diff --git a/mie/include/mie/ir/walk.h b/mie/include/mie/ir/walk.h
index 1f83a1a..0b2ee40 100644
--- a/mie/include/mie/ir/walk.h
+++ b/mie/include/mie/ir/walk.h
@@ -7,7 +7,6 @@
 #include <stddef.h>
 
 struct mie_op;
-struct mie_walker;
 
 enum mie_walker_flags {
 	MIE_WALKER_F_NONE = 0x00u,
@@ -38,7 +37,7 @@ struct mie_walk_item {
 	int _f;
 	b_queue_entry _e;
 	enum mie_walk_item_type i_type;
-	size_t i_index;
+	// size_t i_index;
 	size_t i_depth;
 	union {
 		struct mie_op *i_op;
@@ -47,8 +46,21 @@ struct mie_walk_item {
 	};
 };
 
-MIE_API struct mie_walker *mie_walker_begin(
-	struct mie_op *op, enum mie_walker_flags flags);
+struct mie_walker {
+	enum mie_walker_flags w_flags;
+	union {
+		/* used for recursive walks */
+		b_queue w_stack;
+		/* used for non-recursive walks */
+		struct {
+			struct mie_walk_item w_region, w_block, w_op;
+		};
+	};
+};
+
+MIE_API void mie_walker_begin(
+	struct mie_walker *walker, const struct mie_op *op,
+	enum mie_walker_flags flags);
 MIE_API void mie_walker_end(struct mie_walker *walker);
 
 MIE_API enum mie_status mie_walker_step(struct mie_walker *walker);
diff --git a/mie/ir/walk.c b/mie/ir/walk.c
index f2cbe25..6b931ae 100644
--- a/mie/ir/walk.c
+++ b/mie/ir/walk.c
@@ -11,12 +11,6 @@
 
 #define ITEM_TYPE(f)                 ((f) & 0x0Fu)
 
-struct mie_walker {
-	enum mie_walker_flags w_flags;
-	struct mie_op *w_root;
-	b_queue w_stack;
-};
-
 static bool should_ignore_item(
 	const struct mie_walker *walker, const struct mie_walk_item *item)
 {
@@ -56,15 +50,31 @@ static void pop_walk_item(struct mie_walker *walker)
 
 static struct mie_walk_item *current_item(struct mie_walker *walker)
 {
+	struct mie_walk_item *item = NULL;
+	bool recursive = (walker->w_flags & MIE_WALKER_F_RECURSIVE) != 0;
+	if (!recursive) {
+		if (walker->w_flags & MIE_WALKER_F_INCLUDE_REGIONS) {
+			item = &walker->w_region;
+		} else if (walker->w_flags & MIE_WALKER_F_INCLUDE_BLOCKS) {
+			item = &walker->w_block;
+		} else {
+			item = &walker->w_op;
+		}
+
+		bool eof = !item || item->i_type != MIE_WALK_ITEM_NONE;
+		return eof ? item : NULL;
+	}
+
 	b_queue_entry *entry = b_queue_last(&walker->w_stack);
 	if (!entry) {
 		return NULL;
 	}
 
-	struct mie_walk_item *item = b_unbox(struct mie_walk_item, entry, _e);
+	item = b_unbox(struct mie_walk_item, entry, _e);
 	return item->i_type != MIE_WALK_ITEM_NONE ? item : NULL;
 }
 
+#if 0
 static struct mie_op *get_first_child(struct mie_walker *walker, struct mie_op *op)
 {
 	bool backwards = (walker->w_flags & MIE_WALKER_F_BACKWARD) != 0;
@@ -95,16 +105,17 @@ static struct mie_op *get_first_child(struct mie_walker *walker, struct mie_op *
 
 	return op;
 }
+#endif
 
-static size_t walk_item_get_nr_children(const struct mie_walk_item *item)
+static bool walk_item_has_children(const struct mie_walk_item *item)
 {
 	switch (item->i_type) {
 	case MIE_WALK_ITEM_OP:
-		return MIE_VECTOR_COUNT(item->i_op->op_regions);
+		return mie_op_get_first_region(item->i_op) != NULL;
 	case MIE_WALK_ITEM_BLOCK:
-		return MIE_VECTOR_COUNT(item->i_block->b_ops);
+		return mie_block_get_first_op(item->i_block) != NULL;
 	case MIE_WALK_ITEM_REGION:
-		return MIE_VECTOR_COUNT(item->i_region->r_blocks);
+		return mie_region_get_first_block(item->i_region) != NULL;
 	default:
 		return 0;
 	}
@@ -128,42 +139,68 @@ static bool should_correct_depth(
 	return false;
 }
 
