RFR: 8312749: Generational ZGC: Tests crash with assert(index == 0 || is_power_of_2(index)) [v2]

[Albert Mingkun Yang]

On Tue, 22 Aug 2023 07:53:03 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

>> This changeset ensures that the array copy stub underlying the intrinsic implementation of `Object.clone` only copies its (double-word aligned) payload, excluding the remaining object alignment padding words, when a non-default `ObjectAlignmentInBytes` value is used. This prevents the specialized ZGC stubs for `Object[]` array copy from processing undefined object alignment padding words as valid object pointers. For further details about the specific failure, see [initial analysis](https://bugs.openjdk.org/browse/JDK-8312749?focusedCommentId=14600658&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-14600658) by Erik Österlund and Stefan Karlsson and comments in the regression test included in this changeset.
>> 
>> As a side-benefit, the changeset simplifies the array size computation logic in `GraphKit::new_array()` by decoupling computation of header size and alignment padding size.
>> 
>> #### Testing
>> 
>> ##### Functionality
>> 
>> - tier1-3 (windows-x64, linux-x64, linux-aarch64, macosx-x64, and macosx-aarch64)
>> - tier4-5 (windows-x64, linux-x64, linux-aarch64, macosx-x64, and macosx-aarch64; ZGC-specific tests only)
>> - tier6-9 (linux-x64; ZGC-specific tests only)
>> - tier1-3, and a few custom examples, applying [JDK-8139457](https://github.com/openjdk/jdk/pull/11044) (under review) on top of this changeset
>> 
>> ##### Performance
>> 
>> Tested performance on the following set of OpenJDK micro-benchmarks, on linux-x64 (for both G1 and ZGC, using different ObjectAlignmentInBytes values):
>> 
>> - `openjdk.bench.java.lang.ArrayClone.byteClone`
>> - `openjdk.bench.java.lang.ArrayClone.intClone`
>> - `openjdk.bench.java.lang.ArrayFiddle.simple_clone`
>> - `openjdk.bench.java.lang.Clone.cloneLarge`
>> - `openjdk.bench.java.lang.Clone.cloneThreeDifferent`
>> 
>> No significant regression was observed.
>
> Roberto Castañeda Lozano has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove extra whitespace

> I suggest to integrate this fix as-is and address the simplification in a separate RFE

Sounds reasonable.

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Marked as reviewed by ayang (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/15288#pullrequestreview-1594421310