Fix crash due to non-contiguous EMACS_INT (Bug#10780).

* lisp.h (VALBITS): Move definition up, so that USE_LSB_TAG can use it.
(USE_LSB_TAG): Do not define if UINTPTR_MAX >> VALBITS == 0.
It's useless in that case, and it can cause problems on hosts
that allocate halves of EMACS_INT values separately.
Reported by Dan Horák.  Diagnosed by Andreas Schwab in
<http://debbugs.gnu.org/cgi/bugreport.cgi?bug=10780#30>.
* mem-limits.h (EXCEEDS_LISP_PTR): Define to 0 on hosts where
UINTPTR_MAX >> VALBITS == 0.  This is required by the above change;
it avoids undefined behavior on hosts where shifting right by more
than the word width has undefined behavior.

1 files changed, 13 insertions, 6 deletions

diff --git a/src/lisp.h b/src/lisp.h
index 366d24a737d..8bfd7071e5f 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -168,6 +168,10 @@ extern int suppress_checking EXTERNALLY_VISIBLE;
 #define GCTYPEBITS 3
 #endif
 
+#ifndef VALBITS
+#define VALBITS (BITS_PER_EMACS_INT - GCTYPEBITS)
+#endif
+
 #ifndef NO_DECL_ALIGN
 # ifndef DECL_ALIGN
 #  if HAVE_ATTRIBUTE_ALIGNED
@@ -191,7 +195,15 @@ extern int suppress_checking EXTERNALLY_VISIBLE;
      || defined DARWIN_OS || defined __sun)
 /* We also need to be able to specify mult-of-8 alignment on static vars.  */
 # if defined DECL_ALIGN
-#  define USE_LSB_TAG
+/* mark_maybe_object assumes that EMACS_INT values are contiguous,
+   but this is not true on some hosts where EMACS_INT is wider than a pointer,
+   as they may allocate the halves of an EMACS_INT separately.
+   On these hosts USE_LSB_TAG is not needed because the top bits of an
+   EMACS_INT are unused, so define USE_LSB_TAG only on hosts where it
+   might be useful.  */
+#  if UINTPTR_MAX >> VALBITS != 0
+#   define USE_LSB_TAG
+#  endif
 # endif
 #endif
 
@@ -309,11 +321,6 @@ enum Lisp_Fwd_Type
     Lisp_Fwd_Kboard_Obj,        /* Fwd to a Lisp_Object field of kboards.  */
   };
 
-/* These values are overridden by the m- file on some machines.  */
-#ifndef VALBITS
-#define VALBITS (BITS_PER_EMACS_INT - GCTYPEBITS)
-#endif
-
 #ifdef USE_LISP_UNION_TYPE
 
 #ifndef WORDS_BIGENDIAN