Widen modiff counts to avoid wraparound

Widen modification counts to at least 64 bits, to make
wraparound practically impossible.
* doc/lispref/buffers.texi (Buffer Modification):
Don’t say the modification-count can wrap around.
* src/buffer.c (Frestore_buffer_modified_p, Fbuffer_swap_text)
(modify_overlay):
* src/insdel.c (insert_1_both, insert_from_string_1)
(insert_from_gap, insert_from_buffer_1)
(adjust_after_replace, replace_range, replace_range_2)
(del_range_2, modify_text):
* src/textprop.c (modify_text_properties):
Use modiff_incr instead of incrementing	directly.
(Fbuffer_modified_tick, Fbuffer_chars_modified_tick):
Don’t assume modification counts fit into fixnums.
* src/buffer.h (struct buffer_text, struct buffer):
* src/cmds.c (internal_self_insert):
* src/fileio.c (Finsert_file_contents):
* src/indent.c (last_known_column_modified):
* src/keyboard.c (command_loop_1):
* src/marker.c (cached_modiff):
* src/syntax.c (find_start_modiff, parse_sexp_propertize)
(find_defun_start):
* src/window.h (struct window):
Use modiff_count for modification counts.
* src/editfns.c (Fsubst_char_in_region):
Copy instead of incrementing modification counts,
since integer overflow checking is not needed here.
* src/lisp.h (modiff_count): New type.
(modiff_incr, modiff_to_integer): New inline functions.
* src/pdumper.c (dump_buffer): Update hash.

1 files changed, 22 insertions, 0 deletions

diff --git a/src/lisp.h b/src/lisp.h
index 5159f050a49..5a0de4b12c7 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3490,6 +3490,28 @@ integer_to_uintmax (Lisp_Object num, uintmax_t *n)
     }
 }
 
+/* A modification count.  These are wide enough, and incremented
+   rarely enough, so that they should never overflow a 60-bit counter
+   in practice, and the code below assumes this so a compiler can
+   generate better code if EMACS_INT is 64 bits.  */
+typedef intmax_t modiff_count;
+
+INLINE modiff_count
+modiff_incr (modiff_count *a)
+{
+  modiff_count a0 = *a;
+  bool modiff_overflow = INT_ADD_WRAPV (a0, 1, a);
+  eassert (!modiff_overflow && *a >> 30 >> 30 == 0);
+  return a0;
+}
+
+INLINE Lisp_Object
+modiff_to_integer (modiff_count a)
+{
+  eassume (0 <= a && a >> 30 >> 30 == 0);
+  return make_int (a);
+}
+
 /* Defined in data.c.  */
 extern _Noreturn void wrong_choice (Lisp_Object, Lisp_Object);
 extern void notify_variable_watchers (Lisp_Object, Lisp_Object,