Re: master: Change the ctz pattern to (lognor n (- n))

I'm pretty sure you don't have (ldb (byte 64 0)), though, which makes
it map to hardware.

On Fri, Dec 12, 2025 at 11:21 AM Charles Zhang via Sbcl-commits
<[email protected]> wrote:
>
> Shouldn’t both be recognized? I’m pretty sure I have the logand idiom written in some code somewhere. Not that it’s that important.
>
> > On 12. Dec 2025, at 00:22, stassats via Sbcl-commits <[email protected]> wrote:
> >
> > The branch "master" has been updated in SBCL:
> >       via  7cfd6dbda54b0520ca8488c5ae9e2faabb2b78d5 (commit)
> >      from  c5f6f45800c814805366691207be92715d3a7282 (commit)
> >
> > - Log -----------------------------------------------------------------
> > commit 7cfd6dbda54b0520ca8488c5ae9e2faabb2b78d5
> > Author: Stas Boukarev <[email protected]>
> > Date:   Fri Dec 12 02:18:27 2025 +0300
> >
> >    Change the ctz pattern to (lognor n (- n))
> >
> >    No particular reason other than it's shorter.
> > ---
> > doc/manual/efficiency.texinfo |  2 +-
> > src/compiler/srctran.lisp     | 15 +++++++--------
> > 2 files changed, 8 insertions(+), 9 deletions(-)
> >
> > diff --git a/doc/manual/efficiency.texinfo b/doc/manual/efficiency.texinfo
> > index 08d8afc1d..b13198598 100644
> > --- a/doc/manual/efficiency.texinfo
> > +++ b/doc/manual/efficiency.texinfo
> > @@ -346,7 +346,7 @@ features.
> > @lisp
> > (defun ctz (n)
> >   (declare (type (unsigned-byte 64) n))
> > -  (integer-length (ldb (byte 64 0) (1- (logand n (- n))))))
> > +  (integer-length (ldb (byte 64 0) (lognor n (- n)))))
> > @end lisp
> > is turned into hardware instructions on arm64 and x86-64. It returns 64 when @code{n} is 0.
> > @code{n} can also be @code{(signed-byte 64)} or @code{fixnum}.
> > diff --git a/src/compiler/srctran.lisp b/src/compiler/srctran.lisp
> > index 1693d6643..86a616bbf 100644
> > --- a/src/compiler/srctran.lisp
> > +++ b/src/compiler/srctran.lisp
> > @@ -3003,19 +3003,18 @@
> >                 (when (< hi 0)
> >                   (specifier-type `(integer ,(integer-length hi)))))))))))
> >
> > -;; (integer-length (ldb (byte 64 0) (1- (logand n (- n))))) => ctz
> > +;; (integer-length (ldb (byte 64 0) (lognor n (- n)))) => ctz
> > (when-vop-existsp (:translate count-trailing-zeros)
> >   (deftransform integer-length ((x) (word) * :important nil :node node)
> >     (delay-ir1-transform node :ir1-phases)
> >     (or
> >      (combination-match x
> > -         (sb-vm::--mod64 (:or (logand n ((:or sb-vm::%negate-mod64 sb-vm::%negate-modfx) n))
> > -                              (logand (logand n #.most-positive-word)
> > -                                      (sb-vm::%negate-mod64 (logand n #.most-positive-word)))
> > -                              (logand (logand n #.(ash most-positive-word -1))
> > -                                      (logand (sb-vm::%negate-mod64 (logand n #.(ash most-positive-word -1)))
> > -                                              #.(ash most-positive-word -1))))
> > -                         1)
> > +         (sb-vm::lognot-mod64 (:or
> > +                               (logior n (:or (sb-vm::%negate-mod64 n)
> > +                                              (logand (sb-vm::%negate-modfx n) #.most-positive-word)))
> > +                               (logior (logand n #.most-positive-word)
> > +                                       (:or (sb-vm::%negate-mod64 (logand n #.most-positive-word))
> > +                                            (logand (sb-vm::%negate-modfx n) #.most-positive-word)))))
> >        (when (word-sized-lvar-p n)
> >          (extract-lvar n node)
> >          `(count-trailing-zeros x)))
> >
> > -----------------------------------------------------------------------
> >
> >
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