From: Andi Kleen <ak@suse.de>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
Chuck Ebbert <cebbert@redhat.com>,
Arjan van de Ven <arjan@infradead.org>,
Kirill Korotaev <dev@openvz.org>, Andrew Morton <akpm@osdl.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
devel@openvz.org
Subject: Re: [PATCH] mark read_crX() asm code as volatile
Date: Wed, 3 Oct 2007 10:45:05 +0200 [thread overview]
Message-ID: <200710031045.05234.ak@suse.de> (raw)
In-Reply-To: <200710030149.24914.nickpiggin@yahoo.com.au>
>
> 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
next prev parent reply other threads:[~2007-10-03 8:45 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-10-02 14:08 [PATCH] mark read_crX() asm code as volatile Kirill Korotaev
2007-10-02 14:17 ` Andi Kleen
2007-10-02 15:28 ` Arjan van de Ven
2007-10-02 18:27 ` Chuck Ebbert
2007-10-02 12:14 ` Nick Piggin
2007-10-03 6:18 ` H. Peter Anvin
2007-10-02 15:49 ` Nick Piggin
2007-10-03 8:45 ` Andi Kleen [this message]
2007-10-02 19:21 ` H. Peter Anvin
2007-10-03 8:25 ` Kirill Korotaev
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