From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-return-102967-listarch-gcc=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 32577 invoked by alias); 20 Sep 2004 05:44:20 -0000
Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Archive: <http://gcc.gnu.org/ml/gcc/>
List-Post: <mailto:gcc@gcc.gnu.org>
List-Help: <http://gcc.gnu.org/ml/>
Sender: gcc-owner@gcc.gnu.org
Received: (qmail 32559 invoked from network); 20 Sep 2004 05:44:17 -0000
Received: from unknown (HELO mail.kloo.net) (63.192.214.25)
  by sourceware.org with SMTP; 20 Sep 2004 05:44:17 -0000
Received: by mail.kloo.net (Postfix, from userid 504)
	id 447863B0229; Sun, 19 Sep 2004 22:30:08 -0700 (PDT)
Received: from localhost (localhost [127.0.0.1])
	by mail.kloo.net (Postfix) with ESMTP
	id 449843B4545; Sun, 19 Sep 2004 22:30:08 -0700 (PDT)
Date: Mon, 20 Sep 2004 07:37:00 -0000
From: <tm_gccmail@kloo.net>
To: Nathan Sidwell <nathan@codesourcery.com>
Cc: Jamie Lokier <jamie@shareable.org>, gcc@gcc.gnu.org
Subject: Re: A question about "memory" clobbers in asm
In-Reply-To: <414B0528.1040504@codesourcery.com>
Message-ID: <Pine.LNX.4.21.0409192210390.10468-100000@mail.kloo.net>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
X-SW-Source: 2004-09/txt/msg01154.txt.bz2

On Fri, 17 Sep 2004, Nathan Sidwell wrote:

> Jamie Lokier wrote:
> > I'm having trouble understand when it's appropriate to use a "memory"
> > clobber, when to use volatile, and when to use both.  The manual is
> > unclear to me.  Although I see that someone has tried to clarify it
> > since I last read it, it's still not obvious how "memory" is different
> > from `volatile'.
> 
> my understanding is
> 
> * volatile asm says something to the effect 'this changes state that you
> (the compiler) don't know about' -- such as writing to an IO port.
> 
> * memory clobber says 'this asm will change memory, so don't cache anything
> across this asm'.

This definition of memory clobbers is incomplete.

A memory clobber is required when the inline assembly either reads from or
writes to arbitrary memory or both. Not just "writes to".

I've seen cases like this:

	*ptr = 5;


	/* basically result = (*ptr == 5) */
	__asm__ volatile ("mov.l @%1,%0; mov #5,%2; cmp/eq %1,%2;
			   movt %0"
			   : "r" (result), "r" (temp)
			   : "r" (ptr));

	
	if (!result) {
		...
	}

If the memory clobber is omitted, then the scheduler can move *ptr = 5
AFTER the inline assembly fragment, which will result in the incorrect
result.

So if the inline assembly reads arbitrary memory, then it needs a memory
clobber regardless of whether it writes to memory.

Toshi