linux-kernel - Re: [PATCH] x86, bitops, variable_test_bit should return 1 not -1 on a match

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Date:	Thu, 08 Oct 2015 07:48:34 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>
CC:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86, bitops, variable_test_bit should return 1 not -1
 on a match



On 08/21/2015 04:08 AM, H. Peter Anvin wrote:
> Wrong fix, though.  Instead we should change it to use the set instruction, which would also make it easier to use the CC_SET/CC_OUT proposed macros to use assembly out in the future.
> 
> The downside with set is that it only sets a single byte, the upside is that it always outputs 0 or 1, and apparently if the output variable is your bool gcc can use that for optimization.
> 

hpa, I didn't realize your comment was suggesting a change.  I've done a google
search on "gcc bool optimization" (and various incantations of that) and didn't
find anything that lead me in the direction of making a change.  Could you
elaborate on what the issue is?

Thanks,

P.
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