mfence on i686 with volatile?

[Vladimir Kozlov]

When there are sequential volatile membars
C2 generates instruction only for last one and previous are
declared as "unnecessary" but they are printed for debugging.

Note: during parsing after each store to a volatile
field (or variable) C2 generates several volatile membars - one
per each volatile field (or variable). In the test case there
were 2 volatile variables, so C2 generates 2 volatile membars.

Vladimir

David Holmes - Sun Microsystems wrote:
> Vladimir Kozlov said the following on 11/19/09 06:23:
>> 046     MOV    [ESI + precise klass HelloWorld: 
>> 0x080f2198:Constant:exact *],EBX ! Field  VolatileHelloWorld.v
>> 04c     MEMBAR-volatile (unnecessary so empty encoding)
>> 04c     LOCK ADDL [ESP + #0], 0 ! membar_volatile
> 
> Just as aside but I find the text "unnecessary so empty encoding" very 
> confusing - what exactly is it referring to?
> 
> David
> 
>>
>> Vladimir
>>
>> Dennis Byrne wrote:
>>> Still no luck on this.  There are two "Processor" items under "System
>>> Information"/"System Summary" and Cygwin gives me this:
>>>
>>> $ cat /proc/cpuinfo | grep ^processor | wc -l
>>> 8
>>>
>>> "$jdk1.6.0_18\fastdebug\bin\java.exe -XX:+PrintOptoAssembly -server
>>> -XX:CompileCommand=print,*HelloWorld.run  HelloWorld" yields the
>>> following:
>>>
>>> 01c       CMP    EBP,#1000000
>>> 022       Jge    B13  P=0.000000 C=29521.000000
>>> 022
>>> 028   B2: #    B3 <- B1  Freq: 1
>>> 028       MOV    EDX,EBP
>>> 02a       INC    EDX
>>> 02b       IMUL   EAX,EBP,#333
>>> 031       MOV    ECX,#448
>>> 036       MOV    EBX,[ECX + precise klass HelloWorld:
>>> 0x00a2fc30:Constant:exact *]    # int ! Field HelloWorld.v
>>> 03c       SUB    EBX,EAX
>>> 03e       MOV    ECX,#452
>>> 043       MOV    EDI,[ECX + precise klass HelloWorld:
>>> 0x00a2fc30:Constant:exact *]    # int ! Field HelloWorld.v2
>>> 043
>>> 049   B3: #    B5 B4 <- B2 B4     Loop: B3-B4 inner stride: not 
>>> constant pre
>>> of N112 Freq: 2
>>> 049       MOV    [ESP + #4],EBX
>>> 04d       ADD    EAX,EBX
>>> 04f       ADD    EDI,EAX
>>> 051       MOV    ESI,EBP
>>> 053       INC    ESI
>>> 054       ADD    EDI,#333
>>> 05a       MOV    EBX,#452
>>> 05f       MOV    [EBX + precise klass HelloWorld:
>>> 0x00a2fc30:Constant:exact *],EDI ! Field HelloWorld.v2
>>> 065       ADD    EAX,#333
>>> 06b       MOV    ECX,#448
>>> 070       MOV    [ECX + precise klass HelloWorld:
>>> 0x00a2fc30:Constant:exact *],EAX ! Field HelloWorld.v
>>> 076       CMP    ESI,EDX
>>> 078       Jge,s  B5    # Loop end  P=0.500000 C=29027.000000
>>> 078
>>> 07a   B4: #    B3 <- B3  Freq: 1
>>> 07a       IMUL   EAX,EBP,#333
>>> 080       ADD    EAX,#333
>>> 086       MOV    EBP,ESI
>>> 088       MOV    EBX,[ESP + #4]
>>> 08c       JMP,s  B3
>>> 08c
>>> 08e   B5: #    B8 B6 <- B3  Freq: 1
>>> 08e       MOV    ECX,#1000000
>>> 093       SUB    ECX,ESI
>>> 095       AND    ECX,#-16
>>> 098       ADD    ECX,ESI
>>> 09a       CMP    ESI,ECX
>>> 09c       Jl,s  B8  P=0.999999 C=-1.000000
>>>
>>> Dennis Byrne
>>>
>>> On Wed, Nov 18, 2009 at 8:52 AM, Dennis Byrne 
>>> <dennisbyrne at apache.org> wrote:
>>>> 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
>>>>
>>>
>>>
>>>