RFR(M): 8155949: Support relaxed semantics in cmpxchg
Zhengyu Gu
zgu at redhat.com
Tue May 24 16:03:00 UTC 2016
On 05/24/2016 11:38 AM, Zhengyu Gu wrote:
>
>
> On 05/24/2016 09:06 AM, Doerr, Martin wrote:
>> Hi David,
>>
>> unfortunately, Atomic::add(jlong) is used by mallocTracker.hpp (e.g.
>> line 56). Removing it breaks the build.
> It should be replaced with size_t version in mallocTracker.hpp.
>
I created https://bugs.openjdk.java.net/browse/JDK-8157709 for this.
-Zhengyu
> -Zhengyu
>
>
>
>>
>> But I could change it as follows:
>> inline jlong Atomic::add(jlong add_value, volatile jlong* dest) {
>> #ifdef _LP64
>> return (jlong) add_ptr((intptr_t) add_value, (volatile intptr_t*)
>> dest);
>> #else
>> jlong old = load(dest);
>> jlong new_value = old + add_value;
>> while (old != cmpxchg(new_value, dest, old)) {
>> old = load(dest);
>> new_value = old + add_value;
>> }
>> return new_value;
>> #endif
>> }
>>
>> Best regards,
>> Martin
>>
>>
>> -----Original Message-----
>> From: David Holmes [mailto:david.holmes at oracle.com]
>> Sent: Dienstag, 24. Mai 2016 14:27
>> To: Doerr, Martin <martin.doerr at sap.com>; Andrew Haley
>> (aph at redhat.com) <aph at redhat.com>
>> Cc: Hiroshi H Horii <HORII at jp.ibm.com>; Tim Ellison
>> <Tim_Ellison at uk.ibm.com>; ppc-aix-port-dev at openjdk.java.net;
>> hotspot-gc-dev at openjdk.java.net; hotspot-runtime-dev at openjdk.java.net
>> Subject: Re: RFR(M): 8155949: Support relaxed semantics in cmpxchg
>>
>> Hi Martin,
>>
>> On 24/05/2016 8:21 PM, Doerr, Martin wrote:
>>> Hi David,
>>>
>>> it was moved for the same reason as the jint version of cmpxchg: It
>>> passes the memory order to the jint version.
>>> It may look large in terms of C++ code, but there's not much
>>> substantial content.
>>> I can only see a loop which calls the jint version + a bunch of very
>>> simple operations.
>>> Why shouldn't we give compilers a chance to inline and possibly
>>> optimize some of the simple operations and especially to eliminate
>>> the order check?
>> I think this forces the compiler to inline it, not just "gives it a
>> chance". But I'll leave it to those more knowledgeable about the
>> compiler side of this to comment.
>>
>> But if we're making these changes can you delete the Atomic::add(jlong)
>> - it is unused and incorrect as discussed here:
>>
>> http://mail.openjdk.java.net/pipermail/hotspot-dev/2016-February/021620.html
>>
>>
>> Thanks,
>> David
>>
>>> Best regards,
>>> Martin
>>>
>>> -----Original Message-----
>>> From: David Holmes [mailto:david.holmes at oracle.com]
>>> Sent: Dienstag, 24. Mai 2016 12:04
>>> To: Doerr, Martin <martin.doerr at sap.com>; Andrew Haley
>>> (aph at redhat.com) <aph at redhat.com>
>>> Cc: Hiroshi H Horii <HORII at jp.ibm.com>; Tim Ellison
>>> <Tim_Ellison at uk.ibm.com>; ppc-aix-port-dev at openjdk.java.net;
>>> hotspot-gc-dev at openjdk.java.net; hotspot-runtime-dev at openjdk.java.net
>>> Subject: Re: RFR(M): 8155949: Support relaxed semantics in cmpxchg
>>>
>>> On 24/05/2016 7:37 PM, Doerr, Martin wrote:
>>>> Hi David and Andrew,
>>>>
>>>> sorry for missing this one. There were too many emails.
>>>>
>>>> After moving the jint version as well, there was not much left of
>>>> atomic.cpp.
>>>> I think it doesn't make any sense to keep a couple of trivial
>>>> functions in the cpp file.
>>>> Therefore, I have removed atomic.cpp and moved the remaining small
>>>> functions into the inline file.
>>> Sorry I don't understand why the jbyte cmpxchg_general was moved to the
>>> .inline.hpp file - it seems far too big to be inlined.
