[aarch64-port-dev ] RFR: [8u] 8209835: Aarch64: elide barriers on all volatile operations

[Yangfei (Felix)]

> On 8/28/19 11:39 AM, Yangfei (Felix) wrote:
> >> Having said that, I can sort-of imagine a jcstress failure if we had a test that
> was doing compareAndSwap in one thread and getAndSet in another. That
> would be a seriously compelling reason for a backport!
> > Can you elaborate more please? especially the possible code sequence.
> >
> > Currently, we have C2 JIT for getAndSet like this:
> >
> >  33   ;; membar_release
> >  34   0x0000ffffa416ae74: dmb       ish
> >  35   ;; 0x40000000000
> >  36   0x0000ffffa416ae78: orr       x11, xzr, #0x40000000000
> >  37   0x0000ffffa416ae7c: add       x10, x1, #0x10
> >  38   0x0000ffffa416ae80: prfm      pstl1strm, [x10]
> >  39   0x0000ffffa416ae84: ldxr      x9, [x10]
> >  40   0x0000ffffa416ae88: stxr      w8, x11, [x10]
> >  41   0x0000ffffa416ae8c: cbnz      w8, 0x0000ffffa416ae84
> >  42   0x0000ffffa416ae90: mov       x10, x9
> >  43   ;; membar_acquire
> >  44   0x0000ffffa416ae94: dmb       ishld           ;*invokevirtual
> getAndSetLong
> 
> The important thing is that there should be a StoreLoad barrier between a
> volatile store and a volatile load. If we have a getAndSet followed by a
> CompareAndExchange that might not happen.
> 

Hi, 

  Sorry for the late reply.  
  After some searching in jcstress code, I think the following test might be the test you wanted to see. 

  org.openjdk.jcstress.tests.atomics.longs.AtomicLongArrayPairwiseTests.GetAndSet_CAS

 43     @State
 44     public static class S {
 45         public final int SIZE = 256; // double the maximum cache line
 46         public final AtomicLongArray a = new AtomicLongArray(SIZE);
 47         public final int idx = ThreadLocalRandom.current().nextInt(SIZE);
 48     }

445     @JCStressTest
446     @JCStressMeta(G.class)
447     @Outcome(id = "0, 10",  expect = Expect.ACCEPTABLE, desc = "T1 -> T2 execution")
448     @Outcome(id = "20, 20", expect = Expect.ACCEPTABLE, desc = "T2 -> T1 execution")
449     public static class GetAndSet_CAS {
450         @Actor public void actor1(S s, LongResult2 r) { r.r1 = s.a.getAndSet(s.idx, 5); }
451         @Actor public void actor2(S s, LongResult2 r) { r.r2 = s.a.compareAndSet(s.idx, 0, 20) ? 20 : 10; }
452     }

  AtomicLongArray.getAndSet calls unsafe.getAndSetLong and AtomicLongArray.compareAndSet calls compareAndSetRaw which calls unsafe.compareAndSwapLong. 
  For this test, one thread will do actor1 method. And another thread will do actor2 method concurrently. 

Thanks,
Felix