Message-ID: <4B7C7BE4.9050908@zytor.com>
Date: Wed, 17 Feb 2010 15:29:40 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Zachary Amsden <zamsden@redhat.com>, linux-kernel@vger.kernel.org,
       Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
       x86@kernel.org, Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH] x86 rwsem optimization extreme
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On 02/17/2010 02:10 PM, Linus Torvalds wrote:
> 
> The cost of 'adc' may happen to be identical in this case, but I suspect 
> you didn't test on UP, where the 'lock' prefix goes away. An unlocked 
> 'add' tends to be faster than an unlocked 'adc'.
> 
> (It's possible that some micro-architectures don't care, since it's a 
> memory op, and they can see that 'C' is set. But it's a fragile assumption 
> that it would always be ok).
> 

FWIW, I don't know of any microarchitecture where adc is slower than
add, *as long as* the setup time for the CF flag is already used up.
However, as I already commented, I don't think this is worth it.  This
inline appears to only be instantiated once, and as such, it takes a
whopping six bytes across the entire kernel.

	-hpa

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