Re: [PATCH 03/17] csky: Add __attribute_const__ to ffs()-family implementations

On Mon, Aug 04, 2025 at 09:43:59AM -0700, 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 C-SKY's 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.
LGTM.

Acked-by: Guo Ren <[email protected]>

> 
> Build tested ARCH=csky defconfig with GCC csky-linux 15.1.0.
> 
> Link: https://github.com/KSPP/linux/issues/364 [1]
> Signed-off-by: Kees Cook <[email protected]>
> ---
>  arch/csky/include/asm/bitops.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/csky/include/asm/bitops.h b/arch/csky/include/asm/bitops.h
> index 72e1b2aa29a0..80d67eee6e86 100644
> --- a/arch/csky/include/asm/bitops.h
> +++ b/arch/csky/include/asm/bitops.h
> @@ -9,7 +9,7 @@
>  /*
>   * asm-generic/bitops/ffs.h
>   */
> -static inline int ffs(int x)
> +static inline __attribute_const__ int ffs(int x)
>  {
>  	if (!x)
>  		return 0;
> @@ -26,7 +26,7 @@ static inline int ffs(int x)
>  /*
>   * asm-generic/bitops/__ffs.h
>   */
> -static __always_inline unsigned long __ffs(unsigned long x)
> +static __always_inline __attribute_const__ unsigned long __ffs(unsigned long x)
>  {
>  	asm volatile (
>  		"brev %0\n"
> @@ -39,7 +39,7 @@ static __always_inline unsigned long __ffs(unsigned long x)
>  /*
>   * asm-generic/bitops/fls.h
>   */
> -static __always_inline int fls(unsigned int x)
> +static __always_inline __attribute_const__ int fls(unsigned int x)
>  {
>  	asm volatile(
>  		"ff1 %0\n"
> @@ -52,7 +52,7 @@ static __always_inline int fls(unsigned int x)
>  /*
>   * asm-generic/bitops/__fls.h
>   */
> -static __always_inline unsigned long __fls(unsigned long x)
> +static __always_inline __attribute_const__ unsigned long __fls(unsigned long x)
>  {
>  	return fls(x) - 1;
>  }
> -- 
> 2.34.1
> 
>