bug#22884: 25.0.92; C/l mode editing takes waaaayy too long

[Stefan Monnier]

[ Just to clarify and keep it in the corresponding bug, rather than in
  some side-thread in emacs-devel.  ]

> emacs -Q config.h
> M-x goto-line RET 1661 RET / /

The patch below seems to fix this pathological case, making the "/ /"
instantaneous again.


        Stefan


diff --git a/src/syntax.c b/src/syntax.c
index 8e14bf3..b712e45 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -597,6 +597,26 @@ find_defun_start (ptrdiff_t pos, ptrdiff_t pos_byte)
       && MODIFF == find_start_modiff)
     return find_start_value;
 
+  if (!NILP (Vcomment_use_syntax_ppss))
+    {
+      EMACS_INT modiffs = CHARS_MODIFF;
+      Lisp_Object ppss = call1 (Qsyntax_ppss, make_number (pos));
+      if (modiffs != CHARS_MODIFF)
+       error ("syntax-ppss modified the buffer!");
+      TEMP_SET_PT_BOTH (opoint, opoint_byte);
+      Lisp_Object boc = Fnth (make_number (8), ppss);
+      if (NUMBERP (boc))
+        {
+          find_start_value = XINT (boc);
+          find_start_value_byte = CHAR_TO_BYTE (find_start_value);
+        }
+      else
+        {
+          find_start_value = pos;
+          find_start_value_byte = pos_byte;
+        }
+      goto found;
+    }
   if (!open_paren_in_column_0_is_defun_start)
     {
       find_start_value = BEGV;
@@ -864,6 +884,7 @@ back_comment (ptrdiff_t from, ptrdiff_t from_byte, 
ptrdiff_t stop,
        case Sopen:
          /* Assume a defun-start point is outside of strings.  */
          if (open_paren_in_column_0_is_defun_start
+              && NILP (Vcomment_use_syntax_ppss)
              && (from == stop
                  || (temp_byte = dec_bytepos (from_byte),
                      FETCH_CHAR (temp_byte) == '\n')))
@@ -3647,6 +3668,11 @@ void
 syms_of_syntax (void)
 {
   DEFSYM (Qsyntax_table_p, "syntax-table-p");
+  DEFSYM (Qsyntax_ppss, "syntax-ppss");
+  DEFVAR_LISP ("comment-use-syntax-ppss",
+              Vcomment_use_syntax_ppss,
+              doc: /* Non-nil means `forward-comment' can use `syntax-ppss' 
internally.  */);
+  Vcomment_use_syntax_ppss = Qt;
 
   staticpro (&Vsyntax_code_object);