Re: (if (and (featurep 'xemacs) blah) not optimized anymore

[Stefan Monnier]

> I still see one issue:

> After applying this patch: 

> --- viper-cmd.el      19 Aug 2007 13:47:07 -0000      1.63
> +++ viper-cmd.el      22 Oct 2007 05:37:24 -0000
> @@ -861,7 +861,7 @@
>                                   (1- (length quail-current-str)))))
>                ))
>             ((and viper-special-input-method
> -                 viper-xemacs-p
> +                 nil
>                   (fboundp 'quail-start-translation))
>              ;; same as above but for XEmacs, which doesn't have
>              ;; quail-input-method

> the byte compiler will still warn about about quail-start-translation
> not being defined. I won't warn if the expression is 
> (and nil viper-special-input-method ...)

The patch below seems to fix it, but it does a bit more and I'm not sure
if it behaves well in practice (e.g. performancewise), so please try it
out on a large scale.  If you find it to work well, feel free to
install it.


        Stefan


--- orig/lisp/emacs-lisp/byte-opt.el
+++ mod/lisp/emacs-lisp/byte-opt.el
@@ -185,6 +185,7 @@
 ;;; Code:
 
 (require 'bytecomp)
+(eval-when-compile (require 'cl))
 
 (defun byte-compile-log-lap-1 (format &rest args)
   (if (aref byte-code-vector 0)
@@ -626,12 +628,24 @@
 ;; It is now safe to optimize code such that it introduces new bindings.
 
 ;; I'd like this to be a defsubst, but let's not be self-referential...
-(defmacro byte-compile-trueconstp (form)
-  ;; Returns non-nil if FORM is a non-nil constant.
-  `(cond ((consp ,form) (eq (car ,form) 'quote))
-        ((not (symbolp ,form)))
-        ((eq ,form t))
-        ((keywordp ,form))))
+(defsubst byte-compile-trueconstp (form)
+  "Return non-nil if FORM always evaluates to a non-nil value."
+  (cond ((consp form)
+         (case (car form)
+           (quote (cadr form))
+           (progn (byte-compile-trueconstp (car (last (cdr form)))))))
+        ((not (symbolp form)))
+        ((eq form t))
+        ((keywordp form))))
+
+(defsubst byte-compile-nilconstp (form)
+  "Return non-nil if FORM always evaluates to a nil value."
+  (cond ((consp form)
+         (case (car form)
+           (quote (null (cadr form)))
+           (progn (byte-compile-nilconstp (car (last (cdr form)))))))
+        ((not (symbolp form)) nil)
+        ((null form))))
 
 ;; If the function is being called with constant numeric args,
 ;; evaluate as much as possible at compile-time.  This optimizer
@@ -990,17 +1004,17 @@
     (setq rest form)
     (while (setq rest (cdr rest))
       (cond ((byte-compile-trueconstp (car-safe (car rest)))
-            (cond ((eq rest (cdr form))
-                   (setq form
-                         (if (cdr (car rest))
-                             (if (cdr (cdr (car rest)))
-                                 (cons 'progn (cdr (car rest)))
-                               (nth 1 (car rest)))
-                           (car (car rest)))))
+             ;; This branch will always be taken: kill the subsequent ones.
+            (cond ((eq rest (cdr form)) ;First branch of `cond'.
+                   (setq form `(progn ,@(car rest))))
                   ((cdr rest)
                    (setq form (copy-sequence form))
                    (setcdr (memq (car rest) form) nil)))
-            (setq rest nil)))))
+            (setq rest nil))
+            ((and (consp (car rest))
+                  (byte-compile-nilconstp (caar rest)))
+             ;; This branch will never be taken: kill its body.
+             (setcdr (car rest) nil)))))
   ;;
   ;; Turn (cond (( <x> )) ... ) into (or <x> (cond ... ))
   (if (eq 'cond (car-safe form))
@@ -1031,11 +1045,9 @@
                      (byte-optimize-if
                       `(if ,(car (last clause)) ,@(nthcdr 2 form)))))))
           ((byte-compile-trueconstp clause)
-          (nth 2 form))
-         ((null clause)
-          (if (nthcdr 4 form)
-              (cons 'progn (nthcdr 3 form))
-            (nth 3 form)))
+          `(progn ,clause ,(nth 2 form)))
+         ((byte-compile-nilconstp clause)
+           `(progn ,clause ,@(nthcdr 3 form)))
          ((nth 2 form)
           (if (equal '(nil) (nthcdr 3 form))
               (list 'if clause (nth 2 form))