Re: [PATCH 05/17] powerpc: Add __attribute_const__ to ffs()-family implementations

On 8/4/25 10:14 PM, Kees Cook wrote:
> 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 PowerPC's implementations of
> fls() function. 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=powerpc defconfig with GCC powerpc-linux-gnu 14.2.0.
> 

Also tested with gcc 8.1.

Acked-by: Madhavan Srinivasan <[email protected]>


> Link: https://github.com/KSPP/linux/issues/364 [1]
> Signed-off-by: Kees Cook <[email protected]>
> ---
>  arch/powerpc/include/asm/bitops.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/bitops.h b/arch/powerpc/include/asm/bitops.h
> index 671ecc6711e3..0d0470cd5ac3 100644
> --- a/arch/powerpc/include/asm/bitops.h
> +++ b/arch/powerpc/include/asm/bitops.h
> @@ -276,7 +276,7 @@ static inline void arch___clear_bit_unlock(int nr, volatile unsigned long *addr)
>   * fls: find last (most-significant) bit set.
>   * Note fls(0) = 0, fls(1) = 1, fls(0x80000000) = 32.
>   */
> -static __always_inline int fls(unsigned int x)
> +static __always_inline __attribute_const__ int fls(unsigned int x)
>  {
>  	int lz;
>  
> @@ -294,7 +294,7 @@ static __always_inline int fls(unsigned int x)
>   * 32-bit fls calls.
>   */
>  #ifdef CONFIG_PPC64
> -static __always_inline int fls64(__u64 x)
> +static __always_inline __attribute_const__ int fls64(__u64 x)
>  {
>  	int lz;
>