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

[Peter Zijlstra]

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.