Date: Sat, 30 Aug 2003 12:31:28 +0100
From: Jamie Lokier <jamie@shareable.org>
To: Willy Tarreau <willy@w.ods.org>
Cc: Antonio Vargas <wind@cocodriloo.com>,
       "Richard B. Johnson" <root@chaos.analogic.com>,
       "J.A. Magallon" <jamagallon@able.es>,
       Lista Linux-Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [2.4] gcc3 warns about type-punned pointers ?
Message-ID: <20030830113128.GA18793@mail.jlokier.co.uk>
References: <20030828223511.GA23528@werewolf.able.es> <20030829152418.GB709@wind.cocodriloo.com> <20030829184847.GA2069@werewolf.able.es> <Pine.LNX.4.53.0308291517001.32044@chaos> <20030830062744.GE640@wind.cocodriloo.com> <20030830095625.GN734@alpha.home.local>
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Willy Tarreau wrote:
> On Sat, Aug 30, 2003 at 08:27:44AM +0200, Antonio Vargas wrote:
> > That was my fault for introducing an exchange instruction
> > into an assignement discussion, but I don't know of any
> > x86 instruction which can load 64bits to memory atomically,
> > is there any???
> 
> perhaps "pusha", but it will load fare more than you need, and I don't know
> if it's lockable.

"pusha" does not promise 64 bit writes.  It can't be interrupted, but
I see nothing that ensures the multiple 32 bit words are combined into
atomic 64 bit writes as seen by other CPUs or peripherals.

> Some MMX instruction might do it too, although not sure.

Yes, if you want a 64 bit write and don't want to use cmpxchg8b, MMX
will do it.

You can also do it with the floating-point "fistpll" instruction (also
called "fistpq").

-- Jamie
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