linux-kernel - Re: [PATCH -v2] x86/hweight: Get rid of the special calling convention

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Message-ID: <B403FEFA-78F7-4D8F-A46C-E5174527D79E@zytor.com>
Date:	Wed, 11 May 2016 21:54:50 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Brian Gerst <brgerst@...il.com>
CC:	Borislav Petkov <bp@...e.de>, x86-ml <x86@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Andi Kleen <andi@...stfloor.org>, zengzhaoxiu@....com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH -v2] x86/hweight: Get rid of the special calling convention

On May 11, 2016 4:24:09 AM PDT, Peter Zijlstra <peterz@...radead.org> wrote:
>On Wed, May 11, 2016 at 07:15:19AM -0400, Brian Gerst wrote:
>
>> I think he meant the out of line version would be asm, so you could
>> control what registers were clobbered.
>
>Yeah, it might save a few cycles on the call, but given that most
>machines should have popcnt these days is it worth the hassle/cost of
>duplicating the lib/hweight.c magic in asm (and remember, twice, once
>for 32bit and once for 64bit) ?

I was thinking it isn't really very complex code even in assembly as it is super-regular; you can even crib the gcc-generated code if you wish.
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