Using x86 pause instr in SpinPause

[Vitaly Davidovich]

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).

Thanks

Sent from my phone
On Aug 16, 2012 5:22 PM, "Daniel D. Daugherty" <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
>>
>>
>>
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