On Tue, Jan 12, 2021 at 1:37 PM Nathan Chancellor <natechancel...@gmail.com> wrote: > > > if real_ptr is an unsigned long, do we want to use `__ffs(real_ptr) + > > 1` here rather than ffs which takes an int? It seems the kernel is > > missing a definition of ffsl. :( > > Why the + 1? I think if we use __ffs (which it seems like we should), I > think that needs to become
This came up recently in an internal code review; ffs and __ffs differ in output by one. See also the definition of ffs for alpha in arch/alpha/include/asm/bitops.h. Also, I just confirmed that: ``` #include <stdio.h> // include/asm-generic/bitops/ffs.h static inline int ffs(int x) { int r = 1; if (!x) return 0; if (!(x & 0xffff)) { x >>= 16; r += 16; } if (!(x & 0xff)) { x >>= 8; r += 8; } if (!(x & 0xf)) { x >>= 4; r += 4; } if (!(x & 3)) { x >>= 2; r += 2; } if (!(x & 1)) { x >>= 1; r += 1; } return r; } // include/asm-generic/bitops/__ffs.h static __always_inline unsigned long __ffs(unsigned long word) { int num = 0; if ((word & 0xffffffff) == 0) { num += 32; word >>= 32; } if ((word & 0xffff) == 0) { num += 16; word >>= 16; } if ((word & 0xff) == 0) { num += 8; word >>= 8; } if ((word & 0xf) == 0) { num += 4; word >>= 4; } if ((word & 0x3) == 0) { num += 2; word >>= 2; } if ((word & 0x1) == 0) num += 1; return num; } int main() { int x = 3; unsigned long y = 3; printf("%d\n%lu\n", ffs(x), __ffs(y)); return 0; } ``` will print: 1 0 -- Thanks, ~Nick Desaulniers