Re: [PATCH] bitfield: wire __bf_shf to __builtin_ctzll

[David Laight <david.laight.linux@gmail.com>]

On Mon, 27 Apr 2026 18:57:05 -0400
Yury Norov <ynorov@nvidia.com> wrote:

> __bf_shf() is currently based on built-in ffsll. It's more
> straightforward to wire it to __builtin_ctzll, which makes it a pure
> rename.
> 
> Worth to notice that __builtin_ffsll() is buggy on GCC before 14.1:
> 
>   int main() {
>       sizeof(struct {
>           int t : !(__builtin_ffsll(~0ULL) + 1 < 0);
>       });
>   }
> 
>   test.c: In function 'main':
>   test.c:3:21: error: bit-field 't' width not an integer constant
>       3 |                 int t : !(__builtin_ffsll(~0ULL) + 1 < 0);
>         |                     ^
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124699
> 
> Reported-by: Matt Coster <matt.coster@imgtec.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202603222211.A2XiR1YU-lkp@intel.com/
> Signed-off-by: Yury Norov <ynorov@nvidia.com>
> ---
>  include/linux/bitfield.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index cd44013281c7..ddf6d1a70473 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -44,7 +44,7 @@
>   *  FIELD_MODIFY(REG_FIELD_C, &reg, c);
>   */
>  
> -#define __bf_shf(x) (__builtin_ffsll(x) - 1)
> +#define __bf_shf __builtin_ctzll

I'd leave in the (x).

	David

>  
>  #define __scalar_type_to_unsigned_cases(type)				\
>  		unsigned type:	(unsigned type)0,			\