linux-kernel - Re: [PATCH -v2] x86/hweight: Get rid of the special calling convention

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Date:	Tue, 10 May 2016 12:03:48 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>, Borislav Petkov <bp@...e.de>
CC:	x86-ml <x86@...nel.org>, Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Andi Kleen <andi@...stfloor.org>, zengzhaoxiu@....com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH -v2] x86/hweight: Get rid of the special calling convention

On May 10, 2016 10:23:13 AM PDT, Peter Zijlstra <peterz@...radead.org> wrote:
>On Tue, May 10, 2016 at 06:53:18PM +0200, Borislav Petkov wrote:
>>  static __always_inline unsigned int __arch_hweight32(unsigned int w)
>>  {
>> -	unsigned int res = 0;
>> +	unsigned int res;
>>  
>> -	asm (ALTERNATIVE("call __sw_hweight32", POPCNT32,
>X86_FEATURE_POPCNT)
>> -		     : "="REG_OUT (res)
>> -		     : REG_IN (w));
>> +	if (likely(static_cpu_has(X86_FEATURE_POPCNT))) {
>> +		/* popcnt %eax, %eax */
>> +		asm volatile(POPCNT32
>> +				: "="REG_OUT (res)
>> +				: REG_IN (w));
>>  
>> -	return res;
>> +		return res;
>> +	}
>> +	return __sw_hweight32(w);
>>  }
>
>So what was wrong with using the normal thunk_*.S wrappers for the
>calls? That would allow you to use the alternative() stuff which does
>generate smaller code.

Also, to be fair... if the problem is with these being in C then we could just do it in assembly easily enough.
-- 
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