Re: [Boost-build] x86 CPU spinning should use pause

Subject: Re: [Boost-build] x86 CPU spinning should use pause
From: Hite, Christopher (Christopher.Hite_at_[hidden])
Date: 2011-10-04 07:47:05

Sorry for posting to boost-build.  I mean to do users.

According to this Intel forum they take different times:
http://software.intel.com/en-us/forums/showthread.php?t=48371
That means they can't be the same instruction.  I'd trust their whitepaper.

Pause and no-op are two different use cases.  No-ops can allow a debugger to put jumps into the code.  This pause instruction means flush out memory operations.

Chris

-----Original Message-----
From: boost-build-bounces_at_[hidden] [mailto:boost-build-bounces_at_[hidden]] On Behalf Of Jeremiah Willcock
Sent: Wednesday, September 28, 2011 7:12 PM
To: Boost.Build developer's and user's list
Subject: Re: [Boost-build] x86 CPU spinning should use pause

On Wed, 28 Sep 2011, Hite, Christopher wrote:

> #define BOOST_SMT_PAUSE __asm__ __volatile__( "rep; nop" : : : "memory" );
>  
> We should probably use:
>         __asm__ __volatile__("pause;")
>  
> Why? Because one thread polling( or worse CASing) memory, causes multiple overlapping memory operations to be made:

See 
http://stackoverflow.com/questions/7086220/what-does-rep-nop-mean-in-x86-assembly 
-- basically, "rep; nop" is exactly the same instruction as "pause".

-- Jeremiah Willcock