-static bool walk_item_get_child(
+static bool walk_item_get_first_child(
 	struct mie_walker *walker, const struct mie_walk_item *item,
-	size_t index, struct mie_walk_item *child)
+	struct mie_walk_item *child)
 {
 	bool ok = false;
+	b_queue_entry *entry = NULL;
 
 	switch (item->i_type) {
 	case MIE_WALK_ITEM_OP:
-		if (MIE_VECTOR_COUNT(item->i_op->op_regions) <= index) {
-			ok = false;
-			break;
-		}
-
 		child->i_type = MIE_WALK_ITEM_REGION;
-		child->i_region = &item->i_op->op_regions.items[index];
-		ok = true;
+		child->i_region = mie_op_get_first_region(item->i_op);
+		ok = child->i_region != NULL;
 		break;
 	case MIE_WALK_ITEM_BLOCK:
-		if (MIE_VECTOR_COUNT(item->i_block->b_ops) <= index) {
-			ok = false;
-			break;
-		}
-
 		child->i_type = MIE_WALK_ITEM_OP;
-		child->i_op = &item->i_block->b_ops.items[index];
-		ok = true;
+		child->i_op = mie_block_get_first_op(item->i_block);
+		ok = child->i_op != NULL;
 		break;
 	case MIE_WALK_ITEM_REGION:
-		if (MIE_VECTOR_COUNT(item->i_region->r_blocks) <= index) {
-			ok = false;
-			break;
-		}
-
 		child->i_type = MIE_WALK_ITEM_BLOCK;
-		child->i_block = &item->i_region->r_blocks.items[index];
-		ok = true;
+		child->i_block = mie_region_get_first_block(item->i_region);
+		ok = child->i_block != NULL;
+		break;
+	default:
+		break;
+	}
+
+	if (!ok) {
+		return false;
+	}
+
+	child->i_depth = item->i_depth + 1;
+
+	if (should_correct_depth(walker, item)) {
+		child->i_depth--;
+	}
+
+	return true;
+}
+
+static bool walk_item_get_last_child(
+	struct mie_walker *walker, const struct mie_walk_item *item,
+	struct mie_walk_item *child)
+{
+	bool ok = false;
+	b_queue_entry *entry = NULL;
+
+	switch (item->i_type) {
+	case MIE_WALK_ITEM_OP:
+		child->i_type = MIE_WALK_ITEM_REGION;
+		child->i_region = mie_op_get_last_region(item->i_op);
+		ok = child->i_region != NULL;
+		break;
+	case MIE_WALK_ITEM_BLOCK:
+		child->i_type = MIE_WALK_ITEM_OP;
+		child->i_op = mie_block_get_last_op(item->i_block);
+		ok = child->i_op != NULL;
+		break;
+	case MIE_WALK_ITEM_REGION:
+		child->i_type = MIE_WALK_ITEM_BLOCK;
+		child->i_block = mie_region_get_last_block(item->i_region);
+		ok = child->i_block != NULL;
 		break;
 	default:
 		break;
@@ -174,7 +211,6 @@ static bool walk_item_get_child(
 	}
 
 	child->i_depth = item->i_depth + 1;
-	child->i_index = index;
 
 	if (should_correct_depth(walker, item)) {
 		child->i_depth--;
@@ -192,8 +228,9 @@ static bool walk_item_get_next_sibling(
 
 	sibling->i_depth = item->i_depth;
 	sibling->i_type = item->i_type;
-	sibling->i_index = item->i_index + 1;
 
+	size_t index = 0;
+	b_queue_entry *entry = NULL;
 	struct mie_region *parent_r = NULL;
 	struct mie_block *parent_b = NULL;
 	struct mie_op *parent_o = NULL;
@@ -205,36 +242,26 @@ static bool walk_item_get_next_sibling(
 			return false;
 		}
 
