RFR: 8360654: AArch64: Remove redundant dmb from C1 compareAndSet

[Ruben]

On Sat, 17 Jan 2026 04:28:33 GMT, Ruben <duke at openjdk.org> wrote:

> This is duplicate of the PR #26000 which was originally created by @spchee.
> 
> ===========
> 
> AtomicLong.CompareAndSet has the following assembly dump snippet which gets emitted from the intermediary LIRGenerator::atomic_cmpxchg:
> 
> 
> ;; cmpxchg {
>   0x0000e708d144cf60:   mov	x8, x2
>   0x0000e708d144cf64:   casal	x8, x3, [x0]
>   0x0000e708d144cf68:   cmp	x8, x2
>  ;; 0x1F1F1F1F1F1F1F1F
>   0x0000e708d144cf6c:   mov	x8, #0x1f1f1f1f1f1f1f1f
>  ;; } cmpxchg
>   0x0000e708d144cf70:   cset	x8, ne  // ne = any
>   0x0000e708d144cf74:   dmb	ish
> 
> According to the Oracle Java Specification, AtomicLong.CompareAndSet [1] has the same memory effects as specified by VarHandle.compareAndSet which has the following effects: [2]
> 
>> 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.
> 
> Hence the release on the store 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 release does not happen.
> 
> Hence we remove the dmb from both casl and casw (same logic applies to the non-long variant)
> 
> This is also reflected by C2 not having a dmb for the same respective method.
> 
> [1] https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/util/concurrent/atomic/AtomicLong.html#compareAndSet(long,long)
> [2] https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/lang/invoke/VarHandle.html#compareAndSet(java.lang.Object...)

I've run java -jar jcstress.jar (revision 1d143cbd430f4cca63a8f0c8c1fad3aabc065421) for this PR with -XX:TieredStopAtLevel=1 combined with -XX:-UseLSE / -XX:+UseLSE.

Result for `-XX:TieredStopAtLevel=1 -XX:+UseLSE`:

Jan 28 00:12:16   Failed tests: No matches.
Jan 28 00:12:16
Jan 28 00:12:16   Error tests: No matches.
Jan 28 00:12:16
Jan 28 00:12:16   All remaining tests: 4957 matching test results. Use -v to print them.


Result for `-XX:TieredStopAtLevel=1 -XX:-UseLSE`:

Jan 29 01:43:00   Failed tests: No matches.
Jan 29 01:43:00
Jan 29 01:43:00   Error tests: No matches.
Jan 29 01:43:00
Jan 29 01:43:00   All remaining tests: 4947 matching test results. Use -v to print them.

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PR Comment: https://git.openjdk.org/jdk/pull/29287#issuecomment-3826168430