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

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Date:	Mon, 8 Feb 2010 10:59:45 +0100
From:	Borislav Petkov <petkovbb@...glemail.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	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 Mon, Feb 08, 2010 at 01:35:41AM -0800, H. Peter Anvin wrote:
> 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.

The popcnt-replacement part of the alternative would be as fast as
possible since we're adding the opcode there but the slow version would
add the additional overhead of saving/restoring the registers before
calling the software hweight implementation. I'll do some tracing to see
what a change like that would cost on machines which don't have popcnt.

Let me prep another version when I get back on Wed. (currently
travelling) with all the stuff we discussed to see how it would turn.

Thanks,
Boris.
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