RFR: 8300258: C2: vectorization fails on simple ByteBuffer loop [v2]

[Roland Westrelin]

> The loop that doesn't vectorize is:
> 
> 
> public static void testByteLong4(byte[] dest, long[] src, int start, int stop) {
>     for (int i = start; i < stop; i++) {
>         UNSAFE.putLongUnaligned(dest, 8 * i + baseOffset, src[i]);
>     }
> }
> 
> 
> It's from a micro-benchmark in the panama
> repo. `SuperWord::find_adjacent_refs() `prevents it from vectorizing
> because it finds it cannot properly align the loop and, from the
> comment in the code, that:
> 
> 
> // Can't allow vectorization of unaligned memory accesses with the
> // same type since it could be overlapped accesses to the same array.
> 
> 
> The test for "same type" is implemented by looking at the memory
> operation type which in this case is overly conservative as the loop
> above is reading and writing with long loads/stores but from and to
> arrays of different types that can't overlap. Actually, with such
> mismatched accesses, it's also likely an incorrect test (reading and
> writing could be to the same array with loads/stores that use
> different operand size) eventhough I couldn't write a test case that
> would trigger an incorrect execution.
> 
> As a fix, I propose implementing the "same type" test by looking at
> memory aliases instead.

Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - comments
 - extra test
 - more
 - Merge branch 'master' into JDK-8300258
 - review
 - more
 - fix & test

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12440/files
  - new: https://git.openjdk.org/jdk/pull/12440/files/69895178..67519781

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12440&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12440&range=00-01

  Stats: 70368 lines in 1609 files changed: 31957 ins; 19251 del; 19160 mod
  Patch: https://git.openjdk.org/jdk/pull/12440.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12440/head:pull/12440

PR: https://git.openjdk.org/jdk/pull/12440