Re: [PATCH 09/17] riscv: Add __attribute_const__ to ffs()-family implementations

[Alexandre Ghiti <alex@ghiti.fr>]

Hi Kees,

On 8/4/25 18:44, 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 RISC-V's implementations of
> variable__ffs(), variable__fls(), and variable_ffs() 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=riscv defconfig with GCC riscv64-linux-gnu 14.2.0.
>
> Link: https://github.com/KSPP/linux/issues/364 [1]
> Signed-off-by: Kees Cook <kees@kernel.org>
> ---
>   arch/riscv/include/asm/bitops.h | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/include/asm/bitops.h b/arch/riscv/include/asm/bitops.h
> index d59310f74c2b..77880677b06e 100644
> --- a/arch/riscv/include/asm/bitops.h
> +++ b/arch/riscv/include/asm/bitops.h
> @@ -45,7 +45,7 @@
>   #error "Unexpected BITS_PER_LONG"
>   #endif
>   
> -static __always_inline unsigned long variable__ffs(unsigned long word)
> +static __always_inline __attribute_const__ unsigned long variable__ffs(unsigned long word)
>   {
>   	asm goto(ALTERNATIVE("j %l[legacy]", "nop", 0,
>   				      RISCV_ISA_EXT_ZBB, 1)
> @@ -74,7 +74,7 @@ static __always_inline unsigned long variable__ffs(unsigned long word)
>   	 (unsigned long)__builtin_ctzl(word) :	\
>   	 variable__ffs(word))
>   
> -static __always_inline unsigned long variable__fls(unsigned long word)
> +static __always_inline __attribute_const__ unsigned long variable__fls(unsigned long word)
>   {
>   	asm goto(ALTERNATIVE("j %l[legacy]", "nop", 0,
>   				      RISCV_ISA_EXT_ZBB, 1)
> @@ -103,7 +103,7 @@ static __always_inline unsigned long variable__fls(unsigned long word)
>   	 (unsigned long)(BITS_PER_LONG - 1 - __builtin_clzl(word)) :	\
>   	 variable__fls(word))
>   
> -static __always_inline int variable_ffs(int x)
> +static __always_inline __attribute_const__ int variable_ffs(int x)
>   {
>   	asm goto(ALTERNATIVE("j %l[legacy]", "nop", 0,
>   				      RISCV_ISA_EXT_ZBB, 1)


Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Acked-by: Alexandre Ghiti <alexghiti@rivosinc.com>

Thanks,

Alex