Fix alignment-related core dump during GC.

* configure.in (GC_LISP_OBJECT_ALIGNMENT): Remove.
This is now done by src/alloc.c.
* src/alloc.c (GC_LISP_OBJECT_ALIGNMENT): Use offsetof, not __alignof__
or sizeof.  __alignof__ gives the wrong answer on Fedora x86-64
with GCC 4.6.1 when configured with CC='gcc -m32' --with-wide-int;
this makes Emacs dump core during garbage collection on rare
occasions.  sizeof is obviously inferior to offsetof here, so
stick with offsetof.
(GC_POINTER_ALIGNMENT): New macro.
(mark_memory): Omit 3rd (offset) arg; caller changed.
Don't assume EMACS_INT alignment is the same as pointer alignment.

2 files changed, 29 insertions, 20 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index b697ee43376..d2c069b7351 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,15 @@
+2011-10-07  Paul Eggert  <eggert@cs.ucla.edu>
+
+        * alloc.c (GC_LISP_OBJECT_ALIGNMENT): Use offsetof, not __alignof__
+        or sizeof.  __alignof__ gives the wrong answer on Fedora x86-64
+        with GCC 4.6.1 when configured with CC='gcc -m32' --with-wide-int;
+        this makes Emacs dump core during garbage collection on rare
+        occasions.  sizeof is obviously inferior to offsetof here, so
+        stick with offsetof.
+        (GC_POINTER_ALIGNMENT): New macro.
+        (mark_memory): Omit 3rd (offset) arg; caller changed.
+        Don't assume EMACS_INT alignment is the same as pointer alignment.
+
 2011-10-03  Stefan Monnier  <monnier@iro.umontreal.ca>
 
         * keyboard.c (read_key_sequence_remapped): New var.
diff --git a/src/alloc.c b/src/alloc.c
index ead5c8a8ca4..f4db3abbd93 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -393,7 +393,7 @@ static int live_symbol_p (struct mem_node *, void *);
 static int live_float_p (struct mem_node *, void *);
 static int live_misc_p (struct mem_node *, void *);
 static void mark_maybe_object (Lisp_Object);
-static void mark_memory (void *, void *, int);
+static void mark_memory (void *, void *);
 static void mem_init (void);
 static struct mem_node *mem_insert (void *, void *, enum mem_type);
 static void mem_insert_fixup (struct mem_node *);
@@ -4235,14 +4235,20 @@ mark_maybe_pointer (void *p)
 }
 
 
+#ifndef GC_LISP_OBJECT_ALIGNMENT
+# define GC_LISP_OBJECT_ALIGNMENT offsetof (struct {char a; Lisp_Object b;}, b)
+#endif
+#define GC_POINTER_ALIGNMENT offsetof (struct {char a; void *b;}, b)
+
 /* Mark Lisp objects referenced from the address range START+OFFSET..END
    or END+OFFSET..START. */
 
 static void
-mark_memory (void *start, void *end, int offset)
+mark_memory (void *start, void *end)
 {
   Lisp_Object *p;
   void **pp;
+  int i;
 
 #if GC_MARK_STACK == GC_USE_GCPROS_CHECK_ZOMBIES
   nzombies = 0;
@@ -4258,8 +4264,9 @@ mark_memory (void *start, void *end, int offset)
     }
 
   /* Mark Lisp_Objects.  */
-  for (p = (Lisp_Object *) ((char *) start + offset); (void *) p < end; ++p)
-    mark_maybe_object (*p);
+  for (p = start; (void *) p < end; p++)
+    for (i = 0; i < sizeof *p; i += GC_LISP_OBJECT_ALIGNMENT)
+      mark_maybe_object (*(Lisp_Object *) ((char *) p + i));
 
   /* Mark Lisp data pointed to.  This is necessary because, in some
      situations, the C compiler optimizes Lisp objects away, so that
@@ -4279,8 +4286,9 @@ mark_memory (void *start, void *end, int offset)
      away.  The only reference to the life string is through the
      pointer `s'.  */
 
-  for (pp = (void **) ((char *) start + offset); (void *) pp < end; ++pp)
-    mark_maybe_pointer (*pp);
+  for (pp = start; (void *) pp < end; pp++)
+    for (i = 0; i < sizeof *pp; i += GC_POINTER_ALIGNMENT)
+      mark_maybe_pointer (*(void **) ((char *) pp + i));
 }
 
 /* setjmp will work with GCC unless NON_SAVING_SETJMP is defined in
@@ -4454,15 +4462,11 @@ dump_zombies (void)
    pass starting at the start of the stack + 2.  Likewise, if the
    minimal alignment of Lisp_Objects on the stack is 1, four passes
    would be necessary, each one starting with one byte more offset
-   from the stack start.
-
-   The current code assumes by default that Lisp_Objects are aligned
-   equally on the stack.  */
+   from the stack start.  */
 
 static void
 mark_stack (void)
 {
-  int i;
   void *end;
 
 #ifdef HAVE___BUILTIN_UNWIND_INIT
@@ -4520,15 +4524,8 @@ mark_stack (void)
   /* This assumes that the stack is a contiguous region in memory.  If
      that's not the case, something has to be done here to iterate
      over the stack segments.  */
-#ifndef GC_LISP_OBJECT_ALIGNMENT
-#ifdef __GNUC__
-#define GC_LISP_OBJECT_ALIGNMENT __alignof__ (Lisp_Object)
-#else
-#define GC_LISP_OBJECT_ALIGNMENT sizeof (Lisp_Object)
-#endif
-#endif
-  for (i = 0; i < sizeof (Lisp_Object); i += GC_LISP_OBJECT_ALIGNMENT)
-    mark_memory (stack_base, end, i);
+  mark_memory (stack_base, end);
+
   /* Allow for marking a secondary stack, like the register stack on the
      ia64.  */
 #ifdef GC_MARK_SECONDARY_STACK