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

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Date:	Sun, 14 Feb 2010 21:28:41 +0100
From:	Borislav Petkov <petkovbb@...glemail.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	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 Sun, Feb 14, 2010 at 10:36:48AM -0800, H. Peter Anvin wrote:
> 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.)

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?

-- 
Regards/Gruss,
    Boris.
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