Re: [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions

[Nick Desaulniers]

On Tue, Aug 23, 2022 at 9:23 AM Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> On Fri, Aug 12, 2022 at 08:44:38PM +0900, Vincent Mailhol wrote:
> > __ffs(x) is equivalent to (unsigned long)__builtin_ctzl(x)
>
> Are you sure about this?
>
> My gcc documentation says:
>
> "Built-in Function: int __builtin_ctz (unsigned int x)
>
>     Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined."
>
> Note the undefined part.
>
> Also,
>
> __builtin_ctzl(0): 0x40
> ffs(0): 0x0
>
> I'm using the kernel ffs() version in a small program which is basically
> a wrapper around BSF.

Callers of these need to guard against zero input, as the pre-existing
comment notes:

>> Undefined if no bit exists, so code should check against 0 first.
-- 
Thanks,
~Nick Desaulniers