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

[Andrew Haley]

On Tue, 2 Mar 2021 08:29:14 GMT, Dong Bo <dongbo at openjdk.org> wrote:

> 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`.

No. Try this with and without the acquire:

 Jon1
{ 0:X1=x; 0:X3=y; 1:X1=y; 1:X3=x; 1:X4=z; }
 P0 | P1 ;
 MOV W0,#1 | LDR W0,[X1];
 STR W0,[X1] | CASAL W5,W6,[X4];
 MOV W2,#1 | LDR W2,[X3];
 STLR W2,[X3] | ;
exists
(1:X0=1 /\ 1:X2=0)```

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