strange real timings partly explained

[Matthew Fluet]

> I was looking at the heap, but you are right, the stack is also relevant.
> Does the GC know enough to do the alignment?  I didn't think it knew about
> doubles, just pointer vs. non-pointer.

The heap doesn't know anything about doubles.  But, I think this is
another place where RCPS could really help.  Tune the representation so
that all doubles in an object are aligned mod 8 iff the beginning of the
object is aligned mod 8.  Now we just need one bit of the object header to
tell the GC whether or not it needs to align the object mod 8.  The
initial heap allocation of the object would also need to be tweaked,
although there are subtlties there -- at compile time, we know what kind
of object we're allocating, so we can include or exclude the frontier
alignment check;  but, we can't arbitrarily roll the frontier forward if
we need alignment, because the last limit check won't have taken that into
account.  On the other hand, maybe we can do that -- we just over-estimate
the size of an object taking into account the possible padding needed for
the alignment and use that as the limit check size.