linux-kernel - Re: [tip:perf/core] bitops: Provide compile time HWEIGHT{8,16,32,64}

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Message-ID: <4B672620.2040303@zytor.com>
Date:	Mon, 01 Feb 2010 11:06:08 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>, mingo@...hat.com,
	eranian@...gle.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] bitops: Provide compile time HWEIGHT{8,16,32,64}

On 02/01/2010 04:43 AM, Peter Zijlstra wrote:
> 
> The below does work, but is still a tad ugly in that if you want to use
> any of the HWEIGHT functions that use BUILD_BUG_ON_ZERO() you have to
> have included linux/kernel.h yourself.
> 
> But at least it builds on x86_64, alpha and sparc64 (didn't have a ia64
> compiler around).
> 
> FWIW I was tempted to change the return type of hweight64() from
> unsigned long to unsigned int, its not as if it'll ever return a value
> larger than 64.
> 

That might generate worse code in some cases, though (something that
needs it as a 64-bit value would have to extend it unnecessarily), but
probably the easiest is to just compile and see what happens.

x86 also has a POPCNT instruction now, which we should be able to use
via alternatives.  That's something to do after your cleanup is in.

Anyway, I like the cleanup.

Acked-by: H. Peter Anvin <hpa@...or.com>

	-hpa
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