Using x86 pause instr in SpinPause

[David Holmes]

On 17/08/2012 9:42 AM, Vitaly Davidovich wrote:
> Why would you not want a pause ins? I thought the point of it was to
> inform the CPU that you're busy spinning and not doing anything useful
> so it can make internal adjustments (e.g. scale down frequency on a
> mobile processor).

That's not really applicable in the contexts in which this is used in 
the VM. Spins are used as a back-off strategy for contended actions - 
the fact they are contended indicate there is active work happening in 
other threads so we are not in an "idle" state.

David

>
> Thanks
>
> Sent from my phone
>
> On Aug 16, 2012 5:22 PM, "Daniel D. Daugherty"
> <daniel.daugherty at oracle.com <mailto:daniel.daugherty at oracle.com>> wrote:
>
>     On Win64, SpinPause() has been "return 0" since mid-2005. Way back
>     when Win64 code was in os_win32_amd64.cpp:
>
>          SCCS/s.os_win32_amd64.cpp:
>
>          D 1.9.1.1 05/07/04 03:20:45 dice 12 10  00025/00000/00334
>          MRs:
>          COMMENTS:
>          5030359 -- back-end synchonization improvements - adaptive
>     spinning, etc
>
>     When the i486 and amd64 cpu dirs were merged back in 2007, the code
>     became like it is below (#ifdef'ed):
>
>          D 1.32 07/09/17 09:11:33 sgoldman 37 35 00264/00008/00218
>          MRs:
>          COMMENTS:
>          5108146 Merge i486 and amd64 cpu directories.
>          Macro-ized register names. Inserted amd64 specific code.
>
>     Looks like on Linux-X64, the code has used the PAUSE instruction
>     since mid-2005:
>
>          D 1.3 05/07/04 03:14:09 dice 4 3        00031/00000/00353
>          MRs:
>          COMMENTS:
>          5030359 -- back-end synchonization improvements - adaptive
>     spinning, etc
>
>     We'll have to see if Dave Dice remember why he implemented
>     it this way...
>
>     Dan
>
>
>     On 8/16/12 12:01 PM, Eric Caspole wrote:
>
>         Hi everybody,
>         Does anybody know the reason why SpinPause is simply "return 0"
>         on Win64 but uses PAUSE on Linux in a .s file?
>         We would like to remove PAUSE from linux too.
>
>         Thanks,
>         Eric
>
>
>         ./src/os_cpu/windows_x86/vm/__os_windows_x86.cpp
>
>              548 extern "C" int SpinPause () {
>              549 #ifdef AMD64
>              550    return 0 ;
>              551 #else
>              552    // pause == rep:nop
>              553    // On systems that don't support pause a rep:nop
>              554    // is executed as a nop.  The rep: prefix is ignored.
>              555    _asm {
>              556       pause ;
>              557    };
>              558    return 1 ;
>              559 #endif // AMD64
>              560 }
>
>
>         src/os_cpu/linux_x86/vm/linux___x86_64.s
>
>               63         .globl SpinPause
>               64         .align 16
>               65         .type  SpinPause, at function
>               66 SpinPause:
>               67         rep
>               68         nop
>               69         movq   $1, %rax
>               70         ret
>
>