-		if (item->i_index + 1 >= MIE_VECTOR_COUNT(parent_b->b_ops)) {
-			return false;
-		}
-
-		sibling->i_op = &parent_b->b_ops.items[item->i_index + 1];
-		return true;
+		sibling->i_op = mie_block_get_next_op(parent_b, item->i_op);
+		return sibling->i_op != NULL;
 	case MIE_WALK_ITEM_BLOCK:
 		parent_r = item->i_block->b_parent;
 		if (!parent_r) {
 			return false;
 		}
 
-		if (item->i_index + 1 >= MIE_VECTOR_COUNT(parent_r->r_blocks)) {
-			return false;
-		}
-
-		sibling->i_block = &parent_r->r_blocks.items[item->i_index + 1];
-		return true;
+		sibling->i_block
+			= mie_region_get_next_block(parent_r, item->i_block);
+		return sibling->i_block != NULL;
 	case MIE_WALK_ITEM_REGION:
 		parent_o = item->i_region->r_parent;
 		if (!parent_o) {
 			return false;
 		}
 
-		if (item->i_index + 1 >= MIE_VECTOR_COUNT(parent_o->op_regions)) {
-			return false;
-		}
-
-		sibling->i_region = &parent_o->op_regions.items[item->i_index + 1];
-		return true;
+		sibling->i_region
+			= mie_op_get_next_region(parent_o, item->i_region);
+		return sibling->i_region != NULL;
 	default:
 		return false;
 	}
@@ -243,14 +270,14 @@ static bool walk_item_get_next_sibling(
 static bool walk_item_get_prev_sibling(
 	const struct mie_walk_item *item, struct mie_walk_item *sibling)
 {
-	if (item->i_depth == 0 || item->i_index == 0) {
+	if (item->i_depth == 0) {
 		return false;
 	}
 
 	sibling->i_depth = item->i_depth;
 	sibling->i_type = item->i_type;
-	sibling->i_index = item->i_index - 1;
 
+	size_t index = 0;
 	struct mie_region *parent_r = NULL;
 	struct mie_block *parent_b = NULL;
 	struct mie_op *parent_o = NULL;
@@ -262,24 +289,26 @@ static bool walk_item_get_prev_sibling(
 			return false;
 		}
 
-		sibling->i_op = &parent_b->b_ops.items[item->i_index - 1];
-		return true;
+		sibling->i_op = mie_block_get_prev_op(parent_b, item->i_op);
+		return sibling->i_op != NULL;
 	case MIE_WALK_ITEM_BLOCK:
 		parent_r = item->i_block->b_parent;
 		if (!parent_r) {
 			return false;
 		}
 
-		sibling->i_block = &parent_r->r_blocks.items[item->i_index - 1];
-		return true;
+		sibling->i_block
+			= mie_region_get_prev_block(parent_r, item->i_block);
+		return sibling->i_block != NULL;
 	case MIE_WALK_ITEM_REGION:
 		parent_o = item->i_region->r_parent;
 		if (!parent_o) {
 			return false;
 		}
 
-		sibling->i_region = &parent_o->op_regions.items[item->i_index - 1];
-		return true;
+		sibling->i_region
+			= mie_op_get_prev_region(parent_o, item->i_region);
+		return sibling->i_region != NULL;
 	default:
 		return false;
 	}
@@ -334,9 +363,9 @@ static bool step_pre_f(struct mie_walker *walker)
 	struct mie_walk_item next;
 
 	bool ok = false;
-	size_t nr_children = walk_item_get_nr_children(item);
-	if (nr_children > 0) {
-		ok = walk_item_get_child(walker, item, 0, &next);
+	bool has_children = walk_item_has_children(item);
+	if (has_children) {
+		ok = walk_item_get_first_child(walker, item, &next);
 
 		if (ok) {
 			push_walk_item(walker, &next);
@@ -384,9 +413,9 @@ static bool step_pre_b(struct mie_walker *walker)
 	struct mie_walk_item next;
 
 	bool ok = false;
-	size_t nr_children = walk_item_get_nr_children(item);
-	if (nr_children > 0) {
-		ok = walk_item_get_child(walker, item, nr_children - 1, &next);
+	bool has_children = walk_item_has_children(item);
+	if (has_children) {
+		ok = walk_item_get_last_child(walker, item, &next);
 
 		if (ok) {
 			push_walk_item(walker, &next);
@@ -437,10 +466,10 @@ static bool step_post_f(struct mie_walker *walker)
 
 	while (1) {
 		item = current_item(walker);
-		size_t nr_children = walk_item_get_nr_children(item);
+		bool has_children = walk_item_has_children(item);
 
