8370405: C2: mismatched store from MergeStores wrongly scalarized in allocation elimination

[shipilev]

Fixes JDK 21 -> JDK 25 regression in C2 that can lead to miscompilation. There is a regression test that captures the failure.

Additional testing:

• Linux x86_64 server fastdebug, new regression test fails without the fix, passes with it
• Linux x86_64 server fastdebug, all

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• JDK-8370405 needs maintainer approval
• Commit message must refer to an issue
• JDK-8371385 needs maintainer approval

Issues

• JDK-8370405: C2: mismatched store from MergeStores wrongly scalarized in allocation elimination (Bug - P3)
• JDK-8371385: compiler/escapeAnalysis/TestRematerializeObjects.java fails in case of -XX:-UseUnalignedAccesses (Bug - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk25u.git pull/382/head:pull/382
$ git checkout pull/382

Update a local copy of the PR:
$ git checkout pull/382
$ git pull https://git.openjdk.org/jdk25u.git pull/382/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 382

View PR using the GUI difftool:
$ git pr show -t 382

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk25u/pull/382.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back shade! 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.

[openjdk]

⚠️ @shipilev This change is now ready for you to apply for maintainer approval. This can be done directly in each associated issue or by using the /approval command.

[mlbridge]

Webrevs

• 01: Full - Incremental (0a989a57)
• 00: Full (2ce40615)

[shipilev]

/issue add JDK-8371385

[openjdk]

@shipilev
Adding additional issue to issue list: 8371385: compiler/escapeAnalysis/TestRematerializeObjects.java fails in case of -XX:-UseUnalignedAccesses.