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Subject: Re: [PATCH -v2] x86/hweight: Get rid of the special calling convention
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Tue, 10 May 2016 12:03:48 -0700
To: Peter Zijlstra <peterz@infradead.org>, Borislav Petkov <bp@suse.de>
CC: x86-ml <x86@kernel.org>, Denys Vlasenko <dvlasenk@redhat.com>,
        Brian Gerst <brgerst@gmail.com>, LKML <linux-kernel@vger.kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>, Andi Kleen <andi@firstfloor.org>,
        zengzhaoxiu@163.com, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Zhaoxiu Zeng <zhaoxiu.zeng@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>
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On May 10, 2016 10:23:13 AM PDT, Peter Zijlstra <peterz@infradead.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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