[vector] Vector API -- alignment with value types

[Vladimir Ivanov]

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

In general, I don't think it changes much with value types: special 
support on JIT-compiler side is still needed to strength-reduce vector 
construction operation. Otherwise, a memory load (even from a flattened 
representation) is needed to materialize a value unless JIT knows what 
the value is.

But for particular case of Vector.zero() default value can be used 
(instead of caching).

Best regards,
Vladimir Ivanov

>> 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
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