Re: [Sbcl-commits] master: Recognize (integer-length (ldb (byte 64 0) (1- (logand n (- n))))) as ctz

Without modular arithmetic things turn out pretty badly if it's not
recognized. So it's not a good idea to rely on 0 being excluded.

On Fri, Jan 9, 2026 at 11:35 PM Scott L. Burson <[email protected]> wrote:
>
> On Wed, Dec 10, 2025 at 11:45 AM Stas Boukarev <[email protected]> wrote:
> > I had
> > trouble figuring out how to deal with 0, some code in the wild uses
> > things like
> > (1- (integer-length (logand n (- n)))) or (1- (logcount (logxor n (1-
> > n)))), but they return a different thing on 0. Maybe they should be
> > matched anyway and 0 handled with a conditional move or something.
> > And I decided against (logcount (ldb (byte 64 0) (lognor n (- n))))
> > because popcount is potentially expensive.
>
> Speaking as someone who is actually using this operation -- doesn't the compiler
> know whether n can be zero?  It seems to be so good at tracking the domains
> of integer variables.
>
> I am literally doing (loop while (/= n 0) (let ((b (least-1-bit n)))
> ...)) where least-1-bit
> expands to one of the above expressions.  Surely, under these circumstances,
> the compiler knows that n isn't zero, so it can match all of the three or four
> expressions people use for this?
>
> -- Scott


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