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Re: [PATCH] x86/bitops: adjust partial first word handling in __find_next{,_zero}_bit()
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 19 Feb 2026 17:05:34 +0000
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Thu, 19 Feb 2026 17:06:04 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 19/02/2026 3:42 pm, Jan Beulich wrote:
> There's no need to subtract "bit" in what is passed to __scanbit(), as the
> other bits are zero anyway after the shift (in __find_next_bit()) or can
> be made so (in __find_next_zero_bit()) by flipping negation and shift. (We
> actually leverage the same facts in find_next{,_zero}_bit() as well.) This
> way in __scanbit() the TZCNT alternative can be engaged.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
I agree that we're already doing that in find_next{,_zero}_bit() and
therefore ought to be safe here.
What's really confusing about all of this is that __scanbit() differs
from ffs() by wanting a return value in the range [1, 64). With that in
mind, I think we can do exactly the same as we do in arch_ffs(), but
preloading with BITS_PER_LONG rather than -1. That in turn removes the
alternative and simplifies things further.
~Andrew
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