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, 5 insertions, 0 deletions

diff --git a/lib-src/etags.c b/lib-src/etags.c
index 174c33a7a5f..eee2c596262 100644
--- a/lib-src/etags.c
+++ b/lib-src/etags.c
@@ -124,6 +124,7 @@ University of California, as described above. */
 #include <binary-io.h>
 #include <intprops.h>
 #include <unlocked-io.h>
+#include <verify.h>
 #include <c-ctype.h>
 #include <c-strcase.h>
 
@@ -7310,6 +7311,8 @@ static void *
 xnmalloc (ptrdiff_t nitems, ptrdiff_t item_size)
 {
   ptrdiff_t nbytes;
+  assume (0 <= nitems);
+  assume (0 < item_size);
   if (INT_MULTIPLY_WRAPV (nitems, item_size, &nbytes))
     memory_full ();
   return xmalloc (nbytes);
@@ -7319,6 +7322,8 @@ static void *
 xnrealloc (void *pa, ptrdiff_t nitems, ptrdiff_t item_size)
 {
   ptrdiff_t nbytes;
+  assume (0 <= nitems);
+  assume (0 < item_size);
   if (INT_MULTIPLY_WRAPV (nitems, item_size, &nbytes) || SIZE_MAX < nbytes)
     memory_full ();
   void *result = realloc (pa, nbytes);