bug#3603: 23.0.94; takes much time to save large non-ASCII buffers

[YAMAMOTO Mitsuharu]

Steps to reproduce:

  1. emacs -Q
  2. C-x ( C-x i .../etc/tutorials/TUTORIAL.ja RET C-x )
  3. C-u 20 C-x e
  4. C-x C-s SOME-NEW-FILE-NAME RET

Result:

  It takes much time (~10 sec.) to save this ~1MB buffer.
  Emacs 22 can save it instantly.

The slowness comes from that of select-safe-coding-system, in
particular, find-coding-systems-region(-internal) in it.  The
following patch makes it much faster (a few sec.) than the current
version.

Index: src/coding.c
===================================================================
RCS file: /sources/emacs/emacs/src/coding.c,v
retrieving revision 1.434
diff -c -p -r1.434 coding.c
*** src/coding.c        17 Jun 2009 00:42:07 -0000      1.434
--- src/coding.c        18 Jun 2009 06:05:04 -0000
*************** DEFUN ("find-coding-systems-region-inter
*** 8638,8644 ****
    EMACS_INT start_byte, end_byte;
    const unsigned char *p, *pbeg, *pend;
    int c;
!   Lisp_Object tail, elt;
  
    if (STRINGP (start))
      {
--- 8638,8644 ----
    EMACS_INT start_byte, end_byte;
    const unsigned char *p, *pbeg, *pend;
    int c;
!   Lisp_Object tail, elt, chars_checked;
  
    if (STRINGP (start))
      {
*************** DEFUN ("find-coding-systems-region-inter
*** 8696,8701 ****
--- 8696,8702 ----
    while (p < pend && ASCII_BYTE_P (*p)) p++;
    while (p < pend && ASCII_BYTE_P (*(pend - 1))) pend--;
  
+   chars_checked = Fmake_char_table (Qnil, Qnil);
    while (p < pend)
      {
        if (ASCII_BYTE_P (*p))
*************** DEFUN ("find-coding-systems-region-inter
*** 8703,8708 ****
--- 8704,8711 ----
        else
        {
          c = STRING_CHAR_ADVANCE (p);
+         if (!NILP (char_table_ref (chars_checked, c)))
+           continue;
  
          charset_map_loaded = 0;
          for (tail = coding_attrs_list; CONSP (tail);)
*************** DEFUN ("find-coding-systems-region-inter
*** 8734,8739 ****
--- 8737,8743 ----
              p = pbeg + p_offset;
              pend = pbeg + pend_offset;
            }
+         char_table_set (chars_checked, c, Qt);
        }
      }
  

Some notes:

  1. It's still much slower than Emacs 22.  I guess we need to rewrite
     select-safe-coding-system if we try to make its performance
     comparable with Emacs 22.  But perhaps we should avoid such
     changes at this moment.
  2. If the "if (charset_map_loaded) ..." clause in
     Ffind_coding_systems_region_internal is intended for the
     relocation caused by GC, then maybe `chars_checked' above (and
     also `coding_attrs_list') should be GCPROed.

                                     YAMAMOTO Mitsuharu
                                mituharu@math.s.chiba-u.ac.jp

If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    `bt full' and `xbacktrace'.
If you would like to further debug the crash, please read the file
/usr/local/share/emacs/23.0.94/etc/DEBUG for instructions.


In GNU Emacs 23.0.94.1 (powerpc-apple-darwin9.7.0, X toolkit)
 of 2009-06-18 on yamamoto-mitsuharu-no-power-mac-g5.local
Windowing system distributor `The X.Org Foundation', version 11.0.10402000
configured using `configure  '--without-gif' '--without-jpeg' '--without-tiff''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: ja_JP.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default-enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  tool-bar-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  global-auto-composition-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t