linux-kernel - Re: [PATCH] mark read_crX() asm code as volatile

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Message-Id: <200710031045.05234.ak@suse.de>
Date:	Wed, 3 Oct 2007 10:45:05 +0200
From:	Andi Kleen <ak@...e.de>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Kirill Korotaev <dev@...nvz.org>,
	Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devel@...nvz.org
Subject: Re: [PATCH] mark read_crX() asm code as volatile

> 
> How does the compiler know it doesn't depend on memory?

When it has no m (or equivalent like g) constrained argument 
and no memory clobber.

> How do you say it depends on memory?

You add any of the above.

> You really need something as heavy as volatile?

You could do a memory clobber, but it would be heavier than the volatile
because the memory clobber clobbers all cached variables. volatile essentially 
just says "don't remove; has side effects". Normally gcc does that automatically
for something without outputs, but this one has.

Besides a CRx access does not actually clobber memory.

-Andi
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