8358334: C2/Shenandoah: incorrect execution with Unsafe

[rwestrel]

Patch doesn't apply cleanly:

• jdk21u still has IU barriers: context for some of the changes is
  different.

• Because of IU barriers, there's an extra call to fix_ctrl() for
  which the renaming to nodes_above_barriers must be applied.

• The initial patch makes a subtle change that doesn't affect jdk
  25/26 in the code that was
  refactored. ShenandoahBarrierC2Support::push_data_inputs_at_control()
  is introduced with logic:

    if (in != nullptr && phase->has_ctrl(in) && phase->get_ctrl(in) == ctrl) {

which replaces the same logic in
ShenandoahBarrierC2Support::fix_ctrl() but also a slightly
different logic in
ShenandoahBarrierC2Support::is_dominator_same_ctrl():

    if (m->in(i) != nullptr && phase->ctrl_or_self(m->in(i)) == c) {

that is, it uses ctrl_or_self() which works for both data nodes
and control nodes but the new method uses has_ctrl(in) && get_ctrl(in) which can only be true for data nodes. That change
causes failures in jdk 21 again because of IU barriers that produce
a new memory state when expanded and need the logic from
MemoryGraphFixer. What I propose in this backport to be on the
safe side is to leave
ShenandoahBarrierC2Support::is_dominator_same_ctrl() alone (not
apply that part of the refactoring).

Tested with hotspot_gc_shenandoah + tier1 with -XX:+UseShenandoahGC

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• JDK-8358334 needs maintainer approval
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8358334: C2/Shenandoah: incorrect execution with Unsafe (Bug - P2)

Reviewers

• William Kemper (@earthling-amzn - Author)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk21u-dev.git pull/1901/head:pull/1901
$ git checkout pull/1901

Update a local copy of the PR:
$ git checkout pull/1901
$ git pull https://git.openjdk.org/jdk21u-dev.git pull/1901/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 1901

View PR using the GUI difftool:
$ git pr show -t 1901

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk21u-dev/pull/1901.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]

This backport pull request has now been updated with issue from the original commit.

[mlbridge]

Webrevs

• 00: Full (25cba14f)

[earthling-amzn]

My approval doesn't count for anything here, but in case it helps grease the wheels: I ran this test without the patch and observed it fail. I ran the test with the patch and observed it pass. Thank you for the backport!