From: Alexander Terekhov (terekhov_at_[hidden])
Date: 2005-04-09 09:51:41
< Forward Quoted >
Newsgroups: comp.arch
Subject: Re: lwarx/stwcx on PowerPC 970
References: <4257CC7A.252E5B2_at_[hidden]>
Message-ID: <opsoyu88daqm36vk_at_grunion>
Date: Sat, 09 Apr 2005 09:18:34 -0400
From: "Joe Seigh" <jseigh_01_at_[hidden]>
Joe Seigh wrote:
>
> On Sat, 09 Apr 2005 14:37:14 +0200, Alexander Terekhov <terekhov_at_[hidden]> wrote:
>
> > http://www-306.ibm.com/chips/techlib/techlib.nsf/techdocs/3A2D397F9A3202BD87256D4B007164C0/$file/970Programming_Note_larx_stcx.d20030618.pdf
> >
> > Says that "Every larx executed should have an accompanying stcx to clear
> > the reservation." and that "Additional information regarding Larx/Stcx
> > instructions and usage can be found in the "PowerPC Microprocessor
> > Family: Programming Environments Manual for 64 and 32-Bit Microprocessors".
> >
> > "Additinal information" shows coding examples like this:
> >
> > <quote>
> >
> > loop: lwarx r6,0,r3 #load and reserve
> > cmpw r4,r6 #first 2 operands equal ?
> > bne- exit #skip if not
> > stwcx. r5,0,r3 #store new value if still reserved
> > bne- loop #loop if lost reservation
> > exit: mr r4,r6 #return value from memory
> >
> > </quote>
> >
> > So, who's right here?
> >
>
> Just "strongly recommended", not required. The reservations obviously
> aren't recursively counted since the OS has no way of knowing the count
> in order to "clear" the reservations. Unless there's some performance
> hit for maintaining the reservation, I can't see any reason to worry
> about it.
>
> --
> Joe Seigh
< /Forward Quoted >
On any sensible Power {PC} implementation (especially MP), redundant
stwcx will negatively affect performance, not the other way around.
regards,
alexander.