2010-05-28 Mark Probst <mark.probst@gmail.com>

	* sgen-gc.c, sgen-scan-object.h: Value types in arrays are not
	always aligned to ALLOC_ALIGN.  To handle this, shift the size in
	the GC descriptor by one bit, and don't encode the size for value
	types not containing references.  Also get rid of
	DESC_TYPE_STRING.

svn path=/trunk/mono/; revision=158079

mono/metadata/ChangeLog

@@ -1,3 +1,11 @@
+2010-05-28  Mark Probst  <mark.probst@gmail.com>
+
+	* sgen-gc.c, sgen-scan-object.h: Value types in arrays are not
+	always aligned to ALLOC_ALIGN.  To handle this, shift the size in
+	the GC descriptor by one bit, and don't encode the size for value
+	types not containing references.  Also get rid of
+	DESC_TYPE_STRING.
+
 2010-05-28  Mark Probst  <mark.probst@gmail.com>
 
 	* sgen-gc.c: Don't depend on the size encoded in a GC descriptor

mono/metadata/sgen-gc.c

@@ -953,7 +953,14 @@ static Fragment *nursery_fragments = NULL;
 /* freeelist of fragment structures */
 static Fragment *fragment_freelist = NULL;
 
-/* objects bigger then this go into the large object space */
+/*
+ * Objects bigger then this go into the large object space.  This size
+ * has a few constraints.  It must fit into the major heap, which in
+ * the case of the copying collector means that it must fit into a
+ * pinned chunk.  It must also play well with the GC descriptors, some
+ * of which (DESC_TYPE_RUN_LENGTH, DESC_TYPE_SMALL_BITMAP) encode the
+ * object size.
+ */
 #define MAX_SMALL_OBJ_SIZE 2040
 
 /* Functions supplied by the runtime to be called by the GC */
@@ -1090,9 +1097,13 @@ static void null_ephemerons_for_domain (MonoDomain *domain);
  * inside complex.
  */
 enum {
-	DESC_TYPE_RUN_LENGTH,   /* 16 bits aligned byte size | 1-3 (offset, numptr) bytes tuples */
-	DESC_TYPE_SMALL_BITMAP, /* 16 bits aligned byte size | 16-48 bit bitmap */
-	DESC_TYPE_STRING,       /* nothing */
+	/*
+	 * We don't use 0 so that 0 isn't a valid GC descriptor.  No
+	 * deep reason for this other than to be able to identify a
+	 * non-inited descriptor for debugging.
+	 */
+	DESC_TYPE_RUN_LENGTH = 1, /* 15 bits aligned byte size | 1-3 (offset, numptr) bytes tuples */
+	DESC_TYPE_SMALL_BITMAP, /* 15 bits aligned byte size | 16-48 bit bitmap */
 	DESC_TYPE_COMPLEX,      /* index for bitmap into complex_descriptors */
 	DESC_TYPE_VECTOR,       /* 10 bits element size | 1 bit array | 2 bits desc | element desc */
 	DESC_TYPE_ARRAY,        /* 10 bits element size | 1 bit array | 2 bits desc | element desc */
@@ -1195,7 +1206,7 @@ alloc_complex_descriptor (gsize *bitmap, int numbits)
 void*
 mono_gc_make_descr_for_string (gsize *bitmap, int numbits)
 {
-	return (void*) DESC_TYPE_STRING;
+	return (void*) DESC_TYPE_RUN_LENGTH;
 }
 
