RFR: 8262519: AArch64: Unnecessary acquire semantics of memory-order-conservative atomics in C++ Hotspot code

[Dong Bo]

The aarch64 LSE atomic operations are introduced to C++ hotspot code in JDK-8261027 and optimized in JDK-8261649.
For memory_order_conservative, the acquire semantics in atomic instructions, i.e. ldaddal, swpal, casal, ensure that no subsequent accesses can pass the atomic operations.
We also have a trailing dmb to ensure barrier-ordered-after relationship, it can ensure what the acquire does. So the acquire semantics is no longer needed, {ldaddl, swpl, casl} would be enough.

Checked by using the herd7 consistency model simulator with the test in comments before `gen_cas_entry`:
AArch64 LseCasAfter
{ 0:X1=x; 0:X2=y; 1:X1=x; 1:X2=y; }
P0           | P1                ;
LDR W4, [X2] | MOV W3, #0        ;
DMB LD       | MOV W4, #1        ;
LDR W3, [X1] | CASL W3, W4, [X1] ;
             | DMB ISH           ;
             | STR W4, [X2]      ;
exists
 (0:X3=0 /\ 0:X4=1)
No `X3 == 0 && X4 == 1` witnessed.

Remove the acquire semantics does not allow prior accesses to pass the atomic operations, because the release semantics are still there.
Just in case, checked by herd7 with the testcase below:
AArch64 LseCasPrior
{ 0:X1=x; 0:X2=y; 1:X1=x; 1:X2=y; }
P0           | P1                ;
LDR W3, [X1] | MOV W3, #0        ;
DMB LD       | MOV W4, #1        ;
LDR W4, [X2] | STR W4, [X2]      ;
             | CASL W3, W4, [X1] ;
             | DMB ISH           ;
exists
 (0:X3=1 /\ 0:X4=0)
No `X3 == 1 && X4 == 0` witnessed.

Similarly, the default implementations of `atomic_fetch_add` and `atomic_xchg` via `ldaxr+stlxr+dmb` can be replaced by `ldxr+stlxr+dmb`.

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

Commit messages:
 - 8262519: AArch64: Unnecessary acquire semantics of atomics in C++ Hotspot code

Changes: https://git.openjdk.java.net/jdk/pull/2788/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2788&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262519
  Stats: 7 lines in 2 files changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2788.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2788/head:pull/2788

PR: https://git.openjdk.java.net/jdk/pull/2788