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

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Date:	Sun, 14 Feb 2010 10:36:48 -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 03:24 AM, Borislav Petkov wrote:
> 
> __const_hweightN - for at compile time known constants as arguments
> __arch_hweightN - arch possibly has an optimized hweight version
> __sw_hweightN - fall back when nothing else is there, aka the functions in
> lib/hweight.c
> 
> Now, in the x86 case, when the compiler can't know that the argument is
> a constant, we call the __arch_hweightN versions. The alternative does
> call the __sw_hweightN version in case the CPU doesn't support popcnt.
> In this case, we need to build __sw_hweightN with -fcall-saved* for gcc
> to be able to take care of the regs clobbered ny __sw_hweightN.
> 
> So, if I understand you correctly, your suggestion might work, we
> simply need to rename the lib/hweight.c versions to __sw_hweightN
> and have <asm-generic/bitops/arch_hweight.h> have __arch_hweightN ->
> __sw_hweightN wrappers in the default case, all arches which have an
> optimized version will provide it in their respective bitops header...
> 

I'm not entirely sure what you're asking; if what you're asking what to
name an x86-specific fallback function, it presumably should be
__arch_sw_hweightN (i.e. __arch prefix with a modifier.)

	-hpa

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

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