Subject: Re: bug#10780: 24.0.93 fails to build with 'Invalid function: "DEAD"'

From: Paul Eggert <[email-masked]>
Date: Mon, 20 Feb 2012 15:14:39 -0800

On 02/15/2012 02:09 AM, Andreas Schwab wrote:

> What about local variables in registers?  They are split between two
> registers, but mark_stack does not handle that.

True.  Teerrroooo.  Thanks for diagnosing that.

I pushed the following fix into the trunk.  It's needed
regardless of this particular bug.  Dan, can you please
check whether it indeed fixes the bug for you?  Thanks.

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.
=== modified file 'src/lisp.h'
--- src/lisp.h	2012-01-19 07:21:25 +0000
+++ src/lisp.h	2012-02-20 16:27:49 +0000
@@ -168,6 +168,10 @@
 #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 @@
      || 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 @@
     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

=== modified file 'src/mem-limits.h'
--- src/mem-limits.h	2012-01-19 07:21:25 +0000
+++ src/mem-limits.h	2012-02-20 22:53:46 +0000
@@ -34,7 +34,7 @@
 #endif

 extern char *start_of_data (void);
-#if defined USE_LSB_TAG
+#if defined USE_LSB_TAG || UINTPTR_MAX >> VALBITS == 0
 #define EXCEEDS_LISP_PTR(ptr) 0
 #elif defined DATA_SEG_BITS
 #define EXCEEDS_LISP_PTR(ptr) \