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

[Paul Sandoz]

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`:

    /**
     * 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?

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