[aarch64-port-dev ] RFR: aarch64: minor improvements of atomic operations

[Yangfei (Felix)]

> -----Original Message-----
> From: Andrew Haley [mailto:aph at redhat.com]
> Sent: Monday, November 11, 2019 7:17 PM
> To: Yangfei (Felix) <felix.yang at huawei.com>;
> aarch64-port-dev at openjdk.java.net
> Subject: Re: [aarch64-port-dev ] RFR: aarch64: minor improvements of atomic
> operations
> 
> On 11/5/19 6:20 AM, Yangfei (Felix) wrote:
> > Please review this small improvements of aarch64 atomic operations.
> > This eliminates the use of full memory barriers.
> > Passed tier1-3 testing.
> 
> No, rejected.
> 
> Patch also must go to hotspot-dev.

CCing to hotspot-dev.  

> Are you sure this is safe? The HotSpot internal barriers are specified as being
> full two-way barriers, which these are not. Tier1 testing really isn't going to do
> it. Now, you might argue that none of the uses in HotSpot actually require
> anything stronger that acq/rel, but good luck proving that.

I was also curious about the reason why full memory barrier is used here.  
For add_and_fetch, I was thinking that there is no difference in functionality for the following two code snippet.  
It's interesting to know that this may make a difference.  Can you elaborate more on that please?  

1) without patch
.L2:
        ldxr    x2, [x1]
        add     x2, x2, x0
        stlxr   w3, x2, [x1]
        cbnz    w3, .L2
        dmb     ish
        mov     x0, x2
        ret
-----------------------------------------------
2) with patch
.L2:
        ldaxr   x2, [x1]
        add     x2, x2, x0
        stlxr   w3, x2, [x1]
        cbnz    w3, .L2
        mov     x0, x2
        ret