* [PATCH] bitfield: wire __bf_shf to __builtin_ctzll
@ 2026-04-27 22:57 Yury Norov
2026-04-28 2:04 ` David Laight
0 siblings, 1 reply; 2+ messages in thread
From: Yury Norov @ 2026-04-27 22:57 UTC (permalink / raw)
To: Yury Norov, Rasmus Villemoes, linux-kernel
Cc: Yury Norov, Frank Binns, Alessio Belle, Brajesh Gupta,
Alexandru Dadu, Vincent Mailhol, Matt Coster
__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, ®, c);
*/
-#define __bf_shf(x) (__builtin_ffsll(x) - 1)
+#define __bf_shf __builtin_ctzll
#define __scalar_type_to_unsigned_cases(type) \
unsigned type: (unsigned type)0, \
--
2.51.0
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] bitfield: wire __bf_shf to __builtin_ctzll
2026-04-27 22:57 [PATCH] bitfield: wire __bf_shf to __builtin_ctzll Yury Norov
@ 2026-04-28 2:04 ` David Laight
0 siblings, 0 replies; 2+ messages in thread
From: David Laight @ 2026-04-28 2:04 UTC (permalink / raw)
To: Yury Norov
Cc: Yury Norov, Rasmus Villemoes, linux-kernel, Frank Binns,
Alessio Belle, Brajesh Gupta, Alexandru Dadu, Vincent Mailhol,
Matt Coster
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, ®, 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, \
^ permalink raw reply [flat|nested] 2+ messages in thread
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