Re: [PATCH 14/17] s390: Add __attribute_const__ to ffs()-family implementations

On Mon, Aug 04, 2025 at 09:44:10AM -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 S390'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=s390 defconfig with GCC s390x-linux-gnu 14.2.0.
> 
> Link: https://github.com/KSPP/linux/issues/364 [1]
> Signed-off-by: Kees Cook <[email protected]>
> ---
>  arch/s390/include/asm/bitops.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)

Acked-by: Heiko Carstens <[email protected]>