8318776: Require supports_cx8 to always be true

[dholmes-ora]

As discussed in JBS all platforms (some tweaks to Zero are in progress) actually do support cx8 i.e. 64-bit compare-and-exchange, so we can strip out the locked-based alternatives to using it and just add a guarantee that it is true at runtime. And all platforms except some ARM variants set SUPPORTS_NATIVE_CX8, so we can greatly simplify things. Summary of changes:

• _supports_cx8 field is only needed when SUPPORTS_NATIVE_CX8 is not defined
• Assertions for supports_cx8() are removed
• Compiler predicates requiring supports_cx8() are removed
• Access backend is greatly simplified without the need for lock-based alternative
• java.util.concurrent.AtomicLongFieldUpdater is simplified without the need for a lock-based alternative

I did consider moving all the ARM kuser_helper related code to be only defined when SUPPORTS_NATIVE_CX8 is not defined, but there was a theoretical risk this could change the behaviour if ARMv7 binaries were run on other ARM CPU's. I added a note to that effect in vm_version_linux_arm32.cpp so the ARM port maintainers could clean this up further if desired.

Testing:

• All Oracle tiers 1-5 builds (which includes an ARMv7 build)
• GHA builds/tests
• Oracle tiers 1-3 sanity testing

Zero changes coming in via JDK-8319777 will be merged when they arrive.

Thanks.

---

Progress

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Issue

• JDK-8318776: Require supports_cx8 to always be true (Enhancement - P3)

Reviewers

• Erik Österlund (@fisk - Reviewer) ⚠️ Review applies to 597cef53
• Aleksey Shipilev (@shipilev - Reviewer) ⚠️ Review applies to 2393b9d4
• Daniel D. Daugherty (@dcubed-ojdk - Reviewer) ⚠️ Review applies to aad0a4c4

Reviewing

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[bridgekeeper]

👋 Welcome back dholmes! 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]

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Webrevs

• 06: Full (8870c2d7)
• 05: Full - Incremental (2393b9d4)
• 04: Full - Incremental (aad0a4c4)
• 03: Full - Incremental (597cef53)
• 02: Full - Incremental (65871144)
• 01: Full - Incremental (b6dea4b6)
• 00: Full (3f2ec66f)

[magicus]

/label -build

[openjdk]

@magicus
The build label was successfully removed.

[fisk]

This looks great!

[openjdk]

@dholmes-ora This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8318776: Require supports_cx8 to always be true

Reviewed-by: eosterlund, shade, dcubed

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At the time when this comment was updated there had been no new commits pushed to the master branch. If another commit should be pushed before you perform the /integrate command, your PR will be automatically rebased. If you prefer to avoid any potential automatic rebasing, please check the documentation for the /integrate command for further details.

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[dholmes-ora]

Thanks for the review @fisk ! I have to wait for a few Zero related PRs to get integrated then re-merge, before I can integrate.

[shipilev]

Thanks for the review @fisk ! I have to wait for a few Zero related PRs to get integrated then re-merge, before I can integrate.

Zero patches were pushed, please re-merge. I checked current mainline works well with at least linux-arm-zero-fastdebug, and I would like to re-test it with this patch.

[dholmes-ora]

@shipilev I have re-merged and update the Zero changes (ifdef around _saupports_cx8).

@viktorklang-ora and/or @DougLea could I ask you to look at the java.util.concurrent.AtomicLongFieldUpdater changes please.

[shipilev]

Thanks! Zero tests are running. The PR looks great, except extra safety suggestion in x86 part:

[shipilev]

Zero tests are running.

Caught the guarantee on linux-arm-zero-fastdebug! But that is actually the fault in my previous patch: #16779.

[DougLea]

The deletion of backup code and the check for it in java.util.concurrent.AtomicLongFieldUpdater are clearly OK. We always thought the need for it was transient.

[dcubed-ojdk]

Wow! This PR is much larger than I expected.

Thumbs up!

[dholmes-ora]

Thanks for looking at this @DougLea !

[dholmes-ora]

Wow! This PR is much larger than I expected.

Thumbs up!

Thanks for the Review Dan! Yes lots of code deletion engineering in this one - and even better I got to delete template code with meta-programming stuff! :D

[viktorklang-ora]

@dholmes-ora Just passing by -- impressed by the thorough update!

[dholmes-ora]

Thanks for taking a look @viktorklang-ora !

[shipilev]

Ran full jcstress on linux-arm-zero-release on RPi 4 without problem.

[openjdk]

@dholmes-ora this pull request can not be integrated into master due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout 8318776-supports_cx8
git fetch https://git.openjdk.org/jdk.git master
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge master"
git push

[dholmes-ora]

Thanks for all the reviews. Integrating now.

/integrate

[openjdk]

Going to push as commit c75c388.
Since your change was applied there have been 8 commits pushed to the master branch:

• 14557e7: 8314502: Change the comparator taking version of GrowableArray::find to be a template method
• 2802643: 8314243: Make VM_Exit::wait_for_threads_in_native_to_block wait for user threads time configurable
• 6f26311: 8318490: Increase timeout for JDK tests that are close to the limit when run with libgraal
• cb95e39: 8224261: JProgressBar always with border painted around it
• 6d79e0a: 8318159: RISC-V: Improve itable_stub
• 06f040b: 8320645: DocLint should use javax.lang.model to detect default constructors
• beaa79c: 8320663: Fix C syntax in LIB_SETUP_HSDIS_BINUTILS
• 3787ff8: 8319700: [AArch64] C2 compilation fails with "Field too big for insn"

Your commit was automatically rebased without conflicts.

[openjdk]

@dholmes-ora Pushed as commit c75c388.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.