Re: [PATCH v2] x86_64: Optimize ffsll function code size.

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 31/07/23 15:35, Sunil K Pandey via Libc-alpha wrote:
> Ffsll function size is 17 byte, this patch optimizes size to 16 byte.
> Currently ffsll function randomly regress by ~20%, depending on how
> code get aligned.
> 
> This patch fixes ffsll function random performance regression.
> 
> Changes from v1:
> - Further reduce size ffsll function size to 12 bytes.
> ---
>  sysdeps/x86_64/ffsll.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/sysdeps/x86_64/ffsll.c b/sysdeps/x86_64/ffsll.c
> index a1c13d4906..6a5803c7c1 100644
> --- a/sysdeps/x86_64/ffsll.c
> +++ b/sysdeps/x86_64/ffsll.c
> @@ -26,13 +26,13 @@ int
>  ffsll (long long int x)
>  {
>    long long int cnt;
> -  long long int tmp;
>  
> -  asm ("bsfq %2,%0\n"		/* Count low bits in X and store in %1.  */
> -       "cmoveq %1,%0\n"		/* If number was zero, use -1 as result.  */
> -       : "=&r" (cnt), "=r" (tmp) : "rm" (x), "1" (-1));
> +  asm ("mov $-1,%k0\n"	/* Intialize CNT to -1.  */
> +       "bsf %1,%0\n"	/* Count low bits in X and store in CNT.  */
> +       "inc %k0\n"	/* Increment CNT by 1.  */
> +       : "=&r" (cnt) : "r" (x));
>  
> -  return cnt + 1;
> +  return cnt;
>  }
>  
>  #ifndef __ILP32__



I still prefer if we can just remove this arch-optimized function in favor
in compiler builtins.