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

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Message-ID: <4B78758C.2090808@zytor.com>
Date:	Sun, 14 Feb 2010 14:13:32 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <petkovbb@...glemail.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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>, Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT)

On 02/14/2010 12:28 PM, Borislav Petkov wrote:
> 
> Hmm, basically, what PeterZ suggested is that I drop one indirection
> under __arch_hweightN, which would make x86-specific fallback functions
> superfluous.
> 
> IOW, what we have so far is:
> 
> #define hweightN(w) (__builtin_constant_p(w) ? __const_hweightN(w) : __arch_hweightN(w))
> 
> and have <asm-generic/bitops/arch_hweight.h> provide __arch_hweightN()
> -> __sw_hweightN wrappers per default, where the __sw_hweightN are the
> lib/hweight.c generic versions.
> 
> On architectures/CPUs which provide popcnt in
> hardware, we create __arch_hweightN implementations in
> <arch/[:ARCH_NAME:]/include/asm/bitops.h> overriding the
> <asm-generic/bitops/arch_hweight.h> versions by simply not including
> that last header.
> 
> Is that agreeable?
> 

That makes sense... after all, that's a pretty typical use of asm-generic.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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