RFR: 8256973: Intrinsic creation for VectorMask query (lastTrue, firstTrue, trueCount) APIs

[Paul Sandoz]

On Fri, 7 May 2021 17:45:44 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

> Hi @PaulSandoz , that's a nice suggestion, I think instead of reduction which may emit bulky sequence, VectorMask.toLong() + Long.bitCount() could have been used for trueCount. But since toLong may not work for ARM SVE, so in the mean time intrinsifying at the level of API looked reasonable.

Do you mean that reusing `VectorSupport.reductionCoerced` as the intrinsic entry point may emit bulky sequence?

Note that i was not suggesting to reuse `Long.bitCount()` etc. i was just using that as a example that the bit-wise reduction operations on masks can also apply to integral vectors, suggesting there might be some sharing in C2 just like is done for binary-wise operations, such as logical AND.

For example:

        @Override
        @ForceInline
        public Int256Mask and(VectorMask<Integer> mask) {
            Objects.requireNonNull(mask);
            Int256Mask m = (Int256Mask)mask;
            return VectorSupport.binaryOp(VECTOR_OP_AND, Int256Mask.class, int.class, VLENGTH,
                                             this, m,
                                             (m1, m2) -> m1.bOp(m2, (i, a, b) -> a & b));
        }


And notice that `VECTOR_OP_AND` is reused for vector lane-wise binary and reduction operations on `IntVector` etc. Can we do the same for other bitwise reduction-like operations, first implementing optimal support for masks, then later expanding for integral vectors?

So rather than introducing specific constants, such as `VECTOR_OP_MASK_TRUECOUNT` etc, we can generalize to `VECTOR_OP_BITCOUNT` etc that can apply to both masks and integral vectors, where for masks we interpret `BIT` appropriately to mean `boolean` true value.

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PR: https://git.openjdk.java.net/jdk/pull/3916