[jdk16] RFR: 8259353: VectorReinterpretNode is incorrectly optimized out

[Xiaohong Gong]

Vector reinterpretation just reinterprets the bytes of the vector without performing any value conversions. So normally, optimization like:
  (VectorReinterpret (VectorReinterpret src))  ->  src
can be applied correctly if the logical result and the input `"src"` have the same vector type. However, the results might not be correct if truncation happens after the first reinterpretation: `"(VectorReinterpret src)"`.

Consider the following case:
  byte[] a = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}
  byte[] b = new byte[16]
  byte[] c = new byte[16]

  // load vector from "a" with 128-bits
  ByteVector av = ByteVector.fromArray(SPECIES_128, a, 0);

  // reinterpret to 64-bits
  ByteVector bv = (ByteVector)av.reinterpretShape(SPECIES_64, 0);

  // reinterpret back to 128-bits with the above reinterpreting results
  ByteVector cv = (ByteVector)bv.reinterpretShape(SPECIES_128, 0);
  cv.intoArray(c, 0)
This case:
  1. Reinterprets vector `"av"` from 128-bits to 64-bits. It should only copy the first 8 elements to vector `"bv"` and discard the higher half parts.
  2. Reinterprets vector `"bv"` back to 128-bits. It copies the 64-bits from` "bv"` to` "cv"`, and paddings the rest part of `"cv"` with zero.
  The final values in array `"c"` are expected to be:
  c = [ 0, 1, 2, 3, 4, 5, 6, 7, 0,  0, 0, 0, 0, 0, 0, 0]
However, with the optimization mentioned at the beginning, the second reinterpretation is optimized out. The values in array` "c" `are incorrectly copied from array `"a"`. The values are:
   c = [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
To fix it, this patch adds the vector size constraint for the optimization, that is the first reinterpretation must not be conducted to a shorter vector.

It also fixes a potential issue for the implementation of match rule `"reinterpretX2D (from 16 bytes to 8 bytes)" `on Arm NEON. Specifically, the `"mov"` is always needed even if the` "dst"` and `"src"` are the same register since truncation should be conducted in order to be consistent with the semantics.

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Commit messages:
 - 8259353: VectorReinterpretNode is incorrectly optimized out

Changes: https://git.openjdk.java.net/jdk16/pull/100/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=100&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259353
  Stats: 45 lines in 3 files changed: 19 ins; 1 del; 25 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/100.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/100/head:pull/100

PR: https://git.openjdk.java.net/jdk16/pull/100