RFR (S) 8031818: Experimental VM flag for enforcing safe object construction

[Aleksey Shipilev]

On 01/31/2014 01:53 AM, Vladimir Kozlov wrote:
> hs-comp is closed for almost 2 weeks until SQE done all testing. Only
> ppc merge related fixes are allowed.

Understood, no pressure.

> You check is_initializer() for all types of fields in do_exits(). But
> is_final() is also set for @Stable field which could any methods based
> on the comment in do_put_xxx().

Well, that's disturbing.

In pure spec-induced sense, the block emitting the barrier in do_exits()
is only valid for initializers. That means, @Stable can indeed mimic its
constructor store as the final field store. However, piggybacking on the
same code to produce the barrier for an arbitrary method seems very
error-prone (and even contradicting the comment in do_exits() which
assumes [wrote_final == true] => [method.is_initializer() == true]).

If @Stable writes out the value in an arbitrary method, then it requires
release barrier on it's own. It is a pure luck finals _also_ have
MemBar_Release, while they could have more relaxed form... especially
when/if JMM 9 effort would allow this relaxation. Conflating memory
semantics for finals and @Stable seems wrong.

Hence, I think the change is sound. It keeps @Stable semantics for
constructor stores. @Stable needs more work to emit release barriers for
the arbitrary methods if required (why?), and I think given the failure
scenario affects only non-TSO platforms, it can be done separately.

Thoughts? (Vladimir Ivanov, please report in!)

-Aleksey.

> Thanks,
> Vladimir
> 
> On 1/30/14 12:15 PM, Aleksey Shipilev wrote:
>> On 01/22/2014 11:13 PM, Aleksey Shipilev wrote:
>>> On 01/22/2014 11:07 PM, Vladimir Kozlov wrote:
>>>> Question first: can you wait about 2 weeks when we merge ppc64 changes?
>>>
>>> I think we can wait. I don't want to collide with PPC merge either. In
>>> fact, it would be even more convenient for us to grab PPC C2 in the
>>> experiments from the mainline.
>>
>> Are we open already? I see PPC changes had collided with my patch. Even
>> if it's too early to push the patch, I merged the upstream changes into
>> the updated webrev:
>>    http://cr.openjdk.java.net/~shade/8031818/webrev.01/
>>
>> Instead of introducing a new method, I rewired the predicate and
>> rearranged the comments to make its structure clear. I think extracting
>> method is not required now. Also, a few typos corrected.
>>
>> Testing:
>>   - full cycle JPRT
>>
>> -Aleksey.
>>