bug#35770: [PATCH] Broken duplicate case elimination in switch byte-compilation

["Mattias Engdegård" <mattiase@acm.org>]

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The byte-code compiler attempts to eliminate duplicated cases when turning a `cond' into a switch table, as in

  (cond ((eq x 'a) 1)
         (eq x 'b) 2)
         (eq x 'a) 3)) ; remove

but it doesn't work properly, because of a confusion between expressions and their values, and a logic error that would have discarded the entire table instead of just skipped the duplicate.

This patch attempts to rectify that. I also removed a seemingly redundant condition, the (consp condition), which should always be true at that point.


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From adb7adf2271e65023adc0ce5d6251054b55c6452 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mattias=20Engdeg=C3=A5rd?= <mattiase@acm.org>
Date: Fri, 17 May 2019 11:25:06 +0200
Subject: [PATCH] Correctly eliminate duplicate cases in switch compilation

Fix code mistakes that prevented the correct elimination of duplicated
cases when compiling a `cond' form to a switch bytecode, as in

  (cond ((eq x 'a) 1)
        ((eq x 'b) 2)
        ((eq x 'a) 3))  ; should be elided

* lisp/emacs-lisp/bytecomp.el (byte-compile-cond-vars): Return obj2 eval'ed.
(byte-compile-cond-jump-table-info):
Discard redundant condition.  Use `obj2' as evaluated.
Discard duplicated cases instead of failing the table generation.
---
 lisp/emacs-lisp/bytecomp.el | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index e76baf5ed0..7f7cbe5a68 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -4091,8 +4091,8 @@ that suppresses all warnings during execution of BODY."
   ;; and the other is a constant expression whose value can be
   ;; compared with `eq' (with `macroexp-const-p').
   (or
-   (and (symbolp obj1) (macroexp-const-p obj2) (cons obj1 obj2))
-   (and (symbolp obj2) (macroexp-const-p obj1) (cons obj2 obj1))))
+   (and (symbolp obj1) (macroexp-const-p obj2) (cons obj1 (eval obj2)))
+   (and (symbolp obj2) (macroexp-const-p obj1) (cons obj2 (eval obj1)))))
 
 (defconst byte-compile--default-val (cons nil nil) "A unique object.")
 
@@ -4121,12 +4121,11 @@ Return a list of the form ((TEST . VAR)  ((VALUE BODY) ...))"
                (unless prev-test
                  (setq prev-test test))
                (if (and obj1 (memq test '(eq eql equal))
-                        (consp condition)
                         (eq test prev-test)
-                        (eq obj1 prev-var)
-                        ;; discard duplicate clauses
-                        (not (assq obj2 cases)))
-                   (push (list (if (consp obj2) (eval obj2) obj2) body) cases)
+                        (eq obj1 prev-var))
+                   ;; discard duplicate clauses
+                   (unless (assq obj2 cases)
+                     (push (list obj2 body) cases))
                  (if (and (macroexp-const-p condition) condition)
 		     (progn (push (list byte-compile--default-val
 					(or body `(,condition)))
-- 
2.20.1 (Apple Git-117)