RFR(M): 8155949: Support relaxed semantics in cmpxchg

[David Holmes]

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.
>>>>
>>>>
>>>>
>>>>