Preprocess dominator tree to answer queries in O(1)

Author: tmiasko

compiler/rustc_const_eval/src/transform/validate.rs

@@ -144,7 +144,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         if self.unwind_edge_count <= 1 {
             return;
         }
-        let doms = self.body.basic_blocks.dominators();
+        let dom_tree = self.body.basic_blocks.dominator_tree();
         let mut post_contract_node = FxHashMap::default();
         // Reusing the allocation across invocations of the closure
         let mut dom_path = vec![];
@@ -153,7 +153,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 if let Some(root) = post_contract_node.get(&bb) {
                     break *root;
                 }
-                let parent = doms.immediate_dominator(bb);
+                let parent = dom_tree.immediate_dominator(bb);
                 dom_path.push(bb);
                 if !self.body.basic_blocks[parent].is_cleanup {
                     break bb;

compiler/rustc_data_structures/src/graph/dominators/mod.rs

@@ -25,7 +25,7 @@ rustc_index::newtype_index! {
     struct PreorderIndex {}
 }
 
-pub fn dominators<G: ControlFlowGraph>(graph: G) -> Dominators<G::Node> {
+pub fn dominator_tree<G: ControlFlowGraph>(graph: G) -> DominatorTree<G::Node> {
     // compute the post order index (rank) for each node
     let mut post_order_rank = IndexVec::from_elem_n(0, graph.num_nodes());
 
@@ -201,7 +201,7 @@ pub fn dominators<G: ControlFlowGraph>(graph: G) -> Dominators<G::Node> {
         immediate_dominators[*node] = Some(pre_order_to_real[idom[idx]]);
     }
 
-    Dominators { post_order_rank, immediate_dominators }
+    DominatorTree { post_order_rank, immediate_dominators }
 }
 
 /// Evaluate the link-eval virtual forest, providing the currently minimum semi
@@ -265,13 +265,14 @@ fn compress(
 }
 
 #[derive(Clone, Debug)]
-pub struct Dominators<N: Idx> {
+pub struct DominatorTree<N: Idx> {
     post_order_rank: IndexVec<N, usize>,
+    // Note: immediate_dominators[root] is Some(root)!
     immediate_dominators: IndexVec<N, Option<N>>,
 }
 
-impl<Node: Idx> Dominators<Node> {
-    pub fn is_reachable(&self, node: Node) -> bool {
+impl<Node: Idx> DominatorTree<Node> {
+    fn is_reachable(&self, node: Node) -> bool {
         self.immediate_dominators[node].is_some()
     }
 
@@ -282,25 +283,12 @@ impl<Node: Idx> Dominators<Node> {
 
     pub fn dominators(&self, node: Node) -> Iter<'_, Node> {
         assert!(self.is_reachable(node), "node {node:?} is not reachable");
-        Iter { dominators: self, node: Some(node) }
-    }
-
-    pub fn dominates(&self, dom: Node, node: Node) -> bool {
-        // FIXME -- could be optimized by using post-order-rank
-        self.dominators(node).any(|n| n == dom)
-    }
-
-    /// Provide deterministic ordering of nodes such that, if any two nodes have a dominator
-    /// relationship, the dominator will always precede the dominated. (The relative ordering
-    /// of two unrelated nodes will also be consistent, but otherwise the order has no
-    /// meaning.) This method cannot be used to determine if either Node dominates the other.
-    pub fn rank_partial_cmp(&self, lhs: Node, rhs: Node) -> Option<Ordering> {
-        self.post_order_rank[rhs].partial_cmp(&self.post_order_rank[lhs])
+        Iter { dom_tree: self, node: Some(node) }
     }
 }
 
 pub struct Iter<'dom, Node: Idx> {
-    dominators: &'dom Dominators<Node>,
+    dom_tree: &'dom DominatorTree<Node>,
     node: Option<Node>,
 }
 
