> I recently noticed a behavioral discrepancy in
> jdk.internal.util.ArraysSupport.vectorizedMismatch between the Java
> implementation and the platform intrinsic implementations.
>
> Current behavior
>
> The Java implementation may leave a tail of elements unchecked, returning the
> bitwise complement of the number of remaining elements (i.e., ~remaining).
> The x86_64 intrinsic, by contrast, compares all elements and simply returns
> -1 when no mismatch is found.
> Because of this inconsistency, every caller has to handle the "remaining
> elements" case defensively:
>
>
> int i = vectorizedMismatch(...);
> if (i >= 0) {
> return i; // mismatch found
> } else {
> length -= ~i; // fall through to handle remaining elements
> }
>
> Proposed change
>
> This PR refines the Java implementation so that it always compares all
> elements and returns -1 when no mismatch is found, matching the x86_64
> intrinsic behavior. This also simplify all callsite because it eliminates the
> need for callers to handle remaining elements.
>
> A regression test is included at
> `test/hotspot/jtreg/compiler/intrinsics/VectorizedMismatchReturnDiffTest.java`
> which demonstrates the original behavioral difference.
>
> ## Test
> - [x] tier1 test suites on linux x86_64
> - [x] tier1 test suites on linux aarch64
>
> ---------
> - [x] I confirm that I make this contribution in accordance with the [OpenJDK
> Interim AI Policy](https://openjdk.org/legal/ai).
Kuai Wei has updated the pull request incrementally with one additional commit
since the last revision:
recover some comments
-------------
Changes:
- all: https://git.openjdk.org/jdk/pull/31802/files
- new: https://git.openjdk.org/jdk/pull/31802/files/7febc9f6..b2520207
Webrevs:
- full: https://webrevs.openjdk.org/?repo=jdk&pr=31802&range=01
- incr: https://webrevs.openjdk.org/?repo=jdk&pr=31802&range=00-01
Stats: 14 lines in 1 file changed: 10 ins; 1 del; 3 mod
Patch: https://git.openjdk.org/jdk/pull/31802.diff
Fetch: git fetch https://git.openjdk.org/jdk.git pull/31802/head:pull/31802
PR: https://git.openjdk.org/jdk/pull/31802
