8365799: AArch64: Remove trailing DMB from cmpxchgptr for LSE

[spchee]

According to the Java SE 24 API, CompareAndExchange has the memory semantics as given by VarHandle.compareAndExchange, which has the following effects [1]:

Atomically sets the value of a variable to the newValue with the
memory semantics of setVolatile if the variable's current value,
referred to as the witness value, == the expectedValue, as accessed
with the memory semantics of getVolatile.

Thus, the store-release due to setVolatile only occurs if the compare is successful. Since CASAL already satisfies these requirements, the DMB does not need to occur to ensure memory ordering in case the compare fails and a store-release does not happen.

Therefore, we can remove the DMB from cmpxchgptr when LSE is enabled. We also rename it to cmpxchgptr_barrier to indicate that this method provides trailing barrier semantics (via either LSE CASAL or a DMB).

The unused cmpxchgw is removed.

[1] https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/lang/invoke/VarHandle.html#compareAndExchange(java.lang.Object...)

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Issue

• JDK-8365799: AArch64: Remove trailing DMB from cmpxchgptr for LSE (Enhancement - P4)

Reviewers

• Andrew Haley (@theRealAph - Reviewer)

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8365799: AArch64: Remove trailing DMB from cmpxchgptr for LSE

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At the time when this comment was updated there had been 21 new commits pushed to the master branch:

• 908f3c9: 8356411: Comment tree not reporting correct position for label
• 169d145: 8365188: Remove LockingMode related code from s390
• 70f3469: 8365556: ObjectMonitor::try_lock_or_add_to_entry_list() returns true with the wrong state of the node
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@spchee Please do not rebase or force-push to an active PR as it invalidates existing review comments. Note for future reference, the bots always squash all changes into a single commit automatically as part of the integration. See OpenJDK Developers’ Guide for more information.

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@spchee 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 cmpxchgw_dmb
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[spchee]

Tested and passes jcstress.

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[mlbridge]

Webrevs

• 01: Full - Incremental (e6e46d16)
• 00: Full (4a95340c)

[spchee]

Ok looks like there are some very recent changes to the relevant files since I last touched it. Need to fix my merge-conflict and have a look at whats changed.

[spchee]

Have fixed :)

[dholmes-ora]

Not sure why you are using the specification for the Java API cmpxchg function to determine what the MacroAssembler function does?? Is this the intrinsic for the Java code?

[spchee]

You're right, it is a little inaccurate to use the Java API to determine what cmpxchgptr does. Although the original intent to remove the membar is still functionality correct.

Although looking into it more, it seems that cmpxchgptr can now be removed completely. The recent PR #26594 removed two of its call sites, and the only other existing one is within MacroAssembler::cmpxchg_obj_header - a method which never gets called to.

So I propose:

• We close this pr
• Open new one which removes the methods MacroAssembler::cmpxchg_obj_header, MacroAssembler::cmpxchgptr and MacroAssembler::cmpxchgw completely since they are no longer called to from anywhere.