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

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Message-ID: <1265212054.24455.680.camel@laptop>
Date:	Wed, 03 Feb 2010 16:47:34 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	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>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 2/5] bitops: compile time optimization for
 hweight_long(CONSTANT)

On Wed, 2010-02-03 at 07:42 -0800, Andrew Morton wrote:
> On Wed, 03 Feb 2010 16:15:57 +0100 Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Wed, 2010-02-03 at 07:08 -0800, Andrew Morton wrote:
> > > On Wed, 3 Feb 2010 21:39:51 +0800 Wu Fengguang <fengguang.wu@...el.com> wrote:
> > > 
> > > > Is there an official lowest GCC version that Linux supports?
> > > 
> > > Documentation/Changes says gcc-3.2.
> > 
> > Anyway, its not like that macro I used isn't straight forward. Not much
> > a builtin can do better.
> > 
> > The only benefit the builtin has is possibly use machine popcnt
> > instructions but we already deal with that in the kernel.
> 
> We didn't deal with it on every architecture, which is something which
> the compiler extension takes care of.
> 
> In fact I can't find anywhere where we dealt with it on x86.

>>From what I know the popcnt ins on x86 is relatively new, hpa was going
to look at supporting it using alternatives.

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