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

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Date:	Thu, 7 Apr 2016 11:55:22 +0800
From:	Zeng Zhaoxiu <zhaoxiu.zeng@...il.com>
To:	Borislav Petkov <bp@...e.de>, 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
Subject: Re: [PATCH v2 10/30] Add x86-specific parity functions

在 2016年04月06日 18:13, Borislav Petkov 写道:
> 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.
>

__sw_parity32 is faster than __sw_hweight32.
I don't know how many CPUs do not support the popc, if they are outdated,
use __arch_hweight32 is the easiest way.