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.

Link: https://github.com/KSPP/linux/issues/364 [1]
Signed-off-by: Kees Cook <kees@kernel.org>
---
 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;
 
-- 
2.34.1

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 <maddy@linux.ibm.com>


> Link: https://github.com/KSPP/linux/issues/364 [1]
> Signed-off-by: Kees Cook <kees@kernel.org>
> ---
>  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;
>

Hi!

On Thu, Aug 07, 2025 at 03:16:35PM +0530, Madhavan Srinivasan wrote:
> making them eligible for compiler optimization.

You can instead use GCC for this.  __builtin_ffs () exists since 2003,
and has this attribute built-in, as well as tens of other optimisations
that the kernel thing misses.

Of course using existing stuff instead of cobbling together something
half working prevents you from having a lot of fun ;-)


Segher

On Fri, Aug 15, 2025 at 11:34:56AM -0500, Segher Boessenkool wrote:
> On Thu, Aug 07, 2025 at 03:16:35PM +0530, Madhavan Srinivasan wrote:
> > making them eligible for compiler optimization.
> 
> You can instead use GCC for this.  __builtin_ffs () exists since 2003,

Erm, 1992 actually, but stuff has moved around since then :-)

> and has this attribute built-in, as well as tens of other optimisations
> that the kernel thing misses.
> 
> Of course using existing stuff instead of cobbling together something
> half working prevents you from having a lot of fun ;-)


Segher