8374878: Add Atomic<T>::compare_set #29135

shipilev · 2026-01-09T09:47:18Z

Following up on JDK-8367013 improvement, there is an opportunity to rewrite some of our low-level cas(oldv, newv) == oldv patterns to more straight-forward compare-and-set helper method. This is useful when you do not actually care about the result that used to be in memory or that is currently in memory, you only care if CAS succeeded or not.

Java atomics already have this duality; arguably due to historical timeline of having compare-and-set before introducing compare-and-exchange.

Found this when converting Epsilon to Atomic (JDK-8374876), where this method would simplify the code a bit. I looked around current uses of compare_exchange and rewrote them to compare_set to show what simplifications are possible.

Additional testing:

Linux x86_64 server fastdebug, all

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Issue

JDK-8374878: Add Atomic<T>::compare_set (Enhancement - P4)

Reviewers

Stefan Karlsson (@stefank - Reviewer) Review applies to b06ab798
Kim Barrett (@kimbarrett - Reviewer)

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shipilev · 2026-01-09T09:47:46Z

@kimbarrett, tell me what you think. I can polish and test it further if this is sensible.

bridgekeeper · 2026-01-09T09:48:51Z

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openjdk · 2026-01-09T09:51:08Z

@shipilev 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:

8374878: Add Atomic<T>::compare_set

Reviewed-by: kbarrett, stefank

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 8 new commits pushed to the master branch:

659b53f: 8374923: runtime/cds/ServiceLoaderTest.java fails with mismatch between cds and non-cds
12894a8: 8373643: Test serviceability/jvmti/vthread/ThreadListStackTracesTest/ThreadListStackTracesTest.java still failing
657d5f7: 8374754: jtreg failure handler - replace inline javascript and inline event handlers with same origin javascript files
... and 5 more: https://git.openjdk.org/jdk/compare/f5fa9e40b09b7b6322edb5f057a6350d44980e14...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

openjdk · 2026-01-09T09:51:53Z

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

hotspot

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 · 2026-01-09T09:55:01Z

Webrevs

01: Full - Incremental (3eeec457)
00: Full (b06ab798)

stefank

Looks good to me, but let's at least wait for Kim's review.

shipilev · 2026-01-09T10:32:51Z

/reviewers 2

openjdk · 2026-01-09T10:34:23Z

@shipilev
The total number of required reviews for this PR (including the jcheck configuration and the last /reviewers command) is now set to 2 (with at least 1 Reviewer, 1 Author).

kimbarrett

I like this addition. One optional suggestion.

src/hotspot/share/runtime/atomic.hpp

shipilev · 2026-01-09T17:50:15Z

First run of Linux x86_64 server fastdebug, all yields no failures. I polished the patch a little, and now running the second iteration. If it is still clean, and I have no other comments, I'll integrate.

kimbarrett

Looks good.

Note that the recent updates invalidated prior approvals. So here's a re-approval.

kimbarrett · 2026-01-09T19:09:35Z

src/hotspot/share/gc/shared/taskqueue.inline.hpp


  // Note that using "bottom" here might fail, since a pop_local might
  // have decremented it.
  assert_not_underflow(localBot, newAge.top());


[pre-existing] This assertion seems weird. It is checking newAge.top() after
already using been installed. Maybe it is misplaced? Or maybe it is checking
the wrong thing? Needs to be investigated. I've filed
https://bugs.openjdk.org/browse/JDK-8374915

shipilev · 2026-01-11T11:30:58Z

/reviewers 1

openjdk · 2026-01-11T11:31:31Z

@shipilev
The total number of required reviews for this PR (including the jcheck configuration and the last /reviewers command) is now set to 1 (with at least 1 Reviewer).

shipilev · 2026-01-11T20:35:14Z

Testing is still clean. I am integrating now, thanks.

/integrate

openjdk · 2026-01-11T20:37:05Z

Going to push as commit 3368948.
Since your change was applied there have been 8 commits pushed to the master branch:

659b53f: 8374923: runtime/cds/ServiceLoaderTest.java fails with mismatch between cds and non-cds
12894a8: 8373643: Test serviceability/jvmti/vthread/ThreadListStackTracesTest/ThreadListStackTracesTest.java still failing
657d5f7: 8374754: jtreg failure handler - replace inline javascript and inline event handlers with same origin javascript files
... and 5 more: https://git.openjdk.org/jdk/compare/f5fa9e40b09b7b6322edb5f057a6350d44980e14...master

Your commit was automatically rebased without conflicts.

openjdk · 2026-01-11T20:37:14Z

@shipilev Pushed as commit 3368948.

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

xmas92 · 2026-01-12T10:41:55Z

src/hotspot/share/runtime/atomic.hpp

+    return _value.compare_set(decay(compare_value),
+                              decay(new_value),
+                              order);


This is subtly different from what we usually did with AtomicAccess / Atomic compare_exchange on translated types. We used to check equality with the bool operator==(const T&, constT&) rather than bool operator==(const PrimitiveConversions::Translate<T>::Decayed&, const PrimitiveConversions::Translate<T>::Decayed&).

For the PrimitiveConversions::Translate we currently have I do not think there is an issues as they are memcmp equivalent in both cases. But we may introduce a PrimitiveConversion in the future where this is not the case and this function returns false when using recover and bool operator==(const T&, constT&) would have returned true.

shipilev added 2 commits January 9, 2026 10:44

Fix

12aeb0e

Indenting is a bit off

b06ab79

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src/hotspot/share/runtime/atomic.hpp Outdated Show resolved Hide resolved

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shipilev added 3 commits January 9, 2026 18:12

Merge branch 'master' into JDK-8374878-atomic-compare-set

5ed38f2

Delegate non-translating version

d42406f

Missed another use

3eeec45

openjdk bot removed the ready Pull request is ready to be integrated label Jan 9, 2026

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xmas92 reviewed Jan 12, 2026

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8374878: Add Atomic<T>::compare_set #29135

8374878: Add Atomic<T>::compare_set #29135

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