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To: Konstantin Belousov <kostikbel@gmail.com>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Tue, 18 Sep 2012 19:48:26 +0300."
	<20120918164826.GI37286@deviant.kiev.zoral.com.ua>
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Date: Tue, 18 Sep 2012 17:03:21 +0000
Message-ID: <22549.1347987801@critter.freebsd.dk>
Cc: attilio@freebsd.org, arch@freebsd.org
Subject: Re: Aliasing issue with TAILQ on ppc64 ?
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In message <20120918164826.GI37286@deviant.kiev.zoral.com.ua>, Konstantin Belou
sov writes:

>[...] compiler must not reorder accesses which
>provably causes change in the behaviour.

As I said, I'm not entirely sure why this goes wrong, and I didn't
feel like learning PPC64 assembler constraints just now to find out.

My understanding is that the cast from entry->head effectively
obscures from the compiler what goes on in this case, making that
optimization fair game.


-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.