RFR 8202201: All oop stores in the x64 interpreter are treated as volatile when using G1

Erik Österlund erik.osterlund at oracle.com
Mon Sep 17 12:34:36 UTC 2018

Hi Coleen,

On 2018-09-17 14:02, coleen.phillimore at oracle.com wrote:
> On 9/17/18 12:55 AM, David Holmes wrote:
>> Hi Coleen,
>> Wow. Have you run any performance tests to see what we've gained by 
>> eliding all the unnecessary "lock addl" psuedo-mfence?
> No.  I would be surprised if there's any noticable difference except 
> in microbenchmarks.

I did some Xint measurements when I accidentally found out we had this 
problem, and didn't notice any difference, sadly. I was hoping for some 
speedup. The whole thing made me giggle nevertheless.

>> On 15/09/2018 1:44 AM, coleen.phillimore at oracle.com wrote:
>>> Summary: ran out of registers, generated volatile and non-volatile 
>>> branches.
>>> open webrev at http://cr.openjdk.java.net/~coleenp/8202201.01/webrev
>> Generally looks good. Your webrev seems to have picked up some noise 
>> from Mikael's unused Label changes ??
> I manually made the same changes myself since I'm going to have to 
> merge when he checks his in.
>> One query. Can you comment on the change to the 32-bit code where we 
>> no longer use MO_RELAXED for the store? I'm assuming this is just a 
>> potential optimization in the case that the store might be ordered 
>> somehow by default. Though why this would be 32-bit specific I don't 
>> know.
> Yeah, it looks like an optimization.  Maybe Erik can tell us why he 
> added it.  I assume he added it.

MO_RELAXED is default, so you don't need to add it explicitly.

Also, changes look good to me. Not sure about the unused labels, but if 
you say there is a plan for those, then I guess I am okay with that.


> Thanks,
> Coleen
>> Thanks,
>> David
>>> bug link https://bugs.openjdk.java.net/browse/JDK-8202201
>>> Tested with applications/jcstress, tier1&2 and jvmti tests 
>>> (sometimes they filled up interpreter fixed space).
>>> putfield bytecode size is 2176 bytes now, 1248 bytes before
>>> putstatic bytecode size is 1344 bytes now, 864 before
>>> fast_xputfield is same 96 bytes.
>>> Thanks,
>>> Coleen

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