RFR(M): 8154736: enhancement of cmpxchg and copy_to_survivor for ppc64

David Holmes david.holmes at oracle.com
Wed May 23 08:35:53 UTC 2018


On 22/05/2018 8:16 PM, Doerr, Martin wrote:
> Hi Kim,
> 
> I can't see how a new implicit consume is introduced by Michihiro's change. He just explained how the existing code works.
> 
> If implicit consume has been rejected the current code is wrong:
> "new_obj = o->forwardee();" would need some kind of barrier before using the new_obj.

But if forwardee is set by a CAS with (default) full bi-directional 
fence we have that barrier. The argument is that such a strong barrier 
can be relaxed yet maintain correctness - isn't it?

David


> But this issue is not related to what Michihiro wants to change AFAICS.
> 
> Best regards,
> Martin
> 
> 
> -----Original Message-----
> From: ppc-aix-port-dev [mailto:ppc-aix-port-dev-bounces at openjdk.java.net] On Behalf Of Kim Barrett
> Sent: Montag, 21. Mai 2018 06:00
> To: Michihiro Horie <HORIE at jp.ibm.com>
> Cc: hotspot-dev at openjdk.java.net; hotspot-gc-dev at openjdk.java.net; Gustavo Bueno Romero <gromero at br.ibm.com>; ppc-aix-port-dev at openjdk.java.net; david.holmes at oracle.com
> Subject: Re: RFR(M): 8154736: enhancement of cmpxchg and copy_to_survivor for ppc64
> 
>> On May 18, 2018, at 5:12 PM, Michihiro Horie <HORIE at jp.ibm.com> wrote:
>>
>> Dear all,
>>
>> I update the webrev: http://cr.openjdk.java.net/~mhorie/8154736/webrev.09/
>>
>> With the release barrier before the CAS, new_obj can be observed from other threads. If the CAS succeeds, the current thread can use new_obj without barriers. If the CAS fails, "o->forwardee()" is ordered with respect to CAS by accessing the same memory location "_mark", so no barriers needed. The order of (1) access to the forwardee and (2) access to forwardee's fields is preserved due to Release-Consume ordering on supported platforms. (The ordering between "new_obj = o->forwardee();" and logging or other usages is not changed.)
>>
>> Regarding the maintainability, the requirement is CAS(memory_order_release) as Release-Consume to be consistent with C++11. This requirement is necessary when a weaker platform like DEC Alpha is to be supported. On currently supported platforms, code change can be safe if the code meats this requirement (and the order of (1) access to the forwardee and (2) access to forwardee's fields is the natural way of coding).
> 
> Relying on implicit consume has been been discussed and rejected, in
> the earlier thread on this topic and I think elsewhere too.
> 
> http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-October/021538.html
> 


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