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

[Yangfei (Felix)]

Actually, I didn't see a real world problem related to this.  
As you mentioned before, there should be a StoreLoad barrier between a volatile store and a volatile load.  
I agree with that and I didn't see this kind of barrier when we have a getAndSet followed by a CompareAndSwap.  
I just worried that developers may create code that relies on this memory model requirement.  


Currently we have C2 JIT for getAndSetLong like: 
  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


And C2 JIT for compareAndSwapLong like: 
433   0x0000ffffa6695088: ldaxr     x8, [x10]
434   0x0000ffffa669508c: cmp       x8, x3
435   0x0000ffffa6695090: b.ne      0x0000ffffa669509c
436   0x0000ffffa6695094: stlxr     w8, x4, [x10]
437   0x0000ffffa6695098: cbnz      w8, 0x0000ffffa6695088
438   0x0000ffffa669509c: cset      x0, eq          ;*invokevirtual compareAndSwapLong


Thanks,
Felix

> 
> On 10/28/19 6:11 AM, Yangfei (Felix) wrote:
> > Ping...
> > Is that enough for us to push the 8u backport patch ?
> > It still applies:
> > http://cr.openjdk.java.net/~fyang/8209835-8u-backport/webrev.00/
> 
> Does this change fix any bugs?
> 
> Does this change make a significant performance improvement?
> 
> It is not a low-risk change and I'm not sure it's worth it.
> Andrew Dinn, what do you think?
> 
> --
> Andrew Haley  (he/him)
> Java Platform Lead Engineer
> Red Hat UK Ltd. <https://www.redhat.com> https://keybase.io/andrewhaley
> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671