Avoid unnecessary pointer scanning in garbage collection (Bug#10780).

* alloc.c (POINTERS_MIGHT_HIDE_IN_OBJECTS): New macro.
(mark_memory): Mark Lisp_Objects only if pointers might hide in
objects, as mark_maybe_pointer will catch them otherwise.
(GC_LISP_OBJECT_ALIGNMENT): Remove; no longer needed.
* s/gnu-linux.h (GC_LISP_OBJECT_ALIGNMENT) [__mc68000__]: Likewise.

src/ChangeLog

+2012-04-07  Paul Eggert  <eggert@cs.ucla.edu>
+
+	Avoid unnecessary pointer scanning in garbage collection (Bug#10780).
+	* alloc.c (POINTERS_MIGHT_HIDE_IN_OBJECTS): New macro.
+	(mark_memory): Mark Lisp_Objects only if pointers might hide in
+	objects, as mark_maybe_pointer will catch them otherwise.
+	(GC_LISP_OBJECT_ALIGNMENT): Remove; no longer needed.
+	* s/gnu-linux.h (GC_LISP_OBJECT_ALIGNMENT) [__mc68000__]: Likewise.
+
 2012-04-07  Paul Eggert  <eggert@cs.ucla.edu>
 
 	Fix typo that broke non-Windows builds.

src/alloc.c

@@ -4251,23 +4251,38 @@ mark_maybe_pointer (void *p)
 }
 
 
-/* Alignment of Lisp_Object and pointer values.  Use offsetof, as it
-   sometimes returns a smaller alignment than GCC's __alignof__ and
-   mark_memory might miss objects if __alignof__ were used.  For
-   example, on x86 with WIDE_EMACS_INT, __alignof__ (Lisp_Object) is 8
-   but GC_LISP_OBJECT_ALIGNMENT should be 4.  */
-#ifndef GC_LISP_OBJECT_ALIGNMENT
-# define GC_LISP_OBJECT_ALIGNMENT offsetof (struct {char a; Lisp_Object b;}, b)
-#endif
+/* Alignment of pointer values.  Use offsetof, as it sometimes returns
+   a smaller alignment than GCC's __alignof__ and mark_memory might
+   miss objects if __alignof__ were used.  */
 #define GC_POINTER_ALIGNMENT offsetof (struct {char a; void *b;}, b)
 
+/* Define POINTERS_MIGHT_HIDE_IN_OBJECTS to 1 if marking via C pointers does
+   not suffice, which is the typical case.  A host where a Lisp_Object is
+   wider than a pointer might allocate a Lisp_Object in non-adjacent halves.
+   If USE_LSB_TAG, the bottom half is not a valid pointer, but it should
+   suffice to widen it to to a Lisp_Object and check it that way.  */
+#if defined USE_LSB_TAG || UINTPTR_MAX >> VALBITS != 0
+# if !defined USE_LSB_TAG && UINTPTR_MAX >> VALBITS >> GCTYPEBITS != 0
+  /* If tag bits straddle pointer-word boundaries, neither mark_maybe_pointer
+     nor mark_maybe_object can follow the pointers.  This should not occur on
+     any practical porting target.  */
+#  error "MSB type bits straddle pointer-word boundaries"
+# endif
+  /* Marking via C pointers does not suffice, because Lisp_Objects contain
+     pointer words that hold pointers ORed with type bits.  */
+# define POINTERS_MIGHT_HIDE_IN_OBJECTS 1
+#else
+  /* Marking via C pointers suffices, because Lisp_Objects contain pointer
+     words that hold unmodified pointers.  */
+# define POINTERS_MIGHT_HIDE_IN_OBJECTS 0
+#endif
+
 /* Mark Lisp objects referenced from the address range START+OFFSET..END
    or END+OFFSET..START. */
 
 static void
 mark_memory (void *start, void *end)
 {
-  Lisp_Object *p;
   void **pp;
   int i;
 
@@ -4284,11 +4299,6 @@ mark_memory (void *start, void *end)
       end = tem;
     }
 
-  /* Mark Lisp_Objects.  */
-  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
      only a pointer to them remains.  Example:
@@ -4310,17 +4320,10 @@ mark_memory (void *start, void *end)
   for (pp = start; (void *) pp < end; pp++)
     for (i = 0; i < sizeof *pp; i += GC_POINTER_ALIGNMENT)
       {
-	void *w = *(void **) ((char *) pp + i);
-	mark_maybe_pointer (w);
-
-#ifdef USE_LSB_TAG
-	/* A host where a Lisp_Object is wider than a pointer might
-	   allocate a Lisp_Object in non-adjacent halves.  If
-	   USE_LSB_TAG, the bottom half is not a valid pointer, so
-	   widen it to to a Lisp_Object and check it that way.  */
-	if (sizeof w < sizeof (Lisp_Object))
-	  mark_maybe_object (widen_to_Lisp_Object (w));
-#endif
+	void *p = *(void **) ((char *) pp + i);
+	mark_maybe_pointer (p);
+	if (POINTERS_MIGHT_HIDE_IN_OBJECTS)
+	  mark_maybe_object (widen_to_Lisp_Object (p));
       }
 }
 

src/s/gnu-linux.h

@@ -146,9 +146,6 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
     || defined __ia64__ || defined __sh__
 #define GC_SETJMP_WORKS 1
 #define GC_MARK_STACK GC_MAKE_GCPROS_NOOPS
-#ifdef __mc68000__
-#define GC_LISP_OBJECT_ALIGNMENT 2
-#endif
 #ifdef __ia64__
 #define GC_MARK_SECONDARY_STACK()				\
   do {								\