Warn about calls to `lsh` (bug#56641)

* lisp/subr.el (lsh): Warn when compiled; recommend `ash`.
* etc/NEWS: Add note.

diff --git a/etc/NEWS b/etc/NEWS
index 780dbfa51e8f6bdd5576be90d941fa100a0c5835..a1fea5d6d6809fb0001db5cb35772f36a87bb35b 100644 (file)
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -3230,6 +3230,12 @@ to preserve the old behavior, apply
 '(take N LIST)' returns the first N elements of LIST; 'ntake' does
 the same but works by modifying LIST destructively.
 
+---
+** Calling 'lsh' now elicits a byte-compiler warning.
+'lsh' behaves in somewhat surprising and platform-dependent ways for
+negative arguments and is generally slower than 'ash' which should be
+used instead.
+
 
 \f
 * Changes in Emacs 29.1 on Non-Free Operating Systems

diff --git a/lisp/subr.el b/lisp/subr.el
index 510a77dbc8d1e52534a2d1e020210c26bd1af908..06da5e28730930c726eb06a361689c9216d7c4fa 100644 (file)
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -528,6 +528,11 @@ i.e., subtract 2 * `most-negative-fixnum' from VALUE before shifting it.
 
 This function is provided for compatibility.  In new code, use `ash'
 instead."
+  (declare (compiler-macro
+            (lambda (form)
+              (when (byte-compile-warning-enabled-p 'suspicious 'lsh)
+                (byte-compile-warn-x form "avoid `lsh'; use `ash' instead"))
+              form)))
   (when (and (< value 0) (< count 0))
     (when (< value most-negative-fixnum)
       (signal 'args-out-of-range (list value count)))