[vector] RFC: Add AArch64 SVE 256-byte support for API fromArray with a mask

[Yang Zhang]

Hi, 

I'm adding AArch64 SVE 256-byte support for API fromArray with a mask. 

The issue of current implementation.
In Vector API Java implementations, byte[-128, 127] data type is used to express the lane numbers of a ByteVector.
For AArch64 SVE, the maximum vector length is 256 bytes(2048/8) which is out of the range of byte.

A test case for API ByteVector fromArray with a mask:
This API loads a vector from an array of type {@code byte[]}
starting at an offset and using a mask.
      // Assume vector length is 256 bytes.
      static final VectorSpecies<Byte> SPECIES = ByteVector.SPECIES_MAX;
      int vl = SPECIES.length();  // 256
      static final int LENGTH = 1024;
      byte [] index = new byte[LENGTH];
      boolean [] mask = new boolean[LENGTH];

      for (int i = 0; i < LENGTH; i++) {
          index[i] = (byte) i;
          mask[i] = false;
      }

      for (int i = 0; i < 200; i++) {
          mask[i] = true;
      }
      vmask = VectorMask.fromArray(SPECIES, mask, 0);
      io = ByteVector.fromArray(SPECIES, index, 800, vmask);
      System.out.println("io: " + io);

This test case fails. Error log is: 
Caused by: java.lang.IllegalArgumentException: Vector creation failed: value 255 cannot be represented in ETYPE byte; result of cast is -1
	at jdk.incubator.vector/jdk.incubator.vector.AbstractSpecies.badElementBits(AbstractSpecies.java:411)
	at jdk.incubator.vector/jdk.incubator.vector.ByteVector$ByteSpecies.longToElementBits(ByteVector.java:3644)
	at jdk.incubator.vector/jdk.incubator.vector.ByteVector$ByteSpecies.checkValue(ByteVector.java:3610)
	at jdk.incubator.vector/jdk.incubator.vector.AbstractSpecies.iotaArray(AbstractSpecies.java:435)
	at jdk.incubator.vector/jdk.incubator.vector.ByteMaxVector.<clinit>(ByteMaxVector.java:69)
	... 10 more

I have raised such concern before[1], and there are a lot of discussions.
Based on these, I draft a patch that using two vectors to support 256-byte vector. Could you please help to review it?

Webrev: http://cr.openjdk.java.net/~yzhang/vectorapi/vectorapi.fromarray/webrev.00/index.html

In original version, byte array iota[] is used to store lane numbers.
    for (int i = 0; i < laneCount; i++) {
        iota[i] = i;
    }
In my version, byte array iota[] and iotaMask[] are used.
    for (int i = 0; i < laneCount; i++) {
        if (i < 128) {
            iota[i] = i;
            iotamask[i] = 0;
        } else {
            iota[i] = i - 128;
            iotamask[i] = 1;
        }
    }
When lane numbers are needed, we need to restore real lane numbers
based on iota and iotaMask.

With this patch, byte vector with a mask can be loaded correctly.
correctly.

Regards
Yang

[1] https://mail.openjdk.java.net/pipermail/panama-dev/2019-November/006605.html