mfence on i686 with volatile?

[Vladimir Kozlov]

Dennis,

run fastdebug java with next options

java -XX:+PrintMiscellaneous -XX:+Verbose -version

the "CPU:" line shows number of cores VM see.

On solaris I see barrier when I declared variables as volatile:

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

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
>>
> 
> 
>