Integrated: 8300258: C2: vectorization fails on simple ByteBuffer loop

[Roland Westrelin]

On Mon, 6 Feb 2023 14:15:19 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> 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.

This pull request has now been integrated.

Changeset: dc523a58
Author:    Roland Westrelin <roland at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/dc523a58a6ece87e5865bea0342415a969172c77
Stats:     426 lines in 4 files changed: 415 ins; 1 del; 10 mod

8300258: C2: vectorization fails on simple ByteBuffer loop

Co-authored-by: Emanuel Peter <epeter at openjdk.org>
Reviewed-by: epeter, kvn

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

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