mfence on i686 with volatile?

[Dennis Byrne]

At this point my best guess is that this has something to do with the
fact that I am running on a Virtual Machine.  cat /proc/cpuinfo
indicates the machine does in fact see two processors.  The output I
get for both -client and -server has no locks and no fences.  I will
run the same class through PrintOptoAssembly outside of VM Workstation
later in the day in order to build on my case that this has something
to do with VMWare.  Thanks for your time on this.

Dennis Byrne

On Wed, Nov 18, 2009 at 3:21 AM, David Holmes - Sun Microsystems
<David.Holmes at sun.com> wrote:
> Tim Bell said the following on 11/18/09 18:22:
>>
>> Also - are you running your tests on a single-CPU machine?  I recall
>> HotSpot
>> detects that and will elide any n-way synchronization measures if so...
>
> Yes that should have been my first question :-) Thanks Tim. On x86 this
> stuff is elided if running on only a single core/processor.
>
> I just confirmed -client output showing the lock:addl "fences"
>
> David
>
>  ;;  block B2 [8, 31]
>
>  0xca4f72b8: mov    $0xc61f5590,%edx   ;...ba90551f c6
>                                        ;   {oop('HelloWorld')}
>  0xca4f72bd: mov    0x1c0(%edx),%ecx   ;...8b8ac001 0000
>                                        ;*getstatic v
>                                        ; - HelloWorld::run at 8 (line 10)
>  0xca4f72c3: add    $0x14d,%ecx        ;...81c14d01 0000
>  0xca4f72c9: mov    %ecx,0x1c0(%edx)   ;...898ac001 0000
>  0xca4f72cf: lock addl $0x0,(%esp)     ;...f0830424 00
>                                        ;*putstatic v
>                                        ; - HelloWorld::run at 15 (line 10)
>  0xca4f72d4: mov    0x1c4(%edx),%ecx   ;...8b8ac401 0000
>                                        ;*getstatic v2
>                                        ; - HelloWorld::run at 18 (line 11)
>  0xca4f72da: mov    0x1c0(%edx),%edi   ;...8bbac001 0000
>                                        ;*getstatic v
>                                        ; - HelloWorld::run at 21 (line 11)
>  0xca4f72e0: add    %edi,%ecx          ;...03cf
>  0xca4f72e2: mov    %ecx,0x1c4(%edx)   ;...898ac401 0000
>  0xca4f72e8: lock addl $0x0,(%esp)     ;...f0830424 00
>                                        ;*putstatic v2
>                                        ; - HelloWorld::run at 25 (line 11)
>
>
>
>
>>> For x86 we should see a StoreLoad barrier after the assignment to v2:
>>>
>>> http://gee.cs.oswego.edu/dl/jmm/cookbook.html
>>>
>>> For -server printOptoAssembly shows our "mfence" replacement a lock:addl
>>> (which is the implementation of membar-volatile().
>>>
>>> [ Got to go get hsdis myself to test printAssembly directly ]
>>
>> HTH-
>>
>> Tim
>



-- 
Dennis Byrne