-		if (nr_children > 0 && !(item->_f & WALK_ITEM_F_CHILDREN_VISITED)) {
-			ok = walk_item_get_child(walker, item, 0, &next);
+		if (has_children && !(item->_f & WALK_ITEM_F_CHILDREN_VISITED)) {
+			ok = walk_item_get_first_child(walker, item, &next);
 
 			if (ok) {
 				push_walk_item(walker, &next);
@@ -449,7 +478,7 @@ static bool step_post_f(struct mie_walker *walker)
 			return ok;
 		}
 
-		if (nr_children == 0 && !(item->_f & WALK_ITEM_F_VISITED)) {
+		if (!has_children && !(item->_f & WALK_ITEM_F_VISITED)) {
 			return true;
 		}
 
@@ -469,6 +498,55 @@ static bool step_post_f(struct mie_walker *walker)
 	}
 }
 
+static bool step_flat_f(struct mie_walker *walker)
+{
+	struct mie_walk_item *stack[] = {
+		&walker->w_region,
+		&walker->w_block,
+		&walker->w_op,
+	};
+	const int stack_size = sizeof stack / sizeof stack[0];
+
+	int start;
+	if (walker->w_flags & MIE_WALKER_F_INCLUDE_REGIONS) {
+		start = 0;
+	} else if (walker->w_flags & MIE_WALKER_F_INCLUDE_BLOCKS) {
+		start = 1;
+	} else {
+		start = 2;
+	}
+
+	int i;
+	for (i = start; i >= 0; i--) {
+		struct mie_walk_item *item = stack[i];
+		struct mie_walk_item next;
+		if (walk_item_get_next_sibling(item, &next)) {
+			*item = next;
+			break;
+		}
+	}
+
+	if (i < 0) {
+		return false;
+	}
+
+	for (int k = i; k < stack_size - 1; k++) {
+		struct mie_walk_item *parent = stack[k];
+		struct mie_walk_item *child = stack[k + 1];
+
+		if (!walk_item_get_first_child(walker, parent, child)) {
+			return false;
+		}
+	}
+
+	return true;
+}
+
+static bool step_flat_b(struct mie_walker *walker)
+{
+	return false;
+}
+
 static bool step_post_b(struct mie_walker *walker)
 {
 	/* backward, post-order traversal:
@@ -485,11 +563,10 @@ static bool step_post_b(struct mie_walker *walker)
 
 	while (1) {
 		item = current_item(walker);
-		size_t nr_children = walk_item_get_nr_children(item);
+		bool has_children = walk_item_has_children(item);
 
-		if (nr_children > 0 && !(item->_f & WALK_ITEM_F_CHILDREN_VISITED)) {
-			ok = walk_item_get_child(
-				walker, item, nr_children - 1, &next);
+		if (has_children && !(item->_f & WALK_ITEM_F_CHILDREN_VISITED)) {
+			ok = walk_item_get_last_child(walker, item, &next);
 
 			if (ok) {
 				push_walk_item(walker, &next);
@@ -498,7 +575,7 @@ static bool step_post_b(struct mie_walker *walker)
 			return ok;
 		}
 
-		if (nr_children == 0 && !(item->_f & WALK_ITEM_F_VISITED)) {
+		if (!has_children && !(item->_f & WALK_ITEM_F_VISITED)) {
 			return true;
 		}
 
