Use typed allocations for the hash table entries. This makes sure that the GC won't treat the jump table bitfield as a pointer.

Store the refcount structures inside the hash table, rather than in a chained structure.  This uses less space and should be easier for the GC to scan (less cache used).

ANNOUNCE

@@ -11,6 +11,9 @@ release include:
   collected environment.  This uses the Boehm garbage collector and is enabled
   by default.  To build without garbage collection specify the boehm_gc=no
   option when building.
+- The runtime will now use Boehm GC for several internal data structures, if it
+  is built with GC enabled.  This avoids the need for defensive programming
+  with respect to thread safety in several places.
 - This is the first release to provide a superset of the functionality provided
   by the Mac Objective-C runtime, as shipped with OS X 10.6.
 - The LLVM optimisation passes have been improved and better tested.  Code

gc_boehm.c

@@ -355,17 +355,17 @@ void objc_assertRegisteredThreadWithCollector()
 /**
  * Structure stored for each GC
  */
-struct gc_refcount
+static struct gc_refcount
 {
 	/** Reference count */
-	int refCount;
+	intptr_t refCount;
 	/** Strong pointer */
 	id ptr;
-};
+} null_refcount = {0};
 
-static int refcount_compare(const void *ptr, const struct gc_refcount *rc)
+static int refcount_compare(const void *ptr, struct gc_refcount rc)
 {
-	return ptr == rc->ptr;
+	return ptr == rc.ptr;
 }
 static uint32_t ptr_hash(const void *ptr)
 {
@@ -373,14 +373,23 @@ static uint32_t ptr_hash(const void *ptr)
 	// always be 0, which is not so useful for a hash value
 	return ((uintptr_t)ptr >> 4) | ((uintptr_t)ptr << ((sizeof(id) * 8) - 4));
 }
-static uint32_t refcount_hash(const struct gc_refcount *rc)
+static uint32_t refcount_hash(struct gc_refcount rc)
+{
+	return ptr_hash(rc.ptr);
+}
+static int isEmpty(struct gc_refcount rc)
 {
-	return ptr_hash(rc->ptr);
+	return rc.ptr == NULL;
 }
+#define MAP_TABLE_VALUE_NULL isEmpty
 #define MAP_TABLE_NAME refcount
 #define MAP_TABLE_COMPARE_FUNCTION refcount_compare
 #define MAP_TABLE_HASH_KEY ptr_hash
 #define MAP_TABLE_HASH_VALUE refcount_hash
+#define MAP_TABLE_VALUE_TYPE struct gc_refcount
+#define MAP_TABLE_VALUE_PLACEHOLDER null_refcount
+#define MAP_TABLE_TYPES_BITMAP (1<<(offsetof(struct gc_refcount, ptr) / sizeof(void*)))
+#define MAP_TABLE_ACCESS_BY_REFERENCE
 #include "hash_table.h"
 
 static refcount_table *refcounts;
@@ -394,10 +403,8 @@ id objc_gc_retain(id object)
 		refcount = refcount_table_get(refcounts, object);
 		if (NULL == refcount)
 		{
-			refcount = GC_MALLOC_UNCOLLECTABLE(sizeof(struct gc_refcount));
-			refcount->ptr = object;
-			refcount->refCount = 1;
-			refcount_insert(refcounts, refcount);
+			struct gc_refcount rc = { 1, object};
+			refcount_insert(refcounts, rc);
 			return object;
 		}
 	}
@@ -413,20 +420,14 @@ void objc_gc_release(id object)
 	if (0 == __sync_sub_and_fetch(&(refcount->refCount), 1))
 	{
 		LOCK_FOR_SCOPE(&(refcounts->lock));
-		refcount_remove(refcounts, object);
+		refcount->ptr = 0;
 		__sync_synchronize();
 		// If another thread has incremented the reference count while we were
 		// doing this, then we need to add the count back into the table,
 		// otherwise we can carry on.
-		if (__sync_bool_compare_and_swap(&(refcount->refCount), 0, 0))
+		if (!__sync_bool_compare_and_swap(&(refcount->refCount), 0, 0))
 		{
-			// This doesn't free the object, it just removes the explicit
-			// reference
-			GC_free(refcount);
-		}
-		else
-		{
-			refcount_insert(refcounts, refcount);
+			refcount->ptr = object;
 		}
 	}
 }

