Message-ID: <48AD9614.6060701@qumranet.com>
Date: Thu, 21 Aug 2008 19:21:40 +0300
From: Avi Kivity <avi@qumranet.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: Nick Piggin <nickpiggin@yahoo.com.au>, jmerkey@wolfmountaingroup.com,
       Stefan Richter <stefanr@s5r6.in-berlin.de>, paulmck@linux.vnet.ibm.com,
       linux-kernel@vger.kernel.org,
       Linus Torvalds <torvalds@linux-foundation.org>,
       David Howells <dhowells@redhat.com>
Subject: Re: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger 	2.6.27-rc4	released
References: <200808210250.m7L2obNX028353@wolfmountaingroup.com>	 <48AD5A21.7020801@s5r6.in-berlin.de>	 <43593.166.70.238.46.1219321595.squirrel@webmail.wolfmountaingroup.com>	 <200808212337.38626.nickpiggin@yahoo.com.au> <1219327799.8651.134.camel@twins>
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Peter Zijlstra wrote:
> Not sure - I think all barrier clobber the full register and memory set.
> So if you access a variable after a barrier it will have to issue a
> load.
>   

IIRC a barrier only clobbers memory.  gcc must reload a variable from 
memory unless it can prove the variable's address has not escaped anywhere.

So:

void f()
{
    int v;

    v = g();
    barrier();
    do_domething_with(v);
}

Need not reload v from memory (indeed, v can be in a register for its 
entire lifetime), but

void f()
{
    int v;

    v = g();
    h(&v);
    barrier();
    do_domething_with(v);
}
 
Will force v into memory, and reload it after the barrier.

-- 
error compiling committee.c: too many arguments to function

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