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 M68K's implementations of ffs(), __ffs(), fls(), __fls(), and ffz() 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 ARCH=m68k defconfig with GCC m68k-linux-gnu 14.2.0. Link: https://github.com/KSPP/linux/issues/364 [1] Signed-off-by: Kees Cook <k...@kernel.org> --- arch/m68k/include/asm/bitops.h | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-) diff --git a/arch/m68k/include/asm/bitops.h b/arch/m68k/include/asm/bitops.h index 14c64a6f1217..139ec9289ff2 100644 --- a/arch/m68k/include/asm/bitops.h +++ b/arch/m68k/include/asm/bitops.h @@ -465,7 +465,7 @@ static inline int find_next_bit(const unsigned long *vaddr, int size, * ffz = Find First Zero in word. Undefined if no zero exists, * so code should check against ~0UL first.. */ -static inline unsigned long ffz(unsigned long word) +static inline unsigned long __attribute_const__ ffz(unsigned long word) { int res; @@ -488,7 +488,7 @@ static inline unsigned long ffz(unsigned long word) */ #if (defined(__mcfisaaplus__) || defined(__mcfisac__)) && \ !defined(CONFIG_M68000) -static inline unsigned long __ffs(unsigned long x) +static inline __attribute_const__ unsigned long __ffs(unsigned long x) { __asm__ __volatile__ ("bitrev %0; ff1 %0" : "=d" (x) @@ -496,7 +496,7 @@ static inline unsigned long __ffs(unsigned long x) return x; } -static inline int ffs(int x) +static inline __attribute_const__ int ffs(int x) { if (!x) return 0; @@ -518,7 +518,7 @@ static inline int ffs(int x) * the libc and compiler builtin ffs routines, therefore * differs in spirit from the above ffz (man ffs). */ -static inline int ffs(int x) +static inline __attribute_const__ int ffs(int x) { int cnt; @@ -528,7 +528,7 @@ static inline int ffs(int x) return 32 - cnt; } -static inline unsigned long __ffs(unsigned long x) +static inline __attribute_const__ unsigned long __ffs(unsigned long x) { return ffs(x) - 1; } @@ -536,7 +536,7 @@ static inline unsigned long __ffs(unsigned long x) /* * fls: find last bit set. */ -static inline int fls(unsigned int x) +static inline __attribute_const__ int fls(unsigned int x) { int cnt; @@ -546,7 +546,7 @@ static inline int fls(unsigned int x) return 32 - cnt; } -static inline unsigned long __fls(unsigned long x) +static inline __attribute_const__ unsigned long __fls(unsigned long x) { return fls(x) - 1; } -- 2.34.1