@@ -520,25 +597,106 @@ static bool step_post_b(struct mie_walker *walker)
 
 static bool step(struct mie_walker *walker)
 {
+	bool recursive = (walker->w_flags & MIE_WALKER_F_RECURSIVE) != 0;
 	bool backward = (walker->w_flags & MIE_WALKER_F_BACKWARD) != 0;
 	bool postorder = (walker->w_flags & MIE_WALKER_F_POSTORDER) != 0;
 
-	if (backward) {
-		if (postorder) {
-			return step_post_b(walker);
-		}
-
-		return step_pre_b(walker);
+	if (!recursive) {
+		return backward ? step_flat_b(walker) : step_flat_f(walker);
+	} else if (postorder) {
+		return backward ? step_post_b(walker) : step_post_f(walker);
+	} else {
+		return backward ? step_pre_b(walker) : step_pre_f(walker);
 	}
-
-	if (postorder) {
-		return step_post_f(walker);
-	}
-
-	return step_pre_f(walker);
 }
 
-static enum mie_status prepare_preorder_traversal(struct mie_walker *walker)
+static enum mie_status prepare_flat_traversal(
+	struct mie_walker *walker, const struct mie_op *op)
+{
+	bool backward = (walker->w_flags & MIE_WALKER_F_BACKWARD) != 0;
+	bool ok = false;
+	enum mie_walk_item_type target_type = MIE_WALK_ITEM_NONE;
+	enum mie_status status = MIE_SUCCESS;
+
+	struct mie_walk_item root = {};
+	root.i_op = (struct mie_op *)op;
+	root.i_type = MIE_WALK_ITEM_OP;
+
+	if (walker->w_flags & MIE_WALKER_F_INCLUDE_REGIONS) {
+		target_type = MIE_WALK_ITEM_REGION;
+	} else if (walker->w_flags & MIE_WALKER_F_INCLUDE_BLOCKS) {
+		target_type = MIE_WALK_ITEM_BLOCK;
+	} else {
+		target_type = MIE_WALK_ITEM_OP;
+	}
+
+	bool has_children = walk_item_has_children(&root);
+	if (!has_children) {
+		return MIE_ERR_NO_DATA;
+	}
+
+	if (backward) {
+		ok = walk_item_get_last_child(walker, &root, &walker->w_region);
+		status = ok ? MIE_SUCCESS : MIE_ERR_NO_DATA;
+	} else {
+		ok = walk_item_get_first_child(walker, &root, &walker->w_region);
+		status = ok ? MIE_SUCCESS : MIE_ERR_NO_DATA;
+	}
+
+	if (status != MIE_SUCCESS) {
+		return status;
+	}
+
+	if (target_type == MIE_WALK_ITEM_REGION) {
+		walker->w_region.i_depth = 1;
+		return MIE_SUCCESS;
+	}
+
+	has_children = walk_item_has_children(&walker->w_region);
+	if (!has_children) {
+		return MIE_ERR_NO_DATA;
+	}
+
+	if (backward) {
+		ok = walk_item_get_last_child(
+			walker, &walker->w_region, &walker->w_block);
+		status = ok ? MIE_SUCCESS : MIE_ERR_NO_DATA;
+	} else {
+		ok = walk_item_get_first_child(
+			walker, &walker->w_region, &walker->w_block);
+		status = ok ? MIE_SUCCESS : MIE_ERR_NO_DATA;
+	}
+
+	if (status != MIE_SUCCESS) {
+		return status;
+	}
+
+	if (target_type == MIE_WALK_ITEM_BLOCK) {
+		walker->w_block.i_depth = 1;
+		return MIE_SUCCESS;
+	}
+
+	has_children = walk_item_has_children(&walker->w_block);
+	if (!has_children) {
+		return MIE_ERR_NO_DATA;
+	}
+
+	if (backward) {
+		ok = walk_item_get_last_child(
+			walker, &walker->w_block, &walker->w_op);
+		status = ok ? MIE_SUCCESS : MIE_ERR_NO_DATA;
+	} else {
+		ok = walk_item_get_first_child(
+			walker, &walker->w_block, &walker->w_op);
+		status = ok ? MIE_SUCCESS : MIE_ERR_NO_DATA;
+	}
+
+	walker->w_op.i_depth = 1;
+	return status;
+}
+
+static enum mie_status prepare_preorder_traversal(
+	struct mie_walker *walker, const struct mie_op *root)
 {
 	struct mie_walk_item *item = malloc(sizeof *item);
 	if (!item) {
@@ -546,7 +704,7 @@ static enum mie_status prepare_preorder_traversal(struct mie_walker *walker)
 	}
 