@@ -309,7 +297,7 @@ impl<'dom, Node: Idx> Iterator for Iter<'dom, Node> {
 
     fn next(&mut self) -> Option<Self::Item> {
         if let Some(node) = self.node {
-            let dom = self.dominators.immediate_dominator(node);
+            let dom = self.dom_tree.immediate_dominator(node);
             if dom == node {
                 self.node = None; // reached the root
             } else {
@@ -321,3 +309,101 @@ impl<'dom, Node: Idx> Iterator for Iter<'dom, Node> {
         }
     }
 }
+
+#[derive(Clone, Debug)]
+pub struct Dominators<Node: Idx> {
+    time: IndexVec<Node, Time>,
+    post_order_rank: IndexVec<Node, usize>,
+}
+
+/// Describes the number of vertices discovered at the time when processing of a particular vertex
+/// started and when it finished. Both values are zero for unreachable vertices.
+#[derive(Copy, Clone, Default, Debug)]
+struct Time {
+    start: u32,
+    finish: u32,
+}
+
+impl<Node: Idx> Dominators<Node> {
+    pub fn dummy() -> Self {
+        Self { time: Default::default(), post_order_rank: Default::default() }
+    }
+
+    /// Returns true if `a` dominates `b`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `b` is unreachable.
+    pub fn dominates(&self, a: Node, b: Node) -> bool {
+        let a = self.time[a];
+        let b = self.time[b];
+        assert!(b.start != 0, "node {b:?} is not reachable");
+        a.start <= b.start && b.finish <= a.finish
+    }
+
+    /// Provide deterministic ordering of nodes such that, if any two nodes have a dominator
+    /// relationship, the dominator will always precede the dominated. (The relative ordering
+    /// of two unrelated nodes will also be consistent, but otherwise the order has no
+    /// meaning.) This method cannot be used to determine if either Node dominates the other.
+    pub fn rank_partial_cmp(&self, lhs: Node, rhs: Node) -> Option<Ordering> {
+        self.post_order_rank[rhs].partial_cmp(&self.post_order_rank[lhs])
+    }
+}
+
+pub fn dominators<G: Copy + ControlFlowGraph>(graph: G) -> Dominators<G::Node> {
+    let DominatorTree { mut immediate_dominators, post_order_rank } = dominator_tree(graph);
+
+    immediate_dominators[graph.start_node()] = None;
+
+    // Transpose the dominator tree edges, so that child nodes of vertex v are stored in
+    // node[edges[v].start..edges[y].end].
+    let mut edges: IndexVec<G::Node, std::ops::Range<u32>> =
+        IndexVec::from_elem_n(0..0, graph.num_nodes());
+    for &idom in immediate_dominators.iter() {
+        if let Some(idom) = idom {
+            edges[idom].end += 1;
+        }
+    }
+    let mut m = 0;
+    for e in edges.iter_mut() {
+        m += e.end;
+        e.start = m;
+        e.end = m;
+    }
+    let mut node = IndexVec::from_elem_n(Idx::new(0), m.try_into().unwrap());
+    for (i, &idom) in immediate_dominators.iter_enumerated() {
+        if let Some(idom) = idom {
+            edges[idom].start -= 1;
+            node[edges[idom].start] = i;
+        }
+    }
+
+    // Perform a depth-first search of the dominator tree. Record the number of vertices discovered
+    // when vertex v is discovered first as time[v].start, and when its processing is finished as
+    // time[v].finish.
+    let mut time: IndexVec<G::Node, Time> =
+        IndexVec::from_elem_n(Time::default(), graph.num_nodes());
+    let mut stack = Vec::new();
+
+    let mut discovered = 1;
+    stack.push(graph.start_node());
+    time[graph.start_node()].start = discovered;
+
+    while let Some(&i) = stack.last() {
+        let e = &mut edges[i];
+        if e.start == e.end {
+            // Finish processing vertex i.
+            time[i].finish = discovered;
+            stack.pop();
+        } else {
+            let j = node[e.start];
+            e.start += 1;
+            // Start processing vertex j.
+            discovered += 1;
+            time[j].start = discovered;
+            stack.push(j);
+        }
+    }
+
+    Dominators { time, post_order_rank }
+}

compiler/rustc_data_structures/src/graph/dominators/tests.rs

@@ -6,8 +6,8 @@ use super::super::tests::TestGraph;
 fn diamond() {
     let graph = TestGraph::new(0, &[(0, 1), (0, 2), (1, 3), (2, 3)]);
 
