to have any effect on types or
variables declared with attribute vector_size.  The alignment
set by the latter prevails.

This happens no matter what scope the vector is defined in (i.e.,
file or local).

I have some questions:

1) Is there some reason to align vectors on the same boundary
    as their size no matter how big it is?  I can't find such
    a requirement in the ABIs I looked at.  Or would it be more
    appropriate to align the big ones on the preferred boundary
    for the target?  For instance, does it make more sense to
    align a 64KB vector on a 64KB boundary than on, say,
    a 64-byte boundary (or some other boundary less than 64K?)

2) If not, is it then appropriate to underalign very large
    vectors on a boundary less than their size?

3) Should the aligned attribute not override the default vector
    alignment?

I would like to think the answer to (1) is that vectors should
be aligned on the preferred boundary for the target/ABI.  If
that's feasible, it should also obviate question (2).

I believe the answer to (3) is yes.  If not, GCC should issue
a warning that it doesn't honor the aligned attribute.

Thanks
Martin