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

[Roland Westrelin]

> 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

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27604/files
  - new: https://git.openjdk.org/jdk/pull/27604/files/b6652e04..6dedf517

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27604&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27604&range=01-02

  Stats: 13287 lines in 275 files changed: 10607 ins; 1596 del; 1084 mod
  Patch: https://git.openjdk.org/jdk/pull/27604.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27604/head:pull/27604

PR: https://git.openjdk.org/jdk/pull/27604