From: Linus Torvalds <torvalds@linux-foundation.org>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Harvey Harrison <harvey.harrison@gmail.com>,
Al Viro <viro@ftp.linux.org.uk>,
mchehab@infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] or51132.c: unaligned
Date: Tue, 20 May 2008 19:02:08 -0700 (PDT) [thread overview]
Message-ID: <alpine.LFD.1.10.0805201856200.3081@woody.linux-foundation.org> (raw)
In-Reply-To: <20080521011843.GH28946@ZenIV.linux.org.uk>
On Wed, 21 May 2008, Al Viro wrote:
>
> FWIW, I wonder how they really compare on misaligned and whether it would
> make sense for gcc to try and generate a single load on targets that are
> known to allow that...
It would almost certainly help on x86. The cost of an unaligned integer
access that doesn't cross a cache-fetch boundary (8 bytes on older CPU's,
16 or 32 bytes on newer ones) is zero, last I saw. IOW, there are
misaligned cases that have a higher cost, but they are pretty rare, and
especially so with small data and modern CPU's.
So no disadvantage for 95% of all cases, and the advantage of doing just a
single instruction, rather than four (2 zero-extending loads, a shift and
an add/or, with data dependencies on most of them).
Linus
prev parent reply other threads:[~2008-05-21 2:02 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-05-21 0:33 [PATCH] or51132.c: unaligned Al Viro
2008-05-21 0:41 ` Harvey Harrison
2008-05-21 0:45 ` Al Viro
2008-05-21 0:51 ` Harvey Harrison
2008-05-21 0:56 ` Al Viro
2008-05-21 0:55 ` Linus Torvalds
2008-05-21 1:18 ` Al Viro
2008-05-21 2:02 ` Linus Torvalds [this message]
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