Re: [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user

[Linus Torvalds]

On Tue, Jan 14, 2020 at 1:37 PM Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx> wrote:
>
> On 1/14/20 12:42 PM, Arnd Bergmann wrote:
> >
> > What's wrong with the generic version on little-endian? Any
> > chance you can find a way to make it work as well for you as
> > this copy?
>
> find_zero() by default doesn't use pop count instructions.

Don't you think the generic find_zero() is likely just as fast as the
pop count instruction? On 32-bit, I think it's like a shift and a mask
and a couple of additions.

The 64-bit case has a multiply that is likely expensive unless you
have a good multiplication unit (but what 64-bit architecture
doesn't?), but the generic 32-bit LE code should already be pretty
close to optimal, and it might not be worth it to worry about it.

(The big-endian case is very different, and architectures really can
do much better. But LE allows for bit tricks using the carry chain)

             Linus