[PATCH v1 4/5] rib: track valid descendant count per node

[Maxime Leroy <maxime@leroys.fr>]

Add a uint32_t field to struct rte_rib6_node that counts valid prefixes
in the node's subtree (excluding self). Maintained incrementally on
insert (when a node becomes valid) and remove (when a node becomes
invalid), in O(tree depth) per operation.

The field fits in the padding before ext[]; the node size and ext[]
offset are unchanged. uint32_t is required because a single subtree
may hold more than 65535 BGP routes.

Expose an __rte_internal helper rte_rib6_count_empty_supernets()
via a new private header lib/rib/rib6_internal.h that descends the
tree once and reports how many byte boundaries below a given prefix
have no valid descendant. lib/fib uses this to maintain tbl8
reservation accounting in a single tree walk instead of one
rte_rib6_get_nxt() call per byte boundary.

Signed-off-by: Maxime Leroy <maxime@leroys.fr>
---
 app/test/test_rib6.c    | 92 +++++++++++++++++++++++++++++++++++++++++
 lib/rib/rib6_internal.h | 37 +++++++++++++++++
 lib/rib/rte_rib6.c      | 80 +++++++++++++++++++++++++++++++++++
 3 files changed, 209 insertions(+)
 create mode 100644 lib/rib/rib6_internal.h

diff --git a/app/test/test_rib6.c b/app/test/test_rib6.c
index 0295a9640c..a1a6ab17f4 100644
--- a/app/test/test_rib6.c
+++ b/app/test/test_rib6.c
@@ -8,6 +8,7 @@
 #include <stdlib.h>
 #include <rte_ip6.h>
 #include <rte_rib6.h>
+#include <rib6_internal.h>
 
 #include "test.h"
 
@@ -20,6 +21,7 @@ static int32_t test_insert_invalid(void);
 static int32_t test_get_fn(void);
 static int32_t test_basic(void);
 static int32_t test_tree_traversal(void);
+static int32_t test_empty_supernets(void);
 
 #define MAX_DEPTH 128
 #define MAX_RULES (1 << 22)
@@ -322,6 +324,95 @@ test_tree_traversal(void)
 	return TEST_SUCCESS;
 }
 