 void*
@@ -1204,8 +1215,6 @@ mono_gc_make_descr_for_object (gsize *bitmap, int numbits, size_t obj_size)
 	int first_set = -1, num_set = 0, last_set = -1, i;
 	mword desc = 0;
 	size_t stored_size = obj_size;
-	stored_size += ALLOC_ALIGN - 1;
-	stored_size &= ~(ALLOC_ALIGN - 1);
 	for (i = 0; i < numbits; ++i) {
 		if (bitmap [i / GC_BITS_PER_WORD] & ((gsize)1 << (i % GC_BITS_PER_WORD))) {
 			if (first_set < 0)
@@ -1214,23 +1223,35 @@ mono_gc_make_descr_for_object (gsize *bitmap, int numbits, size_t obj_size)
 			num_set++;
 		}
 	}
+	/*
+	 * We don't encode the size of types that don't contain
+	 * references because they might not be aligned, i.e. the
+	 * bottom two bits might be set, which would clash with the
+	 * bits we need to encode the descriptor type.  Since we don't
+	 * use the encoded size to skip objects, other than for
+	 * processing remsets, in which case only the positions of
+	 * references are relevant, this is not a problem.
+	 */
+	if (first_set < 0)
+		return DESC_TYPE_RUN_LENGTH;
+	g_assert (!(stored_size & 0x3));
 	if (stored_size <= MAX_SMALL_OBJ_SIZE) {
 		/* check run-length encoding first: one byte offset, one byte number of pointers
 		 * on 64 bit archs, we can have 3 runs, just one on 32.
 		 * It may be better to use nibbles.
 		 */
 		if (first_set < 0) {
-			desc = DESC_TYPE_RUN_LENGTH | stored_size;
+			desc = DESC_TYPE_RUN_LENGTH | (stored_size << 1);
 			DEBUG (6, fprintf (gc_debug_file, "Ptrfree descriptor %p, size: %zd\n", (void*)desc, stored_size));
 			return (void*) desc;
 		} else if (first_set < 256 && num_set < 256 && (first_set + num_set == last_set + 1)) {
-			desc = DESC_TYPE_RUN_LENGTH | stored_size | (first_set << 16) | (num_set << 24);
+			desc = DESC_TYPE_RUN_LENGTH | (stored_size << 1) | (first_set << 16) | (num_set << 24);
 			DEBUG (6, fprintf (gc_debug_file, "Runlen descriptor %p, size: %zd, first set: %d, num set: %d\n", (void*)desc, stored_size, first_set, num_set));
 			return (void*) desc;
 		}
 		/* we know the 2-word header is ptr-free */
 		if (last_set < SMALL_BITMAP_SIZE + OBJECT_HEADER_WORDS) {
-			desc = DESC_TYPE_SMALL_BITMAP | stored_size | ((*bitmap >> OBJECT_HEADER_WORDS) << SMALL_BITMAP_SHIFT);
+			desc = DESC_TYPE_SMALL_BITMAP | (stored_size << 1) | ((*bitmap >> OBJECT_HEADER_WORDS) << SMALL_BITMAP_SHIFT);
 			DEBUG (6, fprintf (gc_debug_file, "Smallbitmap descriptor %p, size: %zd, last set: %d\n", (void*)desc, stored_size, last_set));
 			return (void*) desc;
 		}
@@ -1324,11 +1345,11 @@ mono_gc_get_bitmap_for_descr (void *descr, int *numbits)
 	} while (0)
 
 #define OBJ_RUN_LEN_SIZE(size,desc,obj) do { \
-        (size) = (desc) & 0xfff8; \
+		(size) = ((desc) & 0xfff8) >> 1;	\
     } while (0)
 
 #define OBJ_BITMAP_SIZE(size,desc,obj) do { \
-        (size) = (desc) & 0xfff8; \
+		(size) = ((desc) & 0xfff8) >> 1;	\
     } while (0)
 
 //#define PREFETCH(addr) __asm__ __volatile__ ("     prefetchnta     %0": : "m"(*(char *)(addr)))
@@ -6554,7 +6575,7 @@ mono_gc_wbarrier_value_copy (gpointer dest, gpointer src, int count, MonoClass *
 	LOCK_GC;
 	memmove (dest, src, count * mono_class_value_size (klass, NULL));
 	rs = REMEMBERED_SET;
-	if (ptr_in_nursery (dest) || ptr_on_stack (dest)) {
+	if (ptr_in_nursery (dest) || ptr_on_stack (dest) || !klass->has_references) {
 		UNLOCK_GC;
 		return;
 	}

mono/metadata/sgen-scan-object.h

@@ -24,7 +24,9 @@
  *
  * Scans one object, using the OBJ_XXX macros.  The start of the
  * object must be given in the variable "char* start".  Afterwards,
- * "start" will point to the start of the next object.
+ * "start" will point to the start of the next object, if the scanned
+ * object contained references.  If not, the value of "start" should
+ * be considered undefined after executing this code.
  *
  * Modifiers (automatically undefined):
  *
@@ -53,16 +55,8 @@
 	/* gcc should be smart enough to remove the bounds check, but it isn't:( */
 	desc = vt->desc;
 	switch (desc & 0x7) {
-	case DESC_TYPE_STRING:
-		STRING_SIZE (skip_size, start);
-#define SCAN
-		SCAN_OBJECT_ACTION;
-#undef SCAN
-		start += skip_size;
-		break;
 	case DESC_TYPE_RUN_LENGTH:
 		OBJ_RUN_LEN_SIZE (skip_size, desc, start);
-		g_assert (skip_size);
 #define SCAN OBJ_RUN_LEN_FOREACH_PTR (desc, start)
 #ifndef SCAN_OBJECT_NOSCAN
 		SCAN;