linux-kernel - Re: [PATCH 2/5] bitops: compile time optimization for hweight

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Date:	Wed, 03 Feb 2010 11:49:54 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Borislav Petkov <bp@...64.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jamie Lokier <jamie@...reable.org>,
	Roland Dreier <rdreier@...co.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT)

On 02/03/2010 10:47 AM, Peter Zijlstra wrote:
> On Wed, 2010-02-03 at 19:14 +0100, Borislav Petkov wrote:
> 
>> alternative("call hweightXX", "popcnt", X86_FEATURE_POPCNT)
> 
> Make sure to apply a 0xff bitmask to the popcnt r16 call for hweight8(),
> and hweight64() needs a bit of magic for 32bit, but yes, something like
> that ought to work nicely.
> 

Arguably the "best" option is to have the alternative being a jump to an
out-of-line stub which does the necessary parameter marshalling before
calling a stub.  This technique is already used in a few other places.

	-hpa
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