-    let dominators = dominators(&graph);
-    let immediate_dominators = &dominators.immediate_dominators;
+    let dom_tree = dominator_tree(&graph);
+    let immediate_dominators = &dom_tree.immediate_dominators;
     assert_eq!(immediate_dominators[0], Some(0));
     assert_eq!(immediate_dominators[1], Some(0));
     assert_eq!(immediate_dominators[2], Some(0));
@@ -22,8 +22,8 @@ fn paper() {
         &[(6, 5), (6, 4), (5, 1), (4, 2), (4, 3), (1, 2), (2, 3), (3, 2), (2, 1)],
     );
 
-    let dominators = dominators(&graph);
-    let immediate_dominators = &dominators.immediate_dominators;
+    let dom_tree = dominator_tree(&graph);
+    let immediate_dominators = &dom_tree.immediate_dominators;
     assert_eq!(immediate_dominators[0], None); // <-- note that 0 is not in graph
     assert_eq!(immediate_dominators[1], Some(6));
     assert_eq!(immediate_dominators[2], Some(6));
@@ -41,5 +41,5 @@ fn paper_slt() {
         &[(1, 2), (1, 3), (2, 3), (2, 7), (3, 4), (3, 6), (4, 5), (5, 4), (6, 7), (7, 8), (8, 5)],
     );
 
-    dominators(&graph);
+    dominator_tree(&graph);
 }

compiler/rustc_middle/src/mir/basic_blocks.rs

@@ -5,7 +5,9 @@ use crate::mir::traversal::PostorderCache;
 use crate::mir::{BasicBlock, BasicBlockData, Successors, START_BLOCK};
 
 use rustc_data_structures::graph;
-use rustc_data_structures::graph::dominators::{dominators, Dominators};
+use rustc_data_structures::graph::dominators::{
+    dominator_tree, dominators, DominatorTree, Dominators,
+};
 use rustc_index::vec::IndexVec;
 
 #[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable, TypeFoldable, TypeVisitable)]
@@ -35,6 +37,10 @@ impl<'tcx> BasicBlocks<'tcx> {
         self.is_cyclic.is_cyclic(self)
     }
 
+    pub fn dominator_tree(&self) -> DominatorTree<BasicBlock> {
+        dominator_tree(&self)
+    }
+
     #[inline]
     pub fn dominators(&self) -> Dominators<BasicBlock> {
         dominators(&self)

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -652,26 +652,6 @@ pub(super) fn find_loop_backedges(
     let mut backedges = IndexVec::from_elem_n(Vec::<BasicCoverageBlock>::new(), num_bcbs);
 
     // Identify loops by their backedges.
-    //
-    // The computational complexity is bounded by: n(s) x d where `n` is the number of
-    // `BasicCoverageBlock` nodes (the simplified/reduced representation of the CFG derived from the
-    // MIR); `s` is the average number of successors per node (which is most likely less than 2, and
-    // independent of the size of the function, so it can be treated as a constant);
-    // and `d` is the average number of dominators per node.
-    //
-    // The average number of dominators depends on the size and complexity of the function, and
-    // nodes near the start of the function's control flow graph typically have less dominators
-    // than nodes near the end of the CFG. Without doing a detailed mathematical analysis, I
-    // think the resulting complexity has the characteristics of O(n log n).
-    //
-    // The overall complexity appears to be comparable to many other MIR transform algorithms, and I
-    // don't expect that this function is creating a performance hot spot, but if this becomes an
-    // issue, there may be ways to optimize the `dominates` algorithm (as indicated by an
-    // existing `FIXME` comment in that code), or possibly ways to optimize it's usage here, perhaps
-    // by keeping track of results for visited `BasicCoverageBlock`s if they can be used to short
-    // circuit downstream `dominates` checks.
-    //
-    // For now, that kind of optimization seems unnecessarily complicated.
