[aarch64-port-dev ] barrier use in JDK11

[Dave Nuechterlein]

I am new to all of this.  Is there some way to track the progress on this fix?  Can I help?

I have a very simple JMH test that exposes this. Would that be helpful?

Sorry for all of the questions.

Dave


-----Original Message-----
From: Andrew Dinn [mailto:adinn at redhat.com] 
Sent: Friday, June 15, 2018 1:54 AM
To: Andrew Haley <aph at redhat.com>; Dave Nuechterlein <dnuechte at amperecomputing.com>; aarch64-port-dev at openjdk.java.net
Subject: Re: [aarch64-port-dev ] barrier use in JDK11

On 14/06/18 20:37, Andrew Haley wrote:
> On 06/14/2018 06:59 PM, Dave Nuechterlein wrote:
>> I suspect the issue is that there are no customizations for
>> xgene3/emag1 in cpu/aarch64/vm_version_aarch64.cpp.>
> It certainly could be that.

I doubt very much that is the problem. The only customization that affects translation of dmb;ldr;dmb to ldar is the CPU_DMB_ATOMICS feature which, when set, will to update the default value for command line option UseBarriersForVolatile to true. This is only set in one case

  if (_cpu == CPU_CAVIUM && (_model == 0xA1)) {
    if (_variant == 0) _features |= CPU_DMB_ATOMICS;
     . . .

So, unless xgene3/emag1 is masquerading as an A1 Cavium variant 0 . . .

I think a more likely cause is the recent GC changes which have buggered around with the shape of the C2 idela graphs (not for any particularly good reason). If you are really using the latest JDK11 repo tip for your build then that is certainly going to affect some volatile stores and CAS operations. I'm currently working on a patch for these but have not yet tested the ldar translation. I'll take a look and let you know if there is an obvious problem with the latter.

>> Are there any guidelines or best practices on how to do an initial 
>> vm_version variant?  What test should I run before submitting a 
>> patch, etc.
> 
> As long as your processor supports proper lrar/stlr you're good. Test 
> it with jcstress.  Make sure that UseBarriersForVolatile is off.

Yes, if you explicitly set -XX:-UseBarriersForVolatile on the command line the reset in vm_version will be ignored and the translation to ldar will be attempted. If it fails in that case then it means the graph shapes for loads have changed.

regards,


Andrew Dinn
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