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

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Date:	Wed, 11 May 2016 13:24:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Borislav Petkov <bp@...e.de>, "H. Peter Anvin" <hpa@...or.com>,
	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 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) ?