+/*
+ * Exercise rte_rib6_count_empty_supernets() which depends on the
+ * valid_descendants counter maintained on insert/remove.
+ */
+int32_t
+test_empty_supernets(void)
+{
+	struct rte_rib6 *rib = NULL;
+	struct rte_rib6_conf config;
+	struct rte_ipv6_addr ip = RTE_IPV6(0xfcde, 0, 0, 0, 0, 0, 0, 0);
+	struct rte_ipv6_addr sibling = RTE_IPV6(0xfcde, 0, 0, 0, 0, 0, 0, 1);
+	uint8_t cnt;
+
+	config.max_nodes = 64;
+	config.ext_sz = 0;
+
+	rib = rte_rib6_create(__func__, SOCKET_ID_ANY, &config);
+	RTE_TEST_ASSERT(rib != NULL, "Failed to create RIB\n");
+
+	/* depth <= 24: no byte boundaries above to inspect. */
+	cnt = rte_rib6_count_empty_supernets(rib, &ip, 24);
+	RTE_TEST_ASSERT(cnt == 0, "depth 24 must return 0, got %u\n", cnt);
+
+	/* Empty RIB, /128 query: 13 byte boundaries (24..120) all empty. */
+	cnt = rte_rib6_count_empty_supernets(rib, &ip, 128);
+	RTE_TEST_ASSERT(cnt == 13, "empty RIB /128 must return 13, got %u\n", cnt);
+
+	/* Insert a /32 ancestor: level 24 now has a descendant, levels
+	 * 32..120 still empty -> 12.
+	 */
+	RTE_TEST_ASSERT(rte_rib6_insert(rib, &ip, 32) != NULL,
+		"Failed to insert /32\n");
+	cnt = rte_rib6_count_empty_supernets(rib, &ip, 128);
+	RTE_TEST_ASSERT(cnt == 12, "after /32 ADD: expected 12, got %u\n", cnt);
+
+	/* Insert a /48 below: levels 24, 32 and 40 see /48 as descendant,
+	 * 48..120 empty -> 10.
+	 */
+	RTE_TEST_ASSERT(rte_rib6_insert(rib, &ip, 48) != NULL,
+		"Failed to insert /48\n");
+	cnt = rte_rib6_count_empty_supernets(rib, &ip, 128);
+	RTE_TEST_ASSERT(cnt == 10, "after /48 ADD: expected 10, got %u\n", cnt);
+
+	/* Insert a sibling /128 that shares a long common prefix with ip
+	 * but differs in the last bits. This forces creation of a
+	 * common_node and exercises the inherited valid_descendants of
+	 * that synthesized intermediate.
+	 */
+	RTE_TEST_ASSERT(rte_rib6_insert(rib, &sibling, 128) != NULL,
+		"Failed to insert sibling /128\n");
+	/* sibling shares prefix with ip down to bit 127; for the query on
+	 * ip/128, all byte boundaries 24..120 have descendants (the /32,
+	 * /48 and the sibling /128 chain) -> 0 empty levels.
+	 */
+	cnt = rte_rib6_count_empty_supernets(rib, &ip, 128);
+	RTE_TEST_ASSERT(cnt == 0, "fully populated chain: expected 0, got %u\n",
+		cnt);
+
+	/* Remove the /32 ancestor: /48 and /128 sibling still cover all
+	 * byte boundaries 24..120 -> still 0 empty levels.
+	 */
+	rte_rib6_remove(rib, &ip, 32);
+	cnt = rte_rib6_count_empty_supernets(rib, &ip, 128);
+	RTE_TEST_ASSERT(cnt == 0,
+		"after /32 DEL still covered: expected 0, got %u\n", cnt);
+
+	/* Remove the /48: only the /128 sibling remains. For an ip/128
+	 * query, levels 24..120 each see the sibling as descendant, so
+	 * count is still 0.
+	 */
+	rte_rib6_remove(rib, &ip, 48);
+	cnt = rte_rib6_count_empty_supernets(rib, &ip, 128);
+	RTE_TEST_ASSERT(cnt == 0,
+		"after /48 DEL still covered: expected 0, got %u\n", cnt);
+
+	/* Remove the sibling /128: RIB now empty again. */
+	rte_rib6_remove(rib, &sibling, 128);
+	cnt = rte_rib6_count_empty_supernets(rib, &ip, 128);
+	RTE_TEST_ASSERT(cnt == 13,
+		"after final DEL: expected 13, got %u\n", cnt);
+
+	/* Invalid input: NULL rib. */
+	cnt = rte_rib6_count_empty_supernets(NULL, &ip, 128);
+	RTE_TEST_ASSERT(cnt == 0, "NULL rib must return 0, got %u\n", cnt);
+
+	rte_rib6_free(rib);
+	return TEST_SUCCESS;
+}
+
 static struct unit_test_suite rib6_tests = {
 	.suite_name = "rib6 autotest",
 	.setup = NULL,
@@ -333,6 +424,7 @@ static struct unit_test_suite rib6_tests = {
 		TEST_CASE(test_get_fn),
 		TEST_CASE(test_basic),
 		TEST_CASE(test_tree_traversal),
+		TEST_CASE(test_empty_supernets),
 		TEST_CASES_END()
 	}
 };
diff --git a/lib/rib/rib6_internal.h b/lib/rib/rib6_internal.h
new file mode 100644
index 0000000000..8246626de9
--- /dev/null
+++ b/lib/rib/rib6_internal.h
@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: BSD-3-Clause
+ * Copyright(c) 2026 Maxime Leroy, Free Mobile
+ */
+
+#ifndef _RIB6_INTERNAL_H_
+#define _RIB6_INTERNAL_H_
+
+#include <stdint.h>
+
+#include <rte_compat.h>
+#include <rte_ip6.h>
+
+struct rte_rib6;
+
+/**
+ * @internal
+ * Count byte boundaries L in {24, 32, 40, ..., RTE_ALIGN_CEIL(depth, 8) - 8}
+ * for which the supernet of ip at level L has no valid descendant with
+ * depth > L. Used by lib/fib to maintain tbl8 reservation accounting in
+ * a single descent of the binary tree.
+ *
+ * @param rib
+ *  RIB object handle
+ * @param ip
+ *  IPv6 prefix address
+ * @param depth
+ *  prefix length
+ * @return
+ *  number of empty byte boundaries (0 if all levels have descendants
+ *  or depth <= 24)
+ */
+__rte_internal
+uint8_t
+rte_rib6_count_empty_supernets(struct rte_rib6 *rib,
+	const struct rte_ipv6_addr *ip, uint8_t depth);
+
+#endif /* _RIB6_INTERNAL_H_ */
diff --git a/lib/rib/rte_rib6.c b/lib/rib/rte_rib6.c
index ec8ff68e87..ae8aba6563 100644
--- a/lib/rib/rte_rib6.c
+++ b/lib/rib/rte_rib6.c
@@ -18,6 +18,7 @@
 
 #include <rte_rib6.h>
 
+#include "rib6_internal.h"
 #include "rib_log.h"
 
 #define RTE_RIB_VALID_NODE	1
@@ -36,6 +37,7 @@ struct rte_rib6_node {
 	struct rte_rib6_node	*parent;
 	uint64_t		nh;
 	struct rte_ipv6_addr	ip;
+	uint32_t		valid_descendants;
 	uint8_t			depth;
 	uint8_t			flag;
 	uint64_t ext[];
@@ -104,10 +106,35 @@ node_alloc(struct rte_rib6 *rib)
 	ret = rte_mempool_get(rib->node_pool, (void *)&ent);
 	if (unlikely(ret != 0))
 		return NULL;
+	ent->valid_descendants = 0;
 	++rib->cur_nodes;
 	return ent;
 }
 
