optimize adding unwind info entries by hashtable

src/mono/mono/mini/unwind.c

@@ -30,14 +30,15 @@ typedef struct {
 
 typedef struct {
 	guint32 len;
-	guint8 info [MONO_ZERO_LEN_ARRAY];
+	guint8 *info;
 } MonoUnwindInfo;
 
 static mono_mutex_t unwind_mutex;
 
-static MonoUnwindInfo **cached_info;
+static MonoUnwindInfo *cached_info;
 static int cached_info_next, cached_info_size;
 static GSList *cached_info_list;
+static GHashTable *cached_info_ht;
 /* Statistics */
 static int unwind_info_size;
 
@@ -729,6 +730,34 @@ mono_unwind_init (void)
 	mono_counters_register ("Unwind info size", MONO_COUNTER_JIT | MONO_COUNTER_INT, &unwind_info_size);
 }
 
+static guint
+cached_info_hash(gconstpointer key)
+{
+	guint i, a;
+	const guint8 *info = cached_info [GPOINTER_TO_UINT (key)].info;
+	const guint len = cached_info [GPOINTER_TO_UINT (key)].len;
+
+	for (i = a = 0; i != len; ++i)
+		a ^= (((guint)info [i]) << (i & 0xf));
+
+	return a;
+}
+
+static gboolean
+cached_info_eq(gconstpointer a, gconstpointer b)
+{
+	const guint32 lena = cached_info [GPOINTER_TO_UINT (a)].len;
+	const guint32 lenb = cached_info [GPOINTER_TO_UINT (b)].len;
+	if (lena == lenb) {
+		const guint8 *infoa = cached_info [GPOINTER_TO_UINT (a)].info;
+		const guint8 *infob = cached_info [GPOINTER_TO_UINT (b)].info;
+		if (memcmp (infoa, infob, lena) == 0)
+			return TRUE;
+	}
+
+	return FALSE;
+}
+
 /*
  * mono_cache_unwind_info
  *
@@ -742,57 +771,64 @@ mono_unwind_init (void)
 guint32
 mono_cache_unwind_info (guint8 *unwind_info, guint32 unwind_info_len)
 {
-	int i;
-	MonoUnwindInfo *info;
-
+	gpointer orig_key;
+	guint32 i;
 	unwind_lock ();
 
-	if (cached_info == NULL) {
-		cached_info_size = 16;
-		cached_info = g_new0 (MonoUnwindInfo*, cached_info_size);
-	}
-
-	for (i = 0; i < cached_info_next; ++i) {
-		MonoUnwindInfo *cached = cached_info [i];
-
-		if (cached->len == unwind_info_len && memcmp (cached->info, unwind_info, unwind_info_len) == 0) {
-			unwind_unlock ();
-			return i;
-		}
-	}
+	if (!cached_info_ht)
+		cached_info_ht = g_hash_table_new (cached_info_hash, cached_info_eq);
 
-	info = (MonoUnwindInfo *)g_malloc (sizeof (MonoUnwindInfo) + unwind_info_len);
-	info->len = unwind_info_len;
-	memcpy (&info->info, unwind_info, unwind_info_len);
-
-	i = cached_info_next;
-
 	if (cached_info_next >= cached_info_size) {
-		MonoUnwindInfo **new_table;
+		MonoUnwindInfo *new_table;
+		int new_cached_info_size = cached_info_size ? cached_info_size * 2 : 16;
+
+		/* ensure no integer overflow */
+		g_assert (new_cached_info_size > cached_info_size);
 
 		/*
 		 * Avoid freeing the old table so mono_get_cached_unwind_info ()
 		 * doesn't need locks/hazard pointers.
 		 */
+		new_table = g_new0 (MonoUnwindInfo, new_cached_info_size );
 
-		new_table = g_new0 (MonoUnwindInfo*, cached_info_size * 2);
+		/* include array allocations into statistics of memory totally consumed by unwind info */
+		unwind_info_size += sizeof (MonoUnwindInfo) * new_cached_info_size ;
 
-		memcpy (new_table, cached_info, cached_info_size * sizeof (MonoUnwindInfo*));
+		if (cached_info_size)
+			memcpy (new_table, cached_info, sizeof (MonoUnwindInfo) * cached_info_size);
 
 		mono_memory_barrier ();
 
 		cached_info_list = g_slist_prepend (cached_info_list, cached_info);
 
 		cached_info = new_table;
 
-		cached_info_size *= 2;
+		cached_info_size = new_cached_info_size;
 	}
 
-	cached_info [cached_info_next ++] = info;
+	i = cached_info_next;
+
+	/* construct temporary element at array's edge without allocated info copy - it will be used for hashtable lookup */
+	cached_info [i].len = unwind_info_len;
+	cached_info [i].info = unwind_info;
 
-	unwind_info_size += sizeof (MonoUnwindInfo) + unwind_info_len;
+	if (!g_hash_table_lookup_extended (cached_info_ht, GUINT_TO_POINTER (i), &orig_key, NULL) ) {
+		/* hashtable lookup didnt find match - now need to really add new element with allocated copy of unwind info */
+		cached_info [i].info = g_new (guint8, unwind_info_len);
+		memcpy (cached_info [i].info, unwind_info, unwind_info_len);
+
+		/* include allocated memory in stats, note that hashtable allocates struct of 3 pointers per each entry */
+		unwind_info_size += sizeof (void *) * 3 + unwind_info_len;
+		g_hash_table_insert_replace (cached_info_ht, GUINT_TO_POINTER (i), NULL, TRUE);
+
+		cached_info_next = i + 1;
+
+	} else {
+		i = GPOINTER_TO_UINT (orig_key);
+	}
 
 	unwind_unlock ();
+
 	return i;
 }
 
@@ -802,17 +838,14 @@ mono_cache_unwind_info (guint8 *unwind_info, guint32 unwind_info_len)
 guint8*
 mono_get_cached_unwind_info (guint32 index, guint32 *unwind_info_len)
 {
-	MonoUnwindInfo **table;
 	MonoUnwindInfo *info;
 	guint8 *data;
 
 	/*
 	 * This doesn't need any locks/hazard pointers,
 	 * since new tables are copies of the old ones.
 	 */
-	table = cached_info;
-
-	info = table [index];
+	info = &cached_info [index];
 
 	*unwind_info_len = info->len;
 	data = info->info;