RFR: Uses keep_alive_barrier to keep deduped char array alive

[Roman Kennke]

Am 11.10.2017 um 21:50 schrieb Zhengyu Gu:
>>>>
>>> Ok, let me clear my thoughts here.
>>>
>>> If we do SATB barrier, that will ensure existing_value alive.
>> In G1, and Shenandoah (but only during conc-mark, not during 
>> conc-partial).
>>> However, if we do keep_alive_barrier, the existing_value could be 
>>> evacuated, but we still hold old oop here, right? then we can end up 
>>> dead oop after recycle?
>> No, because there cannot be a safepoint between keep_alive_barrier() 
>> and subsequent uses. We may hold a pointer to a from-space object, 
>> but that should be ok if all subsequent uses do their barriers right. 
>> There is only one subsequent use before the oop goes out of scope, 
>> and that is
>>
>> java_lang_String::set_value(java_string, typeArrayOop(existing_value));
>>
>> which does the correct barrier (i.e. storeval-barrier). Everything 
>> should be fine.
> Okay, I see. Thanks for the explanation.
>
> Updated webrev: 
> http://cr.openjdk.java.net/~zgu/shenandoah/strdedup_keep_alive/webrev.01/

It is ok, but I'd just drop the if-else on UseShenandoahGC and 
unconditionally call oopDesc::bs()->keep_alive_barrier(...) that will do 
the right thing for G1 too!

Roman