linux-kernel - Re: [PATCH v2 10/30] Add x86-specific parity functions

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Message-ID: <20160406101300.GA13719@pd.tnic>
Date:	Wed, 6 Apr 2016 12:13:00 +0200
From:	Borislav Petkov <bp@...e.de>
To:	zengzhaoxiu@....com
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	dvlasenk@...hat.com, akpm@...ux-foundation.org, dvyukov@...gle.com,
	keescook@...omium.org, linux-kernel@...r.kernel.org,
	Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>
Subject: Re: [PATCH v2 10/30] Add x86-specific parity functions

On Wed, Apr 06, 2016 at 05:14:45PM +0800, zengzhaoxiu@....com wrote:
> From: Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>
> 
> Use alternatives, lifted from arch_hweight
> 
> Signed-off-by: Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>
> ---
>  arch/x86/include/asm/arch_hweight.h |   5 ++
>  arch/x86/include/asm/arch_parity.h  | 102 ++++++++++++++++++++++++++++++++++++
>  arch/x86/include/asm/bitops.h       |   4 +-
>  arch/x86/lib/Makefile               |   8 +++
>  arch/x86/lib/parity.c               |  32 ++++++++++++
>  5 files changed, 150 insertions(+), 1 deletion(-)
>  create mode 100644 arch/x86/include/asm/arch_parity.h
>  create mode 100644 arch/x86/lib/parity.c

...

> +static __always_inline unsigned int __arch_parity32(unsigned int w)
> +{
> +	unsigned int res;
> +
> +	asm(ALTERNATIVE("call __sw_parity32", POPCNT32 "; and $1, %0", X86_FEATURE_POPCNT)
> +		: "="REG_OUT (res)
> +		: REG_IN (w)
> +		: "cc");

So why all that churn instead of simply doing:

static __always_inline unsigned int __arch_parity32(unsigned int w)
{
	return hweight32(w) & 1;
}

Ditto for the 64-bit version.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--