linux-kernel - Re: [PATCH] x86 rwsem optimization extreme

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Date:	Wed, 17 Feb 2010 17:53:50 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Zachary Amsden <zamsden@...hat.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Avi Kivity <avi@...hat.com>
Subject: Re: [PATCH] x86 rwsem optimization extreme

On Wed, 17 Feb 2010, H. Peter Anvin wrote:
> 
> 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.

Oh, I think there are lots.

Look at just about any x86 latency/throughput table, and you'll see:

 - adc latencies are typically much higher than a single cycle

   But you are right that this is likel not an issue on any out-of-order 
   chip, since the 'stc' will schedule perfectly.

 - but adc _throughput_ is also typically much higher, which indicates 
   that even if you do flag renaming, the 'adc' quite likely only 
   schedules in a single ALU unit.

For example, on a Pentium, adc/sbb can only go in the U pipe, and I think 
the same is true of 'stc'. Now, nobody likely cares about Pentiums any 
more, but the point is, 'adc' does often have constraints that a regular 
'add' does not, and there's an example of a 'stc+adc' pair would at the 
very least have to be scheduled with an instruction in between.

		Linus
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