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 <l...@intel.com> > Closes: > https://lore.kernel.org/oe-kbuild-all/202508031025.dowxtzzc-...@intel.com/ [3] > Signed-off-by: Kees Cook <k...@kernel.org>
Applied in bitmap-for-next, thanks! _______________________________________________ linux-snps-arc mailing list linux-snps-arc@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-snps-arc
