linux-kernel - Re: [PATCH 13/15] x86: add cmpxchg

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Date:	Wed, 24 Aug 2011 16:04:32 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Christoph Lameter <cl@...ux.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 13/15] x86: add cmpxchg_flag() variant

On 08/24/2011 01:38 PM, Jeremy Fitzhardinge wrote:
> On 08/24/2011 01:31 PM, H. Peter Anvin wrote:
>> The sete is actually more expensive than the compare for the single-word
>> case.  The double-word case is a different matter.
> 
> Hm, setcc has a longer latency but a higher throughput, is that
> correct?  Still, its recommended where you can use it to avoid a branch.

Yes, but you don't know when that is (the compiler does, of course.)

> I was also thinking that using it reduces register pressure, since you
> don't need to keep the "old" value around, so it dies sooner.

... but you burn a register for the intermediate flag value, so you're
just as bad off.

	-hpa
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