Allow gap before first non-Lisp pseudovec member

Problem reported by Keith David Bershatsky in:
https://lists.gnu.org/r/emacs-devel/2019-04/msg00259.html
Solution suggested by Stefan Monnier in:
https://lists.gnu.org/r/emacs-devel/2019-04/msg00282.html
* src/buffer.h (BUFFER_LISP_SIZE): Simplify by using PSEUDOVECSIZE.
(BUFFER_REST_SIZE): Simplify by using VECSIZE and BUFFER_LISP_SIZE.
* src/lisp.h (PSEUDOVECSIZE): Base it on the last Lisp field,
not the first non-Lisp field.  All callers changed.  Callers
without Lisp fields changed to use ALLOCATE_PLAIN_PSEUDOVECTOR.
(ALLOCATE_PLAIN_PSEUDOVECTOR): New macro.

1 files changed, 6 insertions, 8 deletions

diff --git a/src/buffer.h b/src/buffer.h
index 63b162161c6..f42c3e97b97 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -741,8 +741,8 @@ struct buffer
      See `cursor-type' for other values.  */
   Lisp_Object cursor_in_non_selected_windows_;
 
-  /* No more Lisp_Object beyond this point.  Except undo_list,
-     which is handled specially in Fgarbage_collect.  */
+  /* No more Lisp_Object beyond cursor_in_non_selected_windows_.
+     Except undo_list, which is handled specially in Fgarbage_collect.  */
 
   /* This structure holds the coordinates of the buffer contents
      in ordinary buffers.  In indirect buffers, this is not used.  */
@@ -1019,14 +1019,12 @@ bset_width_table (struct buffer *b, Lisp_Object val)
    structure, make sure that this is still correct.  */
 
 #define BUFFER_LISP_SIZE                                                \
-  ((offsetof (struct buffer, own_text) - header_size) / word_size)
+  PSEUDOVECSIZE (struct buffer, cursor_in_non_selected_windows_)
 
-/* Size of the struct buffer part beyond leading Lisp_Objects, in word_size
-   units.  Rounding is needed for --with-wide-int configuration.  */
+/* Allocated size of the struct buffer part beyond leading
+   Lisp_Objects, in word_size units.  */
 
-#define BUFFER_REST_SIZE                                                \
-  ((((sizeof (struct buffer) - offsetof (struct buffer, own_text))      \
-     + (word_size - 1)) & ~(word_size - 1)) / word_size)
+#define BUFFER_REST_SIZE (VECSIZE (struct buffer) - BUFFER_LISP_SIZE)
 
 /* Initialize the pseudovector header of buffer object.  BUFFER_LISP_SIZE
    is required for GC, but BUFFER_REST_SIZE is set up just to be consistent