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

[rwestrel]

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 Loads/Stores 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 Stores 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.

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8339526: C2: store incorrectly removed for clone() transformed to series of loads/stores (Bug - P3)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/27604/head:pull/27604
$ git checkout pull/27604

Update a local copy of the PR:
$ git checkout pull/27604
$ git pull https://git.openjdk.org/jdk.git pull/27604/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 27604

View PR using the GUI difftool:
$ git pr show -t 27604

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/27604.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back roland! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

@rwestrel The following label will be automatically applied to this pull request:

• hotspot-compiler

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

[mlbridge]

Webrevs

• 00: Full (cc69d43c)