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).
For arc architecture with CONFIG_ISA_ARCV2=y, the __fls() function uses __builtin_arc_fls() which lacks GCC's const attribute, preventing compile-time constant folding[2]. Fix this by handling compile-time constants with the standard __builtin_clzl() builtin (which has const attribute) while preserving the optimized arc-specific builtin for runtime cases. This has the added benefit of skipping runtime calculation of compile-time constant values. Build tested ARCH=arc allyesconfig with GCC arc-linux 15.2.0. Link: https://github.com/KSPP/linux/issues/364 [1] Link: https://lore.kernel.org/all/202508031025.dowxtzzc-...@intel.com/ Reported-by: kernel test robot <l...@intel.com> Closes: https://lore.kernel.org/oe-kbuild-all/202508031025.dowxtzzc-...@intel.com/ Signed-off-by: Kees Cook <k...@kernel.org> --- Cc: Yury Norov <yury.no...@gmail.com> Cc: Rasmus Villemoes <li...@rasmusvillemoes.dk> Cc: Vineet Gupta <vgu...@kernel.org> Cc: <linux-snps-...@lists.infradead.org> --- arch/arc/include/asm/bitops.h | 2 ++ 1 file changed, 2 insertions(+) diff --git a/arch/arc/include/asm/bitops.h b/arch/arc/include/asm/bitops.h index 5340c2871392..df894235fdbc 100644 --- a/arch/arc/include/asm/bitops.h +++ b/arch/arc/include/asm/bitops.h @@ -133,6 +133,8 @@ static inline __attribute__ ((const)) int fls(unsigned int x) */ static inline __attribute__ ((const)) unsigned long __fls(unsigned long x) { + if (__builtin_constant_p(x)) + return x ? BITS_PER_LONG - 1 - __builtin_clzl(x) : 0; /* FLS insn has exactly same semantics as the API */ return __builtin_arc_fls(x); } -- 2.34.1
