<html><body><p><font size="2">David, Kim, and Marin,</font><br><br><font size="2">Thank you for your question and comments.</font><br><br><font size="2">The important point I should specify earlier is that CAS contains a compiler barrier that prevents moving code around it, e.g. by clobber "memory" in inline asm code. </font><br><br><font size="2">Thus, release with a two-way compiler barrier (CAS) is the prerequisite in my change.</font><br><br><font size="2">Regarding the Release-Consume ordering, I think making use of implicit consume is enough and it is already realized with the existing code (release and CAS).</font><br><br><br><font size="2">Best regards,</font><br><font size="2">--</font><br><font size="2">Michihiro,</font><br><font size="2">IBM Research - Tokyo</font><br><br><img width="16" height="16" src="cid:1__=8FBB0806DFF3BF2B8f9e8a93df938690918c8FB@" border="0" alt="Inactive hide details for "Doerr, Martin" ---2018/05/22 19:17:36---Hi Kim, I can't see how a new implicit consume is introduced"><font size="2" color="#424282">"Doerr, Martin" ---2018/05/22 19:17:36---Hi Kim, I can't see how a new implicit consume is introduced by Michihiro's change. He just explaine</font><br><br><font size="2" color="#5F5F5F">From: </font><font size="2">"Doerr, Martin" <martin.doerr@sap.com></font><br><font size="2" color="#5F5F5F">To: </font><font size="2">Kim Barrett <kim.barrett@oracle.com>, Michihiro Horie <HORIE@jp.ibm.com></font><br><font size="2" color="#5F5F5F">Cc: </font><font size="2">"hotspot-dev@openjdk.java.net" <hotspot-dev@openjdk.java.net>, "hotspot-gc-dev@openjdk.java.net" <hotspot-gc-dev@openjdk.java.net>, "Gustavo Bueno Romero" <gromero@br.ibm.com>, "ppc-aix-port-dev@openjdk.java.net" <ppc-aix-port-dev@openjdk.java.net>, "david.holmes@oracle.com" <david.holmes@oracle.com></font><br><font size="2" color="#5F5F5F">Date: </font><font size="2">2018/05/22 19:17</font><br><font size="2" color="#5F5F5F">Subject: </font><font size="2">RE: RFR(M): 8154736: enhancement of cmpxchg and copy_to_survivor for ppc64</font><br><hr width="100%" size="2" align="left" noshade style="color:#8091A5; "><br><br><br><tt><font size="2">Hi Kim,<br><br>I can't see how a new implicit consume is introduced by Michihiro's change. He just explained how the existing code works.<br><br>If implicit consume has been rejected the current code is wrong:<br>"new_obj = o->forwardee();" would need some kind of barrier before using the new_obj.<br><br>But this issue is not related to what Michihiro wants to change AFAICS.<br><br>Best regards,<br>Martin<br><br><br>-----Original Message-----<br>From: ppc-aix-port-dev [</font></tt><tt><font size="2"><a href="mailto:ppc-aix-port-dev-bounces@openjdk.java.net">mailto:ppc-aix-port-dev-bounces@openjdk.java.net</a></font></tt><tt><font size="2">] On Behalf Of Kim Barrett<br>Sent: Montag, 21. Mai 2018 06:00<br>To: Michihiro Horie <HORIE@jp.ibm.com><br>Cc: hotspot-dev@openjdk.java.net; hotspot-gc-dev@openjdk.java.net; Gustavo Bueno Romero <gromero@br.ibm.com>; ppc-aix-port-dev@openjdk.java.net; david.holmes@oracle.com<br>Subject: Re: RFR(M): 8154736: enhancement of cmpxchg and copy_to_survivor for ppc64<br><br>> On May 18, 2018, at 5:12 PM, Michihiro Horie <HORIE@jp.ibm.com> wrote:<br>> <br>> Dear all,<br>> <br>> I update the webrev: </font></tt><tt><font size="2"><a href="http://cr.openjdk.java.net/~mhorie/8154736/webrev.09/">http://cr.openjdk.java.net/~mhorie/8154736/webrev.09/</a></font></tt><tt><font size="2"><br>> <br>> 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.)<br>> <br>> 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).<br><br>Relying on implicit consume has been been discussed and rejected, in<br>the earlier thread on this topic and I think elsewhere too.<br><br></font></tt><tt><font size="2"><a href="http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-October/021538.html">http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2016-October/021538.html</a></font></tt><tt><font size="2"><br><br><br></font></tt><br><BR>
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