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

[Albert Mingkun Yang]

On Tue, 15 Aug 2023 12:43:56 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.

If I understand it correctly, much of the diff is to ensure that `ArrayCopyNode::make` (in `BarrierSetC2::clone`) gets the correct value for the `length` arg, calculated as `align_up(array-length * elem-size, word-size) / word-size`.

I wonder if it's possible to pass the actual array length (#slots) as `length` and move the merge-bytes-to-words-copying optimization to a lower level, e.g. inside `conjoint_jbytes`. Ofc, `BarrierSetC2::clone_at_expansion` and its derived siblings need to be adjusted accordingly, e.g. to use the actual elem-type.

(Preexisting: having `ArrayCopyNode` to cover both array and instance cloning hinders the readability, IMO.)

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PR Review: https://git.openjdk.org/jdk/pull/15288#pullrequestreview-1586685925