hash_table.h

@@ -25,11 +25,14 @@
 
 #ifdef ENABLE_GC
 #	include <gc/gc.h>
+#	include <gc/gc_typed.h>
 #	define CALLOC(x,y) GC_MALLOC(x*y)
 #	define IF_NO_GC(x)
+#	define IF_GC(x) x
 #else
 #	define CALLOC(x,y) calloc(x,y)
 #	define IF_NO_GC(x) x
+#	define IF_GC(x)
 #endif
 
 #ifndef MAP_TABLE_NAME
@@ -72,6 +75,7 @@ static BOOL PREFIX(_is_null)(void *value)
 {
 	return value == NULL;
 }
+#	define MAP_TABLE_TYPES_BITMAP 1
 #	define MAP_TABLE_VALUE_NULL PREFIX(_is_null)
 #	define MAP_TABLE_VALUE_PLACEHOLDER NULL
 #endif
@@ -105,17 +109,37 @@ typedef struct PREFIX(_table_struct)
 	unsigned int table_size;
 	unsigned int table_used;
 	IF_NO_GC(unsigned int enumerator_count;)
+#	if defined(ENABLE_GC) && defined(MAP_TABLE_TYPES_BITMAP)
+	GC_descr descr;
+#	endif 
 	struct PREFIX(_table_struct) *old;
 	struct PREFIX(_table_cell_struct) *table;
 } PREFIX(_table);
 
+struct PREFIX(_table_cell_struct) *PREFIX(alloc_cells)(PREFIX(_table) *table, int count)
+{
+#	if defined(ENABLE_GC) && defined(MAP_TABLE_TYPES_BITMAP)
+	return GC_CALLOC_EXPLICITLY_TYPED(count,
+			sizeof(struct PREFIX(_table_cell_struct)), table->descr);
+#	else
+	return CALLOC(count, sizeof(struct PREFIX(_table_cell_struct)));
+#	endif
+}
+
+
 PREFIX(_table) *PREFIX(_create)(uint32_t capacity)
 {
 	PREFIX(_table) *table = CALLOC(1, sizeof(PREFIX(_table)));
 #	ifndef MAP_TABLE_NO_LOCK
 	INIT_LOCK(table->lock);
 #	endif
-	table->table = calloc(capacity, sizeof(struct PREFIX(_table_cell_struct)));
+#	if defined(ENABLE_GC) && defined(MAP_TABLE_TYPES_BITMAP)
+	// The low word in the bitmap stores the offsets of the next entries
+	GC_word bitmap = (MAP_TABLE_TYPES_BITMAP << 1);
+	table->descr = GC_make_descriptor(&bitmap,
+			sizeof(struct PREFIX(_table_cell_struct)) / sizeof (void*));
+#	endif
+	table->table = PREFIX(alloc_cells)(table, capacity);
 	table->table_size = capacity;
 	return table;
 }
@@ -170,11 +194,8 @@ static int PREFIX(_insert)(PREFIX(_table) *table, MAP_TABLE_VALUE_TYPE value);
 
 static int PREFIX(_table_resize)(PREFIX(_table) *table)
 {
-	// Note: We multiply the table size, rather than the count, by two so that
-	// we get overflow checking in calloc.  Two times the size of a cell will
-	// never overflow, but two times the table size might.
-	struct PREFIX(_table_cell_struct) *newArray = CALLOC(table->table_size, 2 *
-			sizeof(struct PREFIX(_table_cell_struct)));
+	struct PREFIX(_table_cell_struct) *newArray =
+		PREFIX(alloc_cells)(table, table->table_size * 2);
 	if (NULL == newArray) { return 0; }
 
 	// Allocate a new table structure and move the array into that.  Now
@@ -565,3 +586,5 @@ PREFIX(_current)(PREFIX(_table) *table,
 
 #undef CALLOC
 #undef IF_NO_GC
+#undef IF_GC
+#undef MAP_TABLE_TYPES_BITMAP