Replace Vdead with tagged pointer

This speeds up ‘make compile-always’ by 0.1% on my platform.
Suggested by Pip Cet in:
https://lists.gnu.org/r/emacs-devel/2019-07/msg00257.html
* src/.gdbinit (pwinx, pgx, xbuffer, xprintstr):
Output dead_object () as "DEAD".
* src/alloc.c (Vdead, DEADP): Remove.
All uses replaced by dead_object () / deadp.
(deadp): New function.
(init_alloc_once_for_pdumper): Remove no-longer-needed
initialization.
* src/lisp.h (dead_object): New function.

1 files changed, 9 insertions, 3 deletions

diff --git a/src/lisp.h b/src/lisp.h
index 7641b2aab4d..e93a219625e 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -1247,6 +1247,15 @@ make_lisp_ptr (void *ptr, enum Lisp_Type type)
 #define XSETSYMBOL(a, b) ((a) = make_lisp_symbol (b))
 #define XSETFLOAT(a, b) ((a) = make_lisp_ptr (b, Lisp_Float))
 
+/* Return a Lisp_Object value that does not correspond to any object.
+   This can make some Lisp objects on free lists recognizable in O(1).  */
+
+INLINE Lisp_Object
+dead_object (void)
+{
+  return make_lisp_ptr (NULL, Lisp_String);
+}
+
 /* Pseudovector types.  */
 
 #define XSETPVECTYPE(v, code)                                           \
@@ -3759,9 +3768,6 @@ extern byte_ct const memory_full_cons_threshold;
 #ifdef HAVE_PDUMPER
 extern int number_finalizers_run;
 #endif
-#ifdef ENABLE_CHECKING
-extern Lisp_Object Vdead;
-#endif
 extern Lisp_Object list1 (Lisp_Object);
 extern Lisp_Object list2 (Lisp_Object, Lisp_Object);
 extern Lisp_Object list3 (Lisp_Object, Lisp_Object, Lisp_Object);