+/* Increment valid_descendants along the parent chain when node becomes valid. */
+static inline void
+inc_valid_descendants(struct rte_rib6_node *node)
+{
+	struct rte_rib6_node *p = node->parent;
+
+	while (p != NULL) {
+		p->valid_descendants++;
+		p = p->parent;
+	}
+}
+
+/* Decrement valid_descendants along the parent chain when node becomes invalid. */
+static inline void
+dec_valid_descendants(struct rte_rib6_node *node)
+{
+	struct rte_rib6_node *p = node->parent;
+
+	while (p != NULL) {
+		p->valid_descendants--;
+		p = p->parent;
+	}
+}
+
 static void
 node_free(struct rte_rib6 *rib, struct rte_rib6_node *ent)
 {
@@ -250,6 +277,7 @@ rte_rib6_remove(struct rte_rib6 *rib,
 
 	--rib->cur_routes;
 	cur->flag &= ~RTE_RIB_VALID_NODE;
+	dec_valid_descendants(cur);
 	while (!is_valid_node(cur)) {
 		if ((cur->left != NULL) && (cur->right != NULL))
 			return;
@@ -320,6 +348,7 @@ rte_rib6_insert(struct rte_rib6 *rib,
 			*tmp = new_node;
 			new_node->parent = prev;
 			++rib->cur_routes;
+			inc_valid_descendants(new_node);
 			return *tmp;
 		}
 		/*
@@ -332,6 +361,7 @@ rte_rib6_insert(struct rte_rib6 *rib,
 			node_free(rib, new_node);
 			(*tmp)->flag |= RTE_RIB_VALID_NODE;
 			++rib->cur_routes;
+			inc_valid_descendants(*tmp);
 			return *tmp;
 		}
 
@@ -371,6 +401,9 @@ rte_rib6_insert(struct rte_rib6 *rib,
 			new_node->left = *tmp;
 		new_node->parent = (*tmp)->parent;
 		(*tmp)->parent = new_node;
+		/* new_node inherits *tmp's subtree */
+		new_node->valid_descendants = (is_valid_node(*tmp) ? 1 : 0) +
+			(*tmp)->valid_descendants;
 		*tmp = new_node;
 	} else {
 		/* create intermediate node */
@@ -386,6 +419,11 @@ rte_rib6_insert(struct rte_rib6 *rib,
 		common_node->parent = (*tmp)->parent;
 		new_node->parent = common_node;
 		(*tmp)->parent = common_node;
+		/* common_node inherits *tmp's subtree (new_node will be
+		 * counted by inc_valid_descendants below).
+		 */
+		common_node->valid_descendants = (is_valid_node(*tmp) ? 1 : 0) +
+			(*tmp)->valid_descendants;
 		if (get_dir(&(*tmp)->ip, common_depth) == 1) {
 			common_node->left = new_node;
 			common_node->right = *tmp;
@@ -396,6 +434,7 @@ rte_rib6_insert(struct rte_rib6 *rib,
 		*tmp = common_node;
 	}
 	++rib->cur_routes;
+	inc_valid_descendants(new_node);
 	return new_node;
 }
 
@@ -606,3 +645,44 @@ rte_rib6_free(struct rte_rib6 *rib)
 	rte_free(rib);
 	rte_free(te);
 }
+
+RTE_EXPORT_INTERNAL_SYMBOL(rte_rib6_count_empty_supernets)
+uint8_t
+rte_rib6_count_empty_supernets(struct rte_rib6 *rib,
+	const struct rte_ipv6_addr *ip, uint8_t depth)
+{
+	uint8_t top = RTE_ALIGN_CEIL(depth, 8);
+	struct rte_rib6_node *cur;
+	bool has_descendant;
+	uint8_t level;
+
+	if (unlikely(rib == NULL || ip == NULL || depth > RTE_IPV6_MAX_DEPTH))
+		return 0;
+	if (depth <= 24)
+		return 0;
+
+	cur = rib->tree;
+
+	/* Single descent through the binary tree, checking each byte
+	 * boundary on the way. NULL at level L propagates upward, so we
+	 * stop at the first empty supernet and tally the remaining levels.
+	 */
+	for (level = 24; level < top; level += 8) {
+		while (cur != NULL && cur->depth < level)
+			cur = get_nxt_node(cur, ip);
+
+		if (cur == NULL || !rte_ipv6_addr_eq_prefix(&cur->ip, ip, level))
+			return (top - level) >> 3;
+
+		if (cur->depth > level)
+			has_descendant = is_valid_node(cur) ||
+				cur->valid_descendants > 0;
+		else
+			has_descendant = cur->valid_descendants > 0;
+
+		if (!has_descendant)
+			return (top - level) >> 3;
+	}
+	return 0;
+}
+
-- 
2.43.0