Re: [PATCH v2] arc: Fix __fls() const-foldability via __builtin_clzl()

[Yury Norov <yury.norov@gmail.com>]

On Sat, Aug 30, 2025 at 07:23:53PM -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).
> 
> For arc architecture with CONFIG_ISA_ARCV2=y, the __fls() function
> uses __builtin_arc_fls() which lacks GCC's const attribute, preventing
> compile-time constant folding, and KUnit testing of ffs/fls fails on
> arc[3]. A patch[2] to GCC to solve this has been sent.
> 
> Add a fix for this by handling compile-time constants with the standard
> __builtin_clzl() builtin (which has const attribute) while preserving
> the optimized arc-specific builtin for runtime cases. This has the added
> benefit of skipping runtime calculation of compile-time constant values.
> Even with the GCC bug fixed (which is about "attribute const") this is a
> good change to avoid needless runtime costs, and should be done
> regardless of the state of GCC's bug.
> 
> Build tested ARCH=arc allyesconfig with GCC arc-linux 15.2.0.
> 
> Link: https://github.com/KSPP/linux/issues/364 [1]
> Link: https://gcc.gnu.org/pipermail/gcc-patches/2025-August/693273.html
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202508031025.doWxtzzc-lkp@intel.com/ [3]
> Signed-off-by: Kees Cook <kees@kernel.org>

Applied in bitmap-for-next, thanks!