 	memset(item, 0x0, sizeof *item);
-	item->i_op = walker->w_root;
+	item->i_op = (struct mie_op *)root;
 	item->i_type = MIE_WALK_ITEM_OP;
 
 	push_walk_item(walker, item);
@@ -554,10 +712,11 @@ static enum mie_status prepare_preorder_traversal(struct mie_walker *walker)
 	return MIE_SUCCESS;
 }
 
-static enum mie_status prepare_postorder_traversal(struct mie_walker *walker)
+static enum mie_status prepare_postorder_traversal(
+	struct mie_walker *walker, const struct mie_op *root)
 {
 	struct mie_walk_item item = {};
-	item.i_op = walker->w_root;
+	item.i_op = (struct mie_op *)root;
 	item.i_type = MIE_WALK_ITEM_OP;
 
 	push_walk_item(walker, &item);
@@ -568,18 +727,17 @@ static enum mie_status prepare_postorder_traversal(struct mie_walker *walker)
 	while (1) {
 		struct mie_walk_item *current = current_item(walker);
 		struct mie_walk_item child;
-		size_t nr_children = walk_item_get_nr_children(current);
-		if (!nr_children) {
+		bool has_children = walk_item_has_children(current);
+		if (!has_children) {
 			break;
 		}
 
 		bool ok = false;
 
 		if (backward) {
-			ok = walk_item_get_child(
-				walker, current, nr_children - 1, &child);
+			ok = walk_item_get_last_child(walker, current, &child);
 		} else {
-			ok = walk_item_get_child(walker, current, 0, &child);
+			ok = walk_item_get_first_child(walker, current, &child);
 		}
 
 		if (!ok) {
@@ -592,30 +750,26 @@ static enum mie_status prepare_postorder_traversal(struct mie_walker *walker)
 	return ok ? MIE_SUCCESS : MIE_ERR_NO_DATA;
 }
 
-struct mie_walker *mie_walker_begin(struct mie_op *op, enum mie_walker_flags flags)
+void mie_walker_begin(
+	struct mie_walker *walker, const struct mie_op *op,
+	enum mie_walker_flags flags)
 {
-	struct mie_walker *out = malloc(sizeof *out);
-	if (!out) {
-		return NULL;
-	}
+	memset(walker, 0x0, sizeof *walker);
 
-	memset(out, 0x0, sizeof *out);
+	walker->w_flags = flags;
 
-	out->w_flags = flags;
-	out->w_root = op;
-
-	if (flags & MIE_WALKER_F_POSTORDER) {
-		prepare_postorder_traversal(out);
+	if (!(flags & MIE_WALKER_F_RECURSIVE)) {
+		prepare_flat_traversal(walker, op);
+	} else if (flags & MIE_WALKER_F_POSTORDER) {
+		prepare_postorder_traversal(walker, op);
 	} else {
-		prepare_preorder_traversal(out);
+		prepare_preorder_traversal(walker, op);
 	}
 
-	struct mie_walk_item *cur = current_item(out);
-	if (should_ignore_item(out, cur)) {
-		mie_walker_step(out);
+	struct mie_walk_item *cur = current_item(walker);
+	if (cur && should_ignore_item(walker, cur)) {
+		mie_walker_step(walker);
 	}
-
-	return out;
 }
 
 void mie_walker_end(struct mie_walker *walker)
@@ -631,9 +785,9 @@ enum mie_status mie_walker_step(struct mie_walker *walker)
 	do {
 		ok = step(walker);
 		cur = current_item(walker);
-	} while (cur && should_ignore_item(walker, cur));
+	} while (ok && cur && should_ignore_item(walker, cur));
 
-	return cur ? MIE_SUCCESS : MIE_ERR_NO_DATA;
+	return (ok && cur) ? MIE_SUCCESS : MIE_ERR_NO_DATA;
 }
 
 struct mie_walk_item *mie_walker_get(struct mie_walker *walker)