Pacify GCC 9.2.1 20190927 -O3

Original problem report by N. Jackson in:
https://lists.gnu.org/r/emacs-devel/2020-03/msg00047.html
I found some other warnings when I used gcc, and fixed them
with this patch.
* lib-src/etags.c: Include verify.h.
(xnmalloc, xnrealloc): Tell the compiler that NITEMS is
nononnegative and ITEM_SIZE is positive.
* src/conf_post.h (__has_attribute_returns_nonnull)
(ATTRIBUTE_RETURNS_NONNULL): New macros.
* src/editfns.c (Fuser_full_name): Don’t assume Fuser_login_name
returns non-nil.
* src/intervals.c (rotate_right, rotate_left, update_interval):
* src/intervals.h (LENGTH, LEFT_TOTAL_LENGTH, RIGHT_TOTAL_LENGTH):
Use TOTAL_LENGTH0 or equivalent on intervals that might be null.
* src/intervals.h (TOTAL_LENGTH): Assume arg is nonnull.
(TOTAL_LENGTH0): New macro, with the old TOTAL_LENGTH meaning.
(make_interval, split_interval_right): Add ATTRIBUTE_RETURNS_NONNULL.
* src/pdumper.c (dump_check_dump_off): Now returns void, since
no caller uses the return value.  Redo assert to pacify GCC.
(decode_emacs_reloc): Add a seemingly-random eassume to pacify GCC.
Ugly, and I suspect due to a bug in GCC.

1 files changed, 14 insertions, 10 deletions

diff --git a/src/intervals.h b/src/intervals.h
index a93b10e9fff..9a7ba910a10 100644
--- a/src/intervals.h
+++ b/src/intervals.h
@@ -96,24 +96,27 @@ struct interval
 /* True if this interval has both left and right children.  */
 #define BOTH_KIDS_P(i) ((i)->left != NULL && (i)->right != NULL)
 
-/* The total size of all text represented by this interval and all its
-   children in the tree.   This is zero if the interval is null.  */
-#define TOTAL_LENGTH(i) ((i) == NULL ? 0 : (i)->total_length)
+/* The total size of all text represented by the nonnull interval I
+   and all its children in the tree.  */
+#define TOTAL_LENGTH(i) ((i)->total_length)
+
+/* Likewise, but also defined to be zero if I is null.  */
+#define TOTAL_LENGTH0(i) ((i) ? TOTAL_LENGTH (i) : 0)
 
 /* The size of text represented by this interval alone.  */
-#define LENGTH(i) ((i)->total_length                    \
-                   - TOTAL_LENGTH ((i)->right)          \
-                   - TOTAL_LENGTH ((i)->left))
+#define LENGTH(i) (TOTAL_LENGTH (i)             \
+                   - RIGHT_TOTAL_LENGTH (i)     \
+                   - LEFT_TOTAL_LENGTH (i))
 
 /* The position of the character just past the end of I.  Note that
    the position cache i->position must be valid for this to work.  */
 #define INTERVAL_LAST_POS(i) ((i)->position + LENGTH (i))
 
 /* The total size of the left subtree of this interval.  */
-#define LEFT_TOTAL_LENGTH(i) ((i)->left ? (i)->left->total_length : 0)
+#define LEFT_TOTAL_LENGTH(i) TOTAL_LENGTH0 ((i)->left)
 
 /* The total size of the right subtree of this interval.  */
-#define RIGHT_TOTAL_LENGTH(i) ((i)->right ? (i)->right->total_length : 0)
+#define RIGHT_TOTAL_LENGTH(i) TOTAL_LENGTH0 ((i)->right)
 
 /* These macros are for dealing with the interval properties.  */
 
@@ -234,7 +237,7 @@ set_interval_plist (INTERVAL i, Lisp_Object plist)
 
 /* Declared in alloc.c.  */
 
-extern INTERVAL make_interval (void);
+extern INTERVAL make_interval (void) ATTRIBUTE_RETURNS_NONNULL;
 
 /* Declared in intervals.c.  */
 
@@ -246,7 +249,8 @@ extern void traverse_intervals (INTERVAL, ptrdiff_t,
                                 Lisp_Object);
 extern void traverse_intervals_noorder (INTERVAL,
                                         void (*) (INTERVAL, void *), void *);
-extern INTERVAL split_interval_right (INTERVAL, ptrdiff_t);
+extern INTERVAL split_interval_right (INTERVAL, ptrdiff_t)
+  ATTRIBUTE_RETURNS_NONNULL;
 extern INTERVAL split_interval_left (INTERVAL, ptrdiff_t);
 extern INTERVAL find_interval (INTERVAL, ptrdiff_t);
 extern INTERVAL next_interval (INTERVAL);