Use 0 for Qnil

Fixes Bug#15880.
If USE_LSB_TAG, arrange for the representation of Qnil to be zero so
that NILP (x) is equivalent to testing whether x is 0 at the
machine level.  The overall effects of this and the previous patch
shrink the size of the text segment by 2.3% and speeds up
compilation of all the .elc files by about 0.5% on my platform,
which is Fedora 20 x86-64.
* lib-src/make-docfile.c (compare_globals):
* src/lisp.h (lisp_h_XPNTR, lisp_h_XSYMBOL, lisp_h_XUNTAG)
(make_lisp_symbol) [USE_LSB_TAG]:
Symbols now tag the difference from lispsym, not the pointer.
(lisp_h_XUNTAGBASE, TAG_SYMPTR): New macros.
(Lisp_Int0, Lisp_Int1, Lisp_Symbol, Lisp_Misc, Lisp_String, Lisp_Cons):
Renumber so that Lisp_Symbol is 0, so that Qnil is zero.
(XSYMBOL): New forward decl.
(XUNTAGBASE): New function.
(XUNTAG): Use it.

2 files changed, 13 insertions, 0 deletions

diff --git a/lib-src/ChangeLog b/lib-src/ChangeLog
index 8bdf7d1fb16..9a1fc7a3e9f 100644
--- a/lib-src/ChangeLog
+++ b/lib-src/ChangeLog
@@ -1,5 +1,8 @@
 2015-01-05  Paul Eggert  <eggert@cs.ucla.edu>
 
+        Use 0 for Qnil
+        * make-docfile.c (compare_globals): Consider 'nil' to be the least.
+
         Compute C decls for DEFSYMs automatically
         Fixes Bug#15880.
         * make-docfile.c: Revamp to generate table of symbols, too.
diff --git a/lib-src/make-docfile.c b/lib-src/make-docfile.c
index b05a634eb75..22c4bad2e3f 100644
--- a/lib-src/make-docfile.c
+++ b/lib-src/make-docfile.c
@@ -613,6 +613,16 @@ compare_globals (const void *a, const void *b)
   if (ga->type != gb->type)
     return ga->type - gb->type;
 
+  /* Consider "nil" to be the least, so that aQnil is firat.  That
+     way, Qnil's internal representation is zero, which is a bit faster.  */
+  if (ga->type == SYMBOL)
+    {
+      bool a_nil = strcmp (ga->name, "Qnil") == 0;
+      bool b_nil = strcmp (gb->name, "Qnil") == 0;
+      if (a_nil | b_nil)
+        return b_nil - a_nil;
+    }
+
   return strcmp (ga->name, gb->name);
 }