RFR: 8217576: C1 atomic access handlers use incorrect decorators

[Kim Barrett]

> On Mar 11, 2019, at 3:22 AM, David Holmes <david.holmes at oracle.com> wrote:
> 
> Hi Kim,
> 
> On 10/03/2019 7:47 am, Kim Barrett wrote:
>> Please review this fix of decorator defaulting in C1 atomic access
>> generators.  As noted in the bug, C1 presently ignores the decorators
>> and only generates fully barriered operations, so the incorrect
>> decorators haven't been affecting code generation.
> 
> So by fixing this and potentially using weaker memory ordering we could infact expose hitherto hidden bugs. :) Okay.

It's weirder than that.  The old code produces a decorator set that is
either empty in the MO_ field, or contains two set bits in that
field. Neither of those is valid, so who knows what would happen with
later changes elsewhere.  My guess: either an assertion failure (good)
or quietly ignoring an intentional request for a weaker barrier (not
so good, might lead to some real head-scratching debugging).

> The fix itself looks good.

Thanks.

> Thanks,
> David
> -----
> 
>> CR:
>> https://bugs.openjdk.java.net/browse/JDK-8217576
>> Webrev:
>> http://cr.openjdk.java.net/~kbarrett/8217576/open.00/
>> Testing:
>> mach5 tier1.