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

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Message-ID: <CA+55aFy9r7jHi_9B4zH9LEy4PidWvjs5nDddHA5RyKRvunDehA@mail.gmail.com>
Date:	Wed, 24 Aug 2011 16:11:13 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Christoph Lameter <cl@...ux.com>,
	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 Wed, Aug 24, 2011 at 4:04 PM, H. Peter Anvin <hpa@...or.com> wrote:
>
>> 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.

But that register has much shorter liveness - so I do agree that it *can* help.

Whether it actually *does* help is unclear. I do agree that we might
be better off without introducing yet another (questionable)
interface.

                     Linus
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