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

Borislav Petkov
2022-08-23T18:11:51Z

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.

--
Regards/Gruss,
Boris.