[PATCH 1/1] bitfield.h: Ensure FIELD_PREP_CONST() is constant

david.laight.linux
2026-04-10T09:09:36Z

From: David Laight <david.laight.linux@gmail.com>

Some versions of gcc report that some expressions involving
__builtin_ffsll(1ull << 63) are not integer constant expressions.

Rework FIELD_PREP_CONST() to avoid the issue.

Signed-off-by: David Laight <david.laight.linux@gmail.com>
---

Note that when 'val' is a variable 'val << constant' is likely
to execute faster than 'val * (1 << constant)'.
So the normal FIELD_PREP() is best left alone.

include/linux/bitfield.h | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 54aeeef1f0ec..f21765851d5c 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -45,6 +45,7 @@
*/

#define __bf_shf(x) (__builtin_ffsll(x) - 1)
+#define __bf_low_bit(mask) ((mask) & (~(mask) + 1))

#define __scalar_type_to_unsigned_cases(type) \
unsigned type: (unsigned type)0, \
@@ -138,8 +139,6 @@
__FIELD_PREP(_mask, _val, "FIELD_PREP: "); \
})

-#define __BF_CHECK_POW2(n) BUILD_BUG_ON_ZERO(((n) & ((n) - 1)) != 0)
-
/**
* FIELD_PREP_CONST() - prepare a constant bitfield element
* @_mask: shifted mask defining the field's length and position
@@ -157,11 +156,11 @@
/* mask must be non-zero */ \
BUILD_BUG_ON_ZERO((_mask) == 0) + \
/* check if value fits */ \
- BUILD_BUG_ON_ZERO(~((_mask) >> __bf_shf(_mask)) & (_val)) + \
+ BUILD_BUG_ON_ZERO(~((_mask) / __bf_low_bit(_mask)) & (_val)) + \
/* check if mask is contiguous */ \
- __BF_CHECK_POW2((_mask) + (1ULL << __bf_shf(_mask))) + \
+ BUILD_BUG_ON_ZERO((_mask) & ((_mask) + __bf_low_bit(_mask))) + \
/* and create the value */ \
- (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask)) \
+ (((_val) * __bf_low_bit(_mask)) & (_mask)) \
)

/**
--
2.39.5