GNU bug report logs - #48264
28.0.50; Changing the default for DEFVAR_PER_BUFFER variables takes O(#buffers) time

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From: Spencer Baugh <sbaugh <at> catern.com>
To: 48264 <at> debbugs.gnu.org
Cc: Spencer Baugh <sbaugh <at> catern.com>
Subject: bug#48264: [PATCH v3 10/15] Get rid of buffer_permanent_local_flags array
Date: Thu,  6 May 2021 17:33:41 -0400

* lisp/bindings.el: Update comment to point to
reset_buffer_local_variables for information about
pseudo-permanent-locals.
* src/buffer.c (buffer_permanent_local_flags): Delete.
(reset_buffer_local_variables): Special case two
pseudo-permanent-locals for backwards-compatibility.
(init_buffer_once): Remove use of buffer_permanent_local_flags.
---
 lisp/bindings.el |  3 ++-
 src/buffer.c     | 28 +++++++++-------------------
 2 files changed, 11 insertions(+), 20 deletions(-)

diff --git a/lisp/bindings.el b/lisp/bindings.el
index 6eac528eb6..902528a9ca 100644
--- a/lisp/bindings.el
+++ b/lisp/bindings.el
@@ -766,7 +766,8 @@ okay.  See `mode-line-format'.")
 ;; `kill-all-local-variables', because they have no default value.
 ;; For consistency, we give them the `permanent-local' property, even
 ;; though `kill-all-local-variables' does not actually consult it.
-;; See init_buffer_once in buffer.c for the origins of this list.
+;; See init_buffer_once and reset_buffer_local_variables in buffer.c
+;; for the origins of this list.
 
 (mapc (lambda (sym) (put sym 'permanent-local t))
       '(buffer-file-name default-directory buffer-backed-up
diff --git a/src/buffer.c b/src/buffer.c
index e6b7cd9aa8..9363aabddc 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -93,10 +93,6 @@ struct buffer buffer_local_symbols;
   ((ptrdiff_t) min (MOST_POSITIVE_FIXNUM,				\
 		    min (PTRDIFF_MAX, SIZE_MAX) / word_size))
 
-/* Flags indicating which built-in buffer-local variables
-   are permanent locals.  */
-static char buffer_permanent_local_flags[MAX_PER_BUFFER_VARS];
-
 /* Number of per-buffer variables used.  */
 
 static int last_per_buffer_idx;
@@ -1100,10 +1096,14 @@ reset_buffer_local_variables (struct buffer *b, bool permanent_too)
   /* For each slot that has a default value, copy that into the slot.  */
   FOR_EACH_PER_BUFFER_OBJECT_AT (offset)
     {
-      int idx = PER_BUFFER_IDX (offset);
       if ((BUFFER_DEFAULT_VALUE_P (offset)
-	   && (permanent_too
-	       || buffer_permanent_local_flags[idx] == 0)))
+           && (permanent_too
+               /* Special case these two for backwards-compat; they're
+                  flagged as permanent-locals in bindings.el, even
+                  though they do have default values. */
+               || (offset != PER_BUFFER_VAR_OFFSET (truncate_lines)
+                   && offset !=
+                   PER_BUFFER_VAR_OFFSET (buffer_file_coding_system)))))
         KILL_PER_BUFFER_VALUE (b, offset);
     }
 }
@@ -5133,7 +5133,6 @@ init_buffer_once (void)
 
      buffer_defaults: default values of buffer-locals
      buffer_local_flags: metadata
-     buffer_permanent_local_flags: metadata
      buffer_local_symbols: metadata
 
      There must be a simpler way to store the metadata.
@@ -5141,11 +5140,6 @@ init_buffer_once (void)
 
   int idx;
 
-  /* Items flagged permanent get an explicit permanent-local property
-     added in bindings.el, for clarity.  */
-  PDUMPER_REMEMBER_SCALAR (buffer_permanent_local_flags);
-  memset (buffer_permanent_local_flags, 0, sizeof buffer_permanent_local_flags);
-
   /* 0 means not a lisp var, -1 means always local, else mask.  */
   memset (&buffer_local_flags, 0, sizeof buffer_local_flags);
   bset_filename (&buffer_local_flags, make_fixnum (-1));
@@ -5193,9 +5187,7 @@ init_buffer_once (void)
   XSETFASTINT (BVAR (&buffer_local_flags, selective_display), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, selective_display_ellipses), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, tab_width), idx); ++idx;
-  XSETFASTINT (BVAR (&buffer_local_flags, truncate_lines), idx);
-  /* Make this one a permanent local.  */
-  buffer_permanent_local_flags[idx++] = 1;
+  XSETFASTINT (BVAR (&buffer_local_flags, truncate_lines), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, word_wrap), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, ctl_arrow), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, fill_column), idx); ++idx;
@@ -5209,9 +5201,7 @@ init_buffer_once (void)
   XSETFASTINT (BVAR (&buffer_local_flags, bidi_paragraph_direction), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, bidi_paragraph_separate_re), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, bidi_paragraph_start_re), idx); ++idx;
-  XSETFASTINT (BVAR (&buffer_local_flags, buffer_file_coding_system), idx);
-  /* Make this one a permanent local.  */
-  buffer_permanent_local_flags[idx++] = 1;
+  XSETFASTINT (BVAR (&buffer_local_flags, buffer_file_coding_system), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, left_margin_cols), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, right_margin_cols), idx); ++idx;
   XSETFASTINT (BVAR (&buffer_local_flags, left_fringe_width), idx); ++idx;
-- 
2.31.1

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