>>>
>>> David
>>>
>>>> Webrev is here:
>>>> http://cr.openjdk.java.net/~mdoerr/8155949_relaxed_cas/webrev.05/
>>>>
>>>> Best regards,
>>>> Martin
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: David Holmes [mailto:david.holmes at oracle.com]
>>>> Sent: Dienstag, 24. Mai 2016 05:50
>>>> To: Doerr, Martin <martin.doerr at sap.com>
>>>> Cc: Hiroshi H Horii <HORII at jp.ibm.com>; Tim Ellison
>>>> <Tim_Ellison at uk.ibm.com>; ppc-aix-port-dev at openjdk.java.net;
>>>> hotspot-gc-dev at openjdk.java.net; hotspot-runtime-dev at openjdk.java.net
>>>> Subject: Re: RFR(M): 8155949: Support relaxed semantics in cmpxchg
>>>>
>>>> Hi Martin,
>>>>
>>>> On 23/05/2016 7:29 PM, Doerr, Martin wrote:
>>>>> Hi David,
>>>>>
>>>>> here's the new webrev:
>>>>> http://cr.openjdk.java.net/~mdoerr/8155949_relaxed_cas/webrev.04/
>>>> There seems to be some confusion. You've moved the jbyte
>>>> Atomic::cmpxchg_general from the .cpp file to the .inline/hpp file,
>>>> but
>>>> the comments from Andrew and Kim were about moving the unsigned
>>>> Atomic::cmpxchg version. ??
>>>>
>>>> Aside: In the changeset contributor's have to be specified by "email
>>>> address" or "name <email address>", OpenJDK user names are not
>>>> accepted.
>>>> I think Andrew should also be listed there for the Aarch64 component.
>>>>
>>>> Thanks,
>>>> David
>>>>
>>>>> Btw.: The jbyte version of cmpxchg can be implemented on aarch
>>>>> like on ppc where we emulate the byte access by a 4 byte access
>>>>> (lwarx/stwcx). But that should better be done in a separate change.
>>>>>
>>>>> Thanks for your time and your support.
>>>>>
>>>>> Best regards,
>>>>> Martin
>>>>>
>>>>> -----Original Message-----
>>>>> From: David Holmes [mailto:david.holmes at oracle.com]
>>>>> Sent: Samstag, 21. Mai 2016 01:10
>>>>> To: Doerr, Martin <martin.doerr at sap.com>
>>>>> Cc: Hiroshi H Horii <HORII at jp.ibm.com>; Tim Ellison
>>>>> <Tim_Ellison at uk.ibm.com>; ppc-aix-port-dev at openjdk.java.net;
>>>>> hotspot-gc-dev at openjdk.java.net; hotspot-runtime-dev at openjdk.java.net
>>>>> Subject: Re: RFR(M): 8155949: Support relaxed semantics in cmpxchg
>>>>>
>>>>> Hi Martin,
>>>>>
>>>>> Are you in a position to make the change now suggested by both Kim
>>>>> and
>>>>> Andrew? Can you also include the Aarch64 code that Andrew provided:
>>>>>
>>>>> http://cr.openjdk.java.net/~aph/8154736
>>>>>
>>>>> I'd like to get this finalized so it is ready to push as soon as the
>>>>> process allows it to.
>>>>>
>>>>> Thanks,
>>>>> David
>>>>>
>>>>> On 20/05/2016 8:03 AM, Kim Barrett wrote:
>>>>>>> On May 18, 2016, at 6:12 AM, Doerr, Martin
>>>>>>> <martin.doerr at sap.com> wrote:
>>>>>>>
>>>>>>> Hi Kim,
>>>>>>>
>>>>>>> thank you very much for the detailed review.
>>>>>>>
>>>>>>> I agree with your comments and I have made all your requested
>>>>>>> changes here:
>>>>>>> http://cr.openjdk.java.net/~goetz/wr16/8155949-relaxed_cas/webrev.03/
>>>>>>>
>>>>>>>
>>>>>>> It's correct that the change changes the semantics of the
>>>>>>> conservative cmpxchg. In case of failure, we also execute the
>>>>>>> sync instruction, now.
>>>>>>> Advantage is that the new implementation is maximum conservative
>>>>>>> by default. I think this makes sense as long as the semantics of
>>>>>>> the hotspot C++ cmpxchg are not clearly specified.
>>>>>>>
>>>>>>> For performance optimization, we should better use (or introduce
>>>>>>> additional) enum values.
>>>>>> ------------------------------------------------------------------------------
>>>>>>
>>>>>> There doesn't seem to have been any change for this earlier comment.
>>>>>>
>>>>>> src/share/vm/runtime/atomic.cpp
>>>>>> 59 unsigned Atomic::cmpxchg(unsigned int exchange_value,
>>>>>> 60 volatile unsigned int* dest,
>>>>>> unsigned int compare_value,
>>>>>> 61 cmpxchg_memory_order order) {
>>>>>>
>>>>>> I'm surprised this was ever out-of-line. But with this change it's
>>>>>> quite bad to be out-of-line, as that's going to kill the constant
>>>>>> propogation of the order value.
>>>>>>
>>>>>> ------------------------------------------------------------------------------
>>>>>>
>>>>>>
>>>>>> Other than that, looks good.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>
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