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

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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