revset: do not scan ancestors more than once to evaluate nested children set

lib/src/default_revset_engine.rs

@@ -618,18 +618,28 @@ impl<'index> EvaluationContext<'index> {
                 heads,
                 generation_from_roots,
             } => {
-                let mut root_set = self.evaluate(roots)?;
+                let root_set = self.evaluate(roots)?;
                 let head_set = self.evaluate(heads)?;
-                // TODO: implement generic evaluation path for generation filter
+                if generation_from_roots == &(1..2) {
-                for _ in 0..generation_from_roots.start {
+                    Ok(Box::new(self.walk_children(&*root_set, &*head_set)))
-                    root_set = Box::new(self.walk_children(&*root_set, &*head_set));
+                } else if generation_from_roots == &GENERATION_RANGE_FULL {
-                }
-                if generation_from_roots.end == generation_from_roots.start + 1 {
-                    Ok(root_set)
-                } else {
-                    assert_eq!(generation_from_roots.end, u64::MAX); // TODO
                     let (dag_range_set, _) = self.collect_dag_range(&*root_set, &*head_set);
                     Ok(Box::new(dag_range_set))
+                } else {
+                    // For small generation range, it might be better to build a reachable map
+                    // with generation bit set, which can be calculated incrementally from roots:
+                    //   reachable[pos] = (reachable[parent_pos] | ...) << 1
+                    let root_positions =
+                        root_set.iter().map(|entry| entry.position()).collect_vec();
+                    let walk = self
+                        .walk_ancestors(&*head_set)
+                        .descendants_filtered_by_generation(
+                            &root_positions,
+                            to_u32_generation_range(generation_from_roots)?,
+                        );
+                    let mut index_entries = walk.collect_vec();
+                    index_entries.reverse();
+                    Ok(Box::new(EagerRevset { index_entries }))
                 }
             }
             ResolvedExpression::Heads(candidates) => {

testing/bench-revsets-git.txt

@@ -13,7 +13,6 @@ v2.40.0
 v2.39.0..v2.40.0
 :v2.40.0 ~ :v2.39.0
 v2.39.0:v2.40.0
-v2.40.0+
 # Tags and branches
 tags()
 branches()
@@ -42,16 +41,26 @@ heads(:v2.40.0)
 v1.0.0-
 v1.0.0---
 :v1.0.0---
+# Children and descendants of old commit
+v1.0.0+
+v1.0.0+++
+v1.0.0+++:
 # Parents and ancestors of recent commit
 v2.40.0-
 v2.40.0---
 :v2.40.0---
+# Children and descendants of recent commit
+v2.40.0+
+v2.40.0+++
+v2.40.0+++:
 # Parents and ancestors of small subset
 tags()-
 tags()---
 :tags()---
-# Children of small subset
+# Children and descendants of small subset
 tags()+
+tags()+++
+tags()+++:
 # Filter that doesn't read commit object
 merges()
 ~merges()