[concurrency-interest] a volatile bug?

[Aleksey Shipilev]

Update. GVN is clearly under suspicion -XX:-UseGlobalValueNumbering
mitigates the bug in my setup. Digging through C1 codebase to see
rules for volatiles.

-Aleksey.

On Wed, May 16, 2012 at 11:23 PM, Aleksey Shipilev
<aleksey.shipilev at gmail.com> wrote:
> All right, here's what is on the table.
>
> This bug is reproduced for me on Linux i686 with:
> java version "1.7.0_04"
> Java(TM) SE Runtime Environment (build 1.7.0_04-b20)
> Java HotSpot(TM) Client VM (build 23.0-b21, mixed mode)
>
> It reproduces immediately only with -client.
> Both -server and -Xint do NOT reproduce the bug.
> The code is there in original SO post
> http://stackoverflow.com/questions/10620680/why-volatile-in-java-5-doesnt-synchronize-cached-copies-of-variables-with-main
>
> C1 seems to miscompile run(), and indeed does CSE for local:
>
>  # {method} 'run' '()V' in 'Test$1'
> [Verified Entry Point]
>  0xb4a91e80: mov    %eax,-0x4000(%esp)
>  0xb4a91e87: push   %ebp
>  0xb4a91e88: sub    $0x18,%esp         ;*invokestatic access$000
>                                        ; - Test$1::run at 0 (line 11)
>  0xb4a91e8b: mov    $0xa09c4270,%edx   ;   {oop(a 'java/lang/Class' = 'Test')}
>>>>>>>  0xb4a91e90: mov    0x74(%edx),%edx    ;*getstatic b <<<<<---- loads $b to %edx
>                                        ; - Test::access$000 at 0 (line 1)
>                                        ; - Test$1::run at 0 (line 11)
>  0xb4a91e93: jmp    0xb4a91e9e         ; OopMap{off=40}
>                                        ;*goto
>                                        ; - Test$1::run at 10 (line 13)
>  0xb4a91e98: test   %eax,0xb77a9100    ;*goto
>                                        ; - Test$1::run at 10 (line 13)
>                                        ;   {poll}
>  0xb4a91e9e: mov    $0xa09c4270,%ecx   ;   {oop(a 'java/lang/Class' = 'Test')}
>>>>>>  0xb4a91ea3: mov    0x70(%ecx),%ecx    ;*getstatic a  <<<<< volatile read for $a
>                                        ; - Test::access$100 at 0 (line 1)
>                                        ; - Test$1::run at 4 (line 13)
>  0xb4a91ea6: cmp    $0x0,%ecx     // <---- $a is at %ecx
>  0xb4a91ea9: je     0xb4a91e98         ;*ifne
>                                        ; - Test$1::run at 7 (line 13)
>  >>>> 0xb4a91eab: cmp    $0x0,%edx     // <<<<<<---- $b is cached in %edx here
>  0xb4a91eae: jne    0xb4a91ed8         ;*ifne
>                                        ; - Test$1::run at 16 (line 17)
>  0xb4a91eb4: nopl   0x0(%eax)
>  0xb4a91eb8: jmp    0xb4a91f0e         ;   {no_reloc}
>  0xb4a91ebd: xchg   %ax,%ax
>  0xb4a91ec0: jmp    0xb4a91f28         ; implicit exception:
> dispatches to 0xb4a91f18
>  0xb4a91ec5: nop                       ;*getstatic out
>                                        ; - Test$1::run at 19 (line 18)
>  0xb4a91ec6: cmp    (%ecx),%eax        ; implicit exception:
> dispatches to 0xb4a91f32
>  0xb4a91ec8: mov    $0xa09c6488,%edx   ;*invokevirtual println
>                                        ; - Test$1::run at 24 (line 18)
>                                        ;   {oop("error")}
>
>
> Thanks,
> Aleksey.
>
> On Wed, May 16, 2012 at 11:12 PM, Vitaly Davidovich <vitalyd at gmail.com> wrote:
>> It can be a compiler (mis)optimization that causes this, and not x86 memory
>> ordering.
>>
>> Someone posted the assembly output in the comments on SO and it does seem
>> like there's a place that loads 'b' from the stack rather than memory.
>> Hans' theory of CSE sounds plausible - can someone repro this without that
>> "int tt = b;" line?
>>
>> Adding hotspot compiler guys in case they want to chime in.
>>
>> Sent from my phone
>>
>> On May 16, 2012 3:07 PM, "Aleksey Shipilev" <aleksey.shipilev at gmail.com>
>> wrote:
>>>
>>> On Wed, May 16, 2012 at 10:40 PM, Boehm, Hans <hans.boehm at hp.com> wrote:
>>> > A JDK bug AND a serious test suite omission?
>>>
>>> Stress tests would probably JIT-compile the code in question. See below.
>>>
>>> > But is the problem real?  Can it be reproduced on a mainstream JVM?
>>>
>>> Same question.
>>>
>>> > Note that the example in the original posting also read b before the
>>> > loop,
>>> > so naïve common subexpression elimination would cause the bug.
>>> >  Hopefully
>>> > nobody does CSE in cases like this.
>>>
>>> FWIW, the test case in SO would probably not hit any compilation
>>> threshold in HotSpot, so it could be executed in interpreter. Then,
>>> assuming the interpreter does not reorder Java code, and assuming
>>> original SO poster runs Windows, and hence x86, and hence has TSO,
>>> this bug seems very unlikely. I would be surprised if it actually
>>> *can* be reproduced. That makes the whole story rather interesting.
>>>
>>> -Aleksey.
>>>
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