[vector] Vector cast support

[John Rose]

On Apr 30, 2018, at 3:49 PM, Paul Sandoz <paul.sandoz at oracle.com> wrote:
> 
> I guess this is one of those places where we are better off focusing on optimizing, if need be, the scalar loop over the vector elements?
> 

+1

If there's an operation which is clearly serializing across lanes, then
it shouldn't be in the vector API at all.  This is a borderline decision,
since most of the casts *are* clearly parallel but a few combinations
aren't fully supported.  We are in the shadowy zone between "always
parallelized" and "never parallelized".

Since the vector API is designed as a simpler way to program vectors,
we shouldn't put a sharp edge here, such as throwing an exception if
the operation won't vectorize.  But that's a move we have in our back
pocket, for when the right moment comes.

— John