RFR: 8300258: C2: vectorization fails on simple ByteBuffer loop [v5]

[Doug Simon]

On Mon, 6 Mar 2023 14:26:19 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.
>
> Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   improved test

test/hotspot/jtreg/compiler/c2/irTests/TestVectorizationMismatchedAccess.java line 38:

> 36:  * @bug 8300258
> 37:  * @key randomness
> 38:  * @requires (os.simpleArch == "x64") | (os.simpleArch == "aarch64")

Does this require `@requires compiler.c2`? What is the expectation if this (or any IR) test runs on GraalVM?
cc @chhagedorn

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PR Review Comment: https://git.openjdk.org/jdk/pull/12440#discussion_r1230958136