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

[Samuel Chee]

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...)

-------------

Commit messages:
 - 8360654: AArch64: Remove redundant dmb from C1 compareAndSet

Changes: https://git.openjdk.org/jdk/pull/26000/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26000&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360654
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26000.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26000/head:pull/26000

PR: https://git.openjdk.org/jdk/pull/26000