RFR: 8300258: C2: vectorization fails on simple ByteBuffer loop [v4]

[Emanuel Peter]

On Fri, 3 Mar 2023 15:37:37 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 incrementally with one additional commit since the last revision:
> 
>   more tests

test/hotspot/jtreg/compiler/c2/irTests/TestVectorizationMismatchedAccess.java line 185:

> 183:         baseOffset = 1;
> 184:         testByteByte5(byteArray, byteArray, 0, size-1);
> 185:     }

Suggestion:

    }

test/hotspot/jtreg/compiler/c2/irTests/TestVectorizationMismatchedAccess.java line 186:

> 184:         testByteByte5(byteArray, byteArray, 0, size-1);
> 185:     }
> 186:     

Suggestion:

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

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