On Tue, 29 Apr 2025 at 14:59, Andrew Cooper <andrew.coop...@citrix.com> wrote:
>
> do_variable_ffs() doesn't quite work.
>
> REP BSF is LZCNT, and unconditionally writes it's output operand, and
> defeats the attempt to preload with -1.
>
> Drop the REP prefix, and it should work as intended.
Bah. That's what I get for just doing it blindly without actually
looking at the kernel source. I just copied the __ffs() thing - and
there the 'rep' is not for the zero case - which we don't care about -
but because lzcnt performs better on newer CPUs.
So you're obviously right.
Linus
