Constprop of lexical variables

Lexical variables bound to a constant value (symbol, number or string)
are substituted at their point of use and the variable then eliminated
if possible.  Example:

  (let ((x (+ 2 3))) (f x))  =>  (f 5)

This reduces code size, eliminates stack operations, and enables
further optimisations.  The implementation is conservative, and is
strongly curtailed by the presence of variable mutation, conditions
and loops.

* lisp/emacs-lisp/byte-opt.el
(byte-optimize-enable-variable-constprop)
(byte-optimize-warn-eliminated-variable): New constants.
(byte-optimize--lexvars, byte-optimize--vars-outside-condition)
(byte-optimize--vars-outside-loop, byte-optimize--dynamic-vars):
New dynamic variables.
(byte-optimize--substitutable-p, byte-optimize-let-form):
New functions.
(byte-optimize-form-code-walker): Adapt clauses for variable
constprop, and add clauses for 'setq' and 'defvar'.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-test-var)
(bytecomp-test-get-var, bytecomp-test-identity)
(byte-opt-testsuite-arith-data): Add test cases.

1 files changed, 60 insertions, 1 deletions

diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index 980b402ca2d..bc623d3efca 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -32,6 +32,15 @@
 (require 'bytecomp)
 
 ;;; Code:
+(defvar bytecomp-test-var nil)
+
+(defun bytecomp-test-get-var ()
+  bytecomp-test-var)
+
+(defun bytecomp-test-identity (x)
+  "Identity, but hidden from some optimisations."
+  x)
+
 (defconst byte-opt-testsuite-arith-data
   '(
     ;; some functional tests
@@ -371,7 +380,57 @@
     (assoc 'b '((a 1) (b 2) (c 3)))
     (assoc "b" '(("a" 1) ("b" 2) ("c" 3)))
     (let ((x '((a 1) (b 2) (c 3)))) (assoc 'c x))
-    (assoc 'a '((a 1) (b 2) (c 3)) (lambda (u v) (not (equal u v)))))
+    (assoc 'a '((a 1) (b 2) (c 3)) (lambda (u v) (not (equal u v))))
+
+    ;; Constprop test cases
+    (let ((a 'alpha) (b (concat "be" "ta")) (c nil) (d t) (e :gamma)
+          (f '(delta epsilon)))
+      (list a b c d e f))
+
+    (let ((x 1) (y (+ 3 4)))
+      (list
+       (let (q (y x) (z y))
+         (if q x (list x y z)))))
+
+    (let* ((x 3) (y (* x 2)) (x (1+ y)))
+      x)
+
+    (let ((x 1) (bytecomp-test-var 2) (y 3))
+      (list x bytecomp-test-var (bytecomp-get-test-var) y))
+
+    (progn
+      (defvar d)
+      (let ((x 'a) (y 'b)) (list x y)))
+
+    (let ((x 2))
+      (list x (setq x 13) (setq x (* x 2)) x))
+
+    (let ((x 'a) (y 'b))
+      (setq y x
+            x (cons 'c y)
+            y x)
+      (list x y))
+
+    (let ((x 3))
+      (let ((y x) z)
+        (setq x 5)
+        (setq y (+ y 8))
+        (setq z (if (bytecomp-test-identity t)
+                    (progn
+                      (setq x (+ x 1))
+                      (list x y))
+                  (setq x (+ x 2))
+                  (list x y)))
+        (list x y z)))
+
+    (let ((i 1) (s 0) (x 13))
+      (while (< i 5)
+        (setq s (+ s i))
+        (setq i (1+ i)))
+      (list s x i))
+
+    (let ((x 2))
+      (list (or (bytecomp-identity 'a) (setq x 3)) x)))
   "List of expression for test.
 Each element will be executed by interpreter and with
 bytecompiled code, and their results compared.")