Transform (list) -> nil in source optimiser

This optimisation is already done in the code generator but performing
it at this earlier stage is a useful normalising step that uncovers
more opportunities.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-list): New.

diff --git a/lisp/emacs-lisp/byte-opt.el b/lisp/emacs-lisp/byte-opt.el
index a24a504456233e2b9a505d6b877a0f1c487f3b84..5a138e9fee53b031ae059f00e20a3a8f43da5656 100644 (file)
--- a/lisp/emacs-lisp/byte-opt.el
+++ b/lisp/emacs-lisp/byte-opt.el
@@ -1287,6 +1287,11 @@ See Info node `(elisp) Integer Basics'."
       `(list ,(nth 1 form))
     form))
 
+(put 'list 'byte-optimizer #'byte-optimize-list)
+(defun byte-optimize-list (form)
+  ;; (list) -> nil
+  (and (cdr form) form))
+
 ;; Fixme: delete-char -> delete-region (byte-coded)
 
 (put 'set 'byte-optimizer #'byte-optimize-set)