mfence on i686 with volatile?

[David Holmes - Sun Microsystems]

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