Subject: Re: [PATCH 10/17] openrisc: Add __attribute_const__ to ffs()-family implementations

From: Stafford Horne <[email-masked]>
Date: Mon, 4 Aug 2025 18:15:06 +0100

On Mon, Aug 04, 2025 at 09:44:06AM -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 OpenRISC'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.
> 
> Build tested ARCH=openrisc defconfig with GCC or1k-linux 15.1.0.

THis looks ok to me.

> Link: https://github.com/KSPP/linux/issues/364 [1]
> Signed-off-by: Kees Cook <[email-masked]>

Acked-by: Stafford Horne <[email-masked]>

> ---
>  arch/openrisc/include/asm/bitops/__ffs.h | 2 +-
>  arch/openrisc/include/asm/bitops/__fls.h | 2 +-
>  arch/openrisc/include/asm/bitops/ffs.h   | 2 +-
>  arch/openrisc/include/asm/bitops/fls.h   | 2 +-
>  4 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/openrisc/include/asm/bitops/__ffs.h b/arch/openrisc/include/asm/bitops/__ffs.h
> index 1e224b616fdf..4827b66530b2 100644
> --- a/arch/openrisc/include/asm/bitops/__ffs.h
> +++ b/arch/openrisc/include/asm/bitops/__ffs.h
> @@ -11,7 +11,7 @@
>  
>  #ifdef CONFIG_OPENRISC_HAVE_INST_FF1
>  
> -static inline unsigned long __ffs(unsigned long x)
> +static inline __attribute_const__ unsigned long __ffs(unsigned long x)
>  {
>  	int ret;
>  
> diff --git a/arch/openrisc/include/asm/bitops/__fls.h b/arch/openrisc/include/asm/bitops/__fls.h
> index 9658446ad141..637cc76fe4b7 100644
> --- a/arch/openrisc/include/asm/bitops/__fls.h
> +++ b/arch/openrisc/include/asm/bitops/__fls.h
> @@ -11,7 +11,7 @@
>  
>  #ifdef CONFIG_OPENRISC_HAVE_INST_FL1
>  
> -static inline unsigned long __fls(unsigned long x)
> +static inline __attribute_const__ unsigned long __fls(unsigned long x)
>  {
>  	int ret;
>  
> diff --git a/arch/openrisc/include/asm/bitops/ffs.h b/arch/openrisc/include/asm/bitops/ffs.h
> index b4c835d6bc84..536a60ab9cc3 100644
> --- a/arch/openrisc/include/asm/bitops/ffs.h
> +++ b/arch/openrisc/include/asm/bitops/ffs.h
> @@ -10,7 +10,7 @@
>  
>  #ifdef CONFIG_OPENRISC_HAVE_INST_FF1
>  
> -static inline int ffs(int x)
> +static inline __attribute_const__ int ffs(int x)
>  {
>  	int ret;
>  
> diff --git a/arch/openrisc/include/asm/bitops/fls.h b/arch/openrisc/include/asm/bitops/fls.h
> index 6b77f6556fb9..77da7639bb3e 100644
> --- a/arch/openrisc/include/asm/bitops/fls.h
> +++ b/arch/openrisc/include/asm/bitops/fls.h
> @@ -11,7 +11,7 @@
>  
>  #ifdef CONFIG_OPENRISC_HAVE_INST_FL1
>  
> -static inline int fls(unsigned int x)
> +static inline __attribute_const__ int fls(unsigned int x)
>  {
>  	int ret;
>  
> -- 
> 2.34.1
> 
>