[vector] Vector API -- alignment with value types

[Brian Goetz]

Sure, for now.  When vectors are values, we probably won’t need such inspection?  

> On Feb 11, 2019, at 8:12 PM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
> 
> 
>> I thought this might be the case; zeros are so common that we may well want to just instantiate a zero vector as a species field, which can be shared.  Then we just expose “species.zero()” as a method (which ByteVector.zero() can delegate to.)
> 
> FTR zero() is not cached, but backed by an intrinsic, because it's easier to optimize: it doesn't require JIT-compiler to inspect the boxed value to be able to use faster idiom when constructing vector value at runtime (e.g., xor vs load).
> 
> Best regards,
> Vladimir Ivanov
> 
>> I didn’t want to do this in my first patch because I was mostly intent on providing that we will get the same inlining with a more value-friendly API, but this makes sense to me.
> 
>>> On Feb 8, 2019, at 7:23 PM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
>>> 
>>> 
>>>> http://cr.openjdk.java.net/~sviswanathan/vectorIntrinsics/Brian/zero/webrev.00/
>>> 
>>> I have one comment on implementation:
>>> 
>>> src/jdk.incubator.vector/share/classes/jdk/incubator/vector/ByteVector.java:
>>> 
>>> +    public static ByteVector zero(ByteSpecies species) {
>>> +        return VectorIntrinsics.broadcastCoerced((Class<ByteVector>) species.boxType(), byte.class, species.length(),
>>> +                                                 0,
>>> +                                                 (z -> species.zero()));
>>> +    }
>>> 
>>> There's a slight change in behavior compared to "species.zero()" call and that's because not all arguments to VectorIntrinsics.broadcastCoerced() are guaranteed to be constant.
>>> 
>>> If JIT can't devirtualize & inline "species.boxType()", intrinsification fails and falls back to default implementation.
>>> 
>>> In case of species.zero(), a failure to inline callee leads to a virtual call, but callee contains intrinsified operation.
>>> 
>>> Overall, I believe it shouldn't matter from performance perspective right now, because a call breaks vector box elimination anyway, but in a longer term it may become important.
>>> 
>>> But I'd still prefer to see calls to VectorIntrinsics be fully specialized. In the particular case I referred to, IMO it's better to just call species.zero().
>>> 
>>> Best regards,
>>> Vladimir Ivanov