RFR: 8365799: AArch64: Remove trailing DMB from cmpxchgptr for LSE

[Samuel Chee]

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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Commit messages:
 - 8365799: AArch64: Remove trailing DMB from cmpxchgptr for LSE

Changes: https://git.openjdk.org/jdk/pull/26845/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26845&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365799
  Stats: 113 lines in 3 files changed: 67 ins; 40 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/26845.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26845/head:pull/26845

PR: https://git.openjdk.org/jdk/pull/26845