linux-kernel - Re: cmpxchg and x86 flags output

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Message-ID: <559ea507-b3cb-74e0-7816-26bb8cbc0238@zytor.com>
Date:	Tue, 21 Jun 2016 10:24:53 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: cmpxchg and x86 flags output

On 06/21/16 02:06, David Howells wrote:
> 
> However, there's probably not a great deal of difference to be had if the
> inline asm codes the appropriate instruction in each case for something like
> x86*.  The emitted code ought to look the same.  The second biggest win for
> the intriniscs, I think, is the ability to ask the CMPXCHG instruction whether
> it actually did anything rather than comparing the result.  I added two
> variants, one that only returned the yes/no and one that passed back the value
> as well as the yes/no.
> 

Right, and we want that either way.  The API change that you are
proposing is definitely what we want; the specifics of the x86
implementation is sort of orthogonal.

	-hpa