Make the docstrings-wide check 70x faster

Instead of performing a number of expensive transformations on the
original doc string and then use a dynamically-created regexp to find
wide lines, step through the lines in the unmodified string and only
perform the transformations on lines that exceed the limit.
This is sound because the transformations are contractive.

The new check will usually not cons nor perform any regexp matching.

* lisp/emacs-lisp/bytecomp.el (bytecomp--docstring-line-width): New.
(byte-compile--wide-docstring-p): Cheaper implementation.

1 files changed, 58 insertions, 39 deletions

diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index b3ddc7dd208..f3e27a511da 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -1680,47 +1680,66 @@ This is a heuristic for guessing the width of a documentation
 string: `byte-compile--wide-docstring-p' assumes that any
 `substitute-command-keys' command substitutions are this long.")
 
-(defun byte-compile--wide-docstring-p (docstring col)
-  "Return t if string DOCSTRING is wider than COL.
+(defun bytecomp--docstring-line-width (str)
+  "An approximation of the displayed width of docstring line STR."
+  (when (string-search "\\`" str)
+    (setq str (replace-regexp-in-string
+               (rx "\\`" (group (* (not "'"))) "'")
+               "\\1"
+               str t)))
+  (when (string-search "\\[" str)
+    (setq str (replace-regexp-in-string
+               (rx "\\[" (* (not "]")) "]")
+               (make-string byte-compile--wide-docstring-substitution-len ?x)
+               str t t)))
+  (setq str
+        (replace-regexp-in-string
+         (rx (or
+              ;; Ignore some URLs.
+              (seq "http" (? "s") "://" (* nonl))
+              ;; Ignore these `substitute-command-keys' substitutions.
+              (seq "\\" (or "="
+                            (seq "<" (* (not ">")) ">")
+                            (seq "{" (* (not "}")) "}")))
+              ;; Ignore the function signature that's stashed at the end of
+              ;; the doc string (in some circumstances).
+              (seq bol "(" (+ (any word "-/:[]&"))
+                   ;; One or more arguments.
+                   (+ " " (or
+                           ;; Arguments.
+                           (+ (or (syntax symbol)
+                                  (any word "-/:[]&=()<>.,?^\\#*'\"")))
+                           ;; Argument that is a list.
+                           (seq "(" (* (not ")")) ")")))
+                   ")")))
+         "" str t t))
+  (length str))
+
+(defun byte-compile--wide-docstring-p (docstring max-width)
+  "Whether DOCSTRING contains a line wider than MAX-WIDTH.
 Ignore all `substitute-command-keys' substitutions, except for
 the `\\\\=[command]' ones that are assumed to be of length
-`byte-compile--wide-docstring-substitution-len'.  Also ignore
-URLs."
-  (string-match
-   (format "^.\\{%d,\\}$" (min (1+ col) #xffff)) ; Heed RE_DUP_MAX.
-   (replace-regexp-in-string
-    (rx (or
-         ;; Ignore some URLs.
-         (seq "http" (? "s") "://" (* nonl))
-         ;; Ignore these `substitute-command-keys' substitutions.
-         (seq "\\" (or "="
-                       (seq "<" (* (not ">")) ">")
-                       (seq "{" (* (not "}")) "}")))
-         ;; Ignore the function signature that's stashed at the end of
-         ;; the doc string (in some circumstances).
-         (seq bol "(" (+ (any word "-/:[]&"))
-              ;; One or more arguments.
-              (+ " " (or
-                      ;; Arguments.
-                      (+ (or (syntax symbol)
-                             (any word "-/:[]&=()<>.,?^\\#*'\"")))
-                      ;; Argument that is a list.
-                      (seq "(" (* (not ")")) ")")))
-              ")")))
-    ""
-    ;; Heuristic: We can't reliably do `substitute-command-keys'
-    ;; substitutions, since the value of a keymap in general can't be
-    ;; known at compile time.  So instead, we assume that these
-    ;; substitutions are of some length N.
-    (replace-regexp-in-string
-     (rx "\\[" (* (not "]")) "]")
-     (make-string byte-compile--wide-docstring-substitution-len ?x)
-     ;; For literal key sequence substitutions (e.g. "\\`C-h'"), just
-     ;; remove the markup as `substitute-command-keys' would.
-     (replace-regexp-in-string
-      (rx "\\`" (group (* (not "'"))) "'")
-      "\\1"
-      docstring)))))
+`byte-compile--wide-docstring-substitution-len'.  Also ignore URLs."
+  (let ((string-len (length docstring))
+        (start 0)
+        (too-wide nil))
+    (while (< start string-len)
+      (let ((eol (or (string-search "\n" docstring start)
+                     string-len)))
+        ;; Since `bytecomp--docstring-line-width' is almost always
+        ;; contractive, we can safely assume that if the raw length is
+        ;; within the allowed width, then so is the transformed width.
+        ;; This allows us to avoid the very expensive transformation in
+        ;; most cases.
+        (if (and (> (- eol start) max-width)
+                 (> (bytecomp--docstring-line-width
+                     (substring docstring start eol))
+                    max-width))
+            (progn
+              (setq too-wide t)
+              (setq start string-len))
+          (setq start (1+ eol)))))
+    too-wide))
 
 (defcustom byte-compile-docstring-max-column 80
   "Recommended maximum width of doc string lines.