RFR: 8339526: C2: store incorrectly removed for clone() transformed to series of loads/stores [v3]

[Roberto Castañeda Lozano]

On Mon, 13 Oct 2025 15:28:01 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> In the `test1()` method of the test case:
>> 
>> `inlined2()` calls `clone()` for an object loaded from field `field`
>> that has inexact type `A` at parse time. The intrinsic for `clone()`
>> inserts an `Allocate` and an `ArrayCopy` nodes. When igvn runs, the
>> load of `field` is optimized out because it reads back a newly
>> allocated `B` written to `field` in the same method. `ArrayCopy` can
>> now be optimized because the type of its `src` input is known. The
>> type of its `dest` input is the `CheckCastPP` from the allocation of
>> the cloned object created at parse time. That one has type `A`. A
>> series of `Load`s/`Store`s are created to copy the fields of class `B`
>> from `src` (of type `B`) to `dest` of (type `A`).
>> 
>> Writting to `dest` with offsets for fields that don't exist in `A`,
>> causes this code in `Compile::flatten_alias_type()`:
>> 
>> 
>>     } else if (offset < 0 || offset >= ik->layout_helper_size_in_bytes()) {
>>       // Static fields are in the space above the normal instance
>>       // fields in the java.lang.Class instance.
>>       if (ik != ciEnv::current()->Class_klass()) {
>>         to = nullptr;
>>         tj = TypeOopPtr::BOTTOM;
>>         offset = tj->offset();
>>       }
>> 
>> 
>> to assign it some slice that doesn't match the one that's used at the
>> same offset in `B`.
>> 
>> That causes an assert in `ArrayCopyNode::try_clone_instance()` to
>> fire. With a release build, execution proceeds. `test1()` also has a
>> non escaping allocation. That one causes EA to run and
>> `ConnectionGraph::split_unique_types()` to move the store to the non
>> escaping allocation to a new slice. In the process, when it iterates
>> over `MergeMem` nodes, it notices the stores added by
>> `ArrayCopyNode::try_clone_instance()`, finds that some are not on the
>> right slice, tries to move them to the correct slice (expecting they
>> are from a non escaping EA). That causes some of the `Store`s to be
>> disconnected. When the resulting code runs, execution fails as some
>> fields are not copied.
>> 
>> The fix I propose is to skip `ArrayCopyNode::try_clone_instance()`
>> when `src` and `dest` classes don't match as this seems like a rare
>> enough corner case.
>
> 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 six additional commits since the last revision:
> 
>  - review
>  - Merge branch 'master' into JDK-8339526
>  - review
>  - Merge branch 'master' into JDK-8339526
>  - Update src/hotspot/share/opto/arraycopynode.cpp
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - test & fix

Test results look good. Approving, but please consider my suggestions about the test file. Thanks!

-------------

Marked as reviewed by rcastanedalo (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/27604#pullrequestreview-3361966729