RFR: 8262096: Vector API fails to work due to VectorShape initialization exception [v5]

[Jie Fu]

On Sun, 28 Feb 2021 13:31:38 GMT, Jie Fu <jiefu at openjdk.org> wrote:

>> `@requires vm.compiler2.enabled` had been added.
>> Thanks.
>
> @PaulSandoz , are you also OK with the latest version?
> Thanks.

> @DamonFool I think Vladimir is correct in the layering, in this respect i think we can make things a littler clearer. This seems like a small thing but i think its worth making very explicit as there is some hidden complexity.
> 
> What if we add the following method to `VectorShape`:
> 
> ```java
>     /**
>      * Returns the maximum vector bit size for a given element type.
>      *
>      * @param etype the element type.
>      * @return the maximum vector bit.
>      */
>      /*package-private*/
>     static int getMaxVectorBitSize(Class<?> etype) {
>         // May return -1 if C2 is not enabled,
>         // or a value smaller than the S_64_BIT.vectorBitSize / elementSizeInBits, on say 32-bit platforms
>         // If so default to S_64_BIT
>         int maxLaneCount = VectorSupport.getMaxLaneCount(etype);
>         int elementSizeInBits = LaneType.of(etype).elementSize;
>         return Math.max(maxLaneCount * elementSizeInBits, S_64_BIT.vectorBitSize);
>     }
> ```
> 
> It is package private so it can be tested explicitly if need be.
> 
> Then we can reuse that method:
> 
> ```
>     S_Max_BIT(getMaxVectorBitSize(byte.class));
> ```
> 
> ```
>     static VectorShape largestShapeFor(Class<?> etype) {
>         return VectorShape.forBitSize(getMaxVectorBitSize(etype));
>     }
> ```
> 
> I think that's correct, but i have not tested. WDYT?

Good suggestion.
Updated.

Testing:
 - jdk/incubator/vector with MaxVectorSize=default/8/4 on Linux/x64
 - jdk/incubator/vector without C2 on Linux/x64

Thanks.

-------------

PR: https://git.openjdk.java.net/jdk/pull/2722