[vectorIntrinsics] [PATCH] Elemental shifts and rotates speedup

[John Rose]

On May 30, 2019, at 9:35 AM, John Rose <john.r.rose at oracle.com> wrote:
> 
> I looked at the Java changes.  They look good.

One more comment at the API level.  I have been
thinking about  the Java rules for <<n and >>n,
versus our vector-level shifting operations,
and I think the Java rules are not for the best
here. So in Java x<<n is reduced modulo bitsize(x)
(by and-ing with n-1).  But this doesn't seem
to make sense (to me) when working with lanes.
If you are shifting lanes by a large number,
I think you should just shift in zeroes or
(even better) throw an exception.

Reason:  Lane bundles *sometimes* wrap modulo the
lane-count but *usually* do not, because a bundle
of lanes always represents an oriented segment in
a much larger data workload.  In this setting, an
overflowing lane shift n in shiftLanesRight(n) is probably
unintentional and will reorder the workload unpredictably.
The user can say v.shiftLanesRight(n%LC) if the n parameter
really needs to be wrapped.

Comments?

— John