diff options
| author | Kees Cook <kees@kernel.org> | 2025-08-04 19:43:58 +0300 |
|---|---|---|
| committer | Kees Cook <kees@kernel.org> | 2025-09-09 00:58:50 +0300 |
| commit | 6606c8c7e81886565f5cbdb0c0ce82e280c2b229 (patch) | |
| tree | 7295c6aafb146a62d116db93fbda8af690709a94 /include/linux | |
| parent | b3a7bb71bfcd56c8266af8cf2a5dee3802e7a449 (diff) | |
| download | linux-6606c8c7e81886565f5cbdb0c0ce82e280c2b229.tar.xz | |
bitops: Add __attribute_const__ to generic ffs()-family implementations
While tracking down a problem where constant expressions used by
BUILD_BUG_ON() suddenly stopped working[1], we found that an added static
initializer was convincing the compiler that it couldn't track the state
of the prior statically initialized value. Tracing this down found that
ffs() was used in the initializer macro, but since it wasn't marked with
__attribute__const__, the compiler had to assume the function might
change variable states as a side-effect (which is not true for ffs(),
which provides deterministic math results).
Add missing __attribute_const__ annotations to generic implementations of
ffs(), __ffs(), fls(), and __fls() functions. These are pure mathematical
functions that always return the same result for the same input with no
side effects, making them eligible for compiler optimization.
Build tested with x86_64 defconfig using GCC 14.2.0, which should validate
the implementations when used by ARM, ARM64, LoongArch, Microblaze,
NIOS2, and SPARC32 architectures.
Link: https://github.com/KSPP/linux/issues/364 [1]
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250804164417.1612371-2-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>
Diffstat (limited to 'include/linux')
| -rw-r--r-- | include/linux/bitops.h | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/include/linux/bitops.h b/include/linux/bitops.h index 9be2d50da09a..ea7898cc5903 100644 --- a/include/linux/bitops.h +++ b/include/linux/bitops.h @@ -267,7 +267,7 @@ static inline int parity8(u8 val) * The result is not defined if no bits are set, so check that @word * is non-zero before calling this. */ -static inline unsigned int __ffs64(u64 word) +static inline __attribute_const__ unsigned int __ffs64(u64 word) { #if BITS_PER_LONG == 32 if (((u32)word) == 0UL) |