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

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Date:	Mon, 08 Feb 2010 01:35:41 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <petkovbb@...glemail.com>,
	Borislav Petkov <bp@...64.org>,
	Peter Zijlstra <peterz@...radead.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>, Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT)

On 02/08/2010 01:28 AM, Borislav Petkov wrote:
>
> Well, in the second version I did replace a 'call _hweightXX' with
> the actual popcnt opcode so the alternatives is only needed to do the
> replacement during boot. We might just as well do
>
> if (X86_FEATURE_POPCNT)
> 	__hw_popcnt()
> else
> 	__software_hweight()
>
> The only advantage of the alternatives is that it would save us the
> if-else test above each time we do cpumask_weight. However, the if-else
> approach is much more readable and obviates the need for all that macro
> magic and taking special care of calling c function from within asm. And
> since we do not call cpumask_weight all that often I'll honestly opt for
> alternative-less solution...
>

The highest performance will be gotten by alternatives, but it only make 
sense if they are inlined at the point of use... otherwise it's 
basically pointless.

	-hpa
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