Re: [PATCH 11/17] m68k: Add __attribute_const__ to ffs()-family implementations

On Mon, 4 Aug 2025 at 18:44, Kees Cook <[email protected]> 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 M68K's implementations
> of ffs(), __ffs(), fls(), __fls(), and ffz() 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=m68k defconfig with GCC m68k-linux-gnu 14.2.0.
>
> Link: https://github.com/KSPP/linux/issues/364 [1]
> Signed-off-by: Kees Cook <[email protected]>

Reviewed-by: Geert Uytterhoeven <[email protected]>
Acked-by: Geert Uytterhoeven <[email protected]>

# ffs: pass:9 fail:0 skip:0 total:9

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds