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

[Emanuel Peter]

On Tue, 21 Feb 2023 08:26:59 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.
>
> 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

Why are you not changing it here?
https://github.com/openjdk/jdk/blob/6751978122afc5102db84c7f040a0b0ab76e70b6/src/hotspot/share/opto/superword.cpp#L728-L739

If we just removed the memory slice where `best_align_to_mem_ref` was in, then should we not find a new `best`?

src/hotspot/share/opto/superword.cpp line 670:

> 668:         create_pack = false;
> 669:       } else {
> 670:         // Allow independent (different type) unaligned memory operations

Suggestion:

        // Allow independent (different memory slice) unaligned memory operations

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

Changes requested by epeter (Committer).

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