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

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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