Would it make a difference if the static fields were declared to be public
instead of their current package-level visibility? Then it might be a
little harder for the JIT to justify "knowing" that the variables were
never accessed concurrently.
--dave
Andreas Zwinkau <zwin...@kit.edu> wrote on 03/11/2016 07:31:07 AM:
>
> Am 04.03.2016 um 17:27 schrieb David P Grove:
> >
> > Andreas Zwinkau <zwin...@kit.edu> wrote on 03/04/2016 07:02:30 AM:
> >
> >> From: Andreas Zwinkau <zwin...@kit.edu>
> >> To: x10-users@lists.sourceforge.net
> >> Date: 03/04/2016 07:03 AM
> >> Subject: [X10-users] Fences Java implementation
> >>
> >> The x10.util.concurrent.Fences implementation looks reasonable for C+
+,
> >> but the Java implementation seems weird to me.
> >>
> >> For example, loadLoadBarrier is implemented via two volatile reads.
Why
> >> *two*? I could ask more questions ...
> >>
> >> The code was added by David Grove in 2009 it seems:
> >> https://github.com/x10-lang/x10/commit/
> >> f4f5420ce476f2538f358509ad13ef52b8c2c4c1
> >>
> >> Any idea where that came from? Any sources? Any reasons why it is the
> >> way it is?
> >
> > Hi Andreas,
> >
> > Best as I can remember, the Java code that is there was based on
> > discussion being had in 2009 about how to get memory fences given the
> > definition of the Java memory model and java.util.concurrent as it
existed
> > at the time.
> >
> > --dave
>
> I compared it with the JMM Cookbook [0] and it follows it very directly.
> For example, "volatile load then volatile store means loadStore barrier".
>
> The problem is the JMM considers volatile variables separately, so the
> JVM can optimize volatiles. If the JVM figures out the volatile
> variables in FencesUtils.java are never accessed concurrently, then they
> can be optimized away. I made a toy example and this became
> loadLoadBarrier assembly:
>
> # {method} 'loadLoadBarrier' '()V' in 'FencesUtils'
> # [sp+0x20] (sp of caller)
> 0x00007fa3f905f5c0: sub $0x18,%rsp
> 0x00007fa3f905f5c7: mov %rbp,0x10(%rsp) ;*synchronization entry
> ; -
> FencesUtils::loadLoadBarrier@-1 (line 38)
> 0x00007fa3f905f5cc: mov $0x7acd4eee8,%r10 ; {oop(a
> 'java/lang/Class' = 'FencesUtils')}
> 0x00007fa3f905f5d6: mov 0x58(%r10),%r8d
> 0x00007fa3f905f5da: mov %r8d,0x60(%r10) ;*getstatic v1
> ; -
> FencesUtils::loadLoadBarrier@0 (line 38)
> 0x00007fa3f905f5de: mov 0x5c(%r10),%r11d
> 0x00007fa3f905f5e2: mov %r11d,0x64(%r10) ;*getstatic v2
> ; -
> FencesUtils::loadLoadBarrier@6 (line 39)
> 0x00007fa3f905f5e6: add $0x10,%rsp
> 0x00007fa3f905f5ea: pop %rbp
> 0x00007fa3f905f5eb: test %eax,0xc967a0f(%rip) #
> 0x00007fa4059c7000
> ; {poll_return}
> 0x00007fa3f905f5f1: retq
>
> No synchronization instruction. The other three barriers contained a
> "lock addl" instruction.
>
> I think this sometimes might work, but not reliably. Afaik there is no
> way to correctly implement FencesUtils.java. This implies
> x10.util.concurrent.Fences cannot be provided with the Java backend.
>
> [0] http://gee.cs.oswego.edu/dl/jmm/cookbook.html
> --
> Andreas Zwinkau
>
> KIT IPD Snelting
> Web: http://pp.ipd.kit.edu/personhp/andreas_zwinkau.php
>
> [attachment "signature.asc" deleted by David P Grove/Watson/IBM]
>
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