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

[Emanuel Peter]

On Tue, 21 Feb 2023 16:05:27 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> 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
>
> Maybe we should first collect all legal packs and then try to find the best alignment, dropping packs that are unaligned on those platforms that support it.

Hi @rwestrel @vnkozlov .

I have another general question. Should this really vectorize?

    @Test
    @IR(counts = { IRNode.LOAD_VECTOR, ">=1", IRNode.STORE_VECTOR, ">=1" })
    public static void testOffHeapLong1(long dest, long[] src) {
        for (int i = 0; i < src.length; i++) {
            UNSAFE.putLongUnaligned(null, dest + 8 * i, src[i]);
        }
    }


I talked with @TobiHartmann about this. It is apparently possible to get the address of an array, and store in a `long`. In that case, we could play with that rawptr a bit, and create a cyclic dependency.

Pseudocode:

long[] arr = new long[1000];
long ptr = unsafe.getTheArrayAddress(arr); // does not exist directly, but it is possible
ptr += 8; // shift it one long forward
testOffHeapLong1(ptr, arr);


This should have different behavior if it is vectorized or not.

But maybe this is expected, and just a bad use of `Unsafe`. Probably we want this to vectorize, for the extra performance, and people who use `Unsafe` just have to be careful not to do such strange things.

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