optimize key librlist functions to improve flux resource list performance

src/common/libidset/idset.c

@@ -410,6 +410,7 @@ bool idset_equal (const struct idset *idset1,
                   const struct idset *idset2)
 {
     unsigned int id;
+    bool count_checked = false;
 
     if (!idset1 || !idset2)
         return false;
@@ -421,6 +422,7 @@ bool idset_equal (const struct idset *idset1,
         && !(idset2->flags & IDSET_FLAG_COUNT_LAZY)) {
         if (idset_count (idset1) != idset_count (idset2))
             return false;
+        count_checked = true;
     }
 
     id = vebsucc (idset1->T, 0);
@@ -429,6 +431,13 @@ bool idset_equal (const struct idset *idset1,
             return false; // id in idset1 not set in idset2
         id = vebsucc (idset1->T, id + 1);
     }
+
+    /* No need to iterate idset2 if counts were equal and all ids in idset1
+     * were found in idset2.
+     */
+    if (count_checked)
+        return true;
+
     id = vebsucc (idset2->T, 0);
     while (id < idset2->T.M) {
         if (vebsucc (idset1->T, id) != id)

src/common/librlist/rnode.c

@@ -29,6 +29,8 @@
 static struct idset *util_idset_add_check (const struct idset *a,
                                            const struct idset *b)
 {
+    if (idset_count (a) == 0)
+        return idset_copy (b);
     if (idset_has_intersection (a, b)) {
         errno = EEXIST;
         return NULL;
@@ -81,6 +83,20 @@ static struct rnode_child *rnode_child_copy (const struct rnode_child *c)
     return rnode_child_idset (c->name, c->ids, c->avail);
 }
 
+static int rnode_child_clear (struct rnode_child *c)
+{
+    /* clearing idsets manually is expensive. It is cheaper to destroy
+     * and recraete an empty idset. Update this code when that is no longer
+     * the case.
+     */
+    idset_destroy (c->avail);
+    idset_destroy (c->ids);
+    if (!(c->avail = idset_create (0, IDSET_FLAG_AUTOGROW))
+        || !(c->ids = idset_create (0, IDSET_FLAG_AUTOGROW)))
+        return -1;
+    return 0;
+}
+
 static void rn_child_free (void **x)
 {
     if (x) {
@@ -463,8 +479,16 @@ struct rnode *rnode_diff (const struct rnode *a, const struct rnode *b)
     while (c) {
         struct rnode_child *nc = zhashx_lookup (n->children, c->name);
         if (nc) {
-            if (idset_subtract (nc->ids, c->ids) < 0
-                || idset_subtract (nc->avail, c->avail) < 0)
+            if (idset_equal (nc->ids, c->ids)) {
+                /* Optimization: if nc->ids == c->ids, then just replace
+                 * nc with empty idsets. This will be much faster than
+                 * idset_subtract().
+                 */
+                if (rnode_child_clear (nc) < 0)
+                    goto err;
+            }
+            else if (idset_subtract (nc->ids, c->ids) < 0
+                     || idset_subtract (nc->avail, c->avail) < 0)
                 goto err;
 
             /*  For non-core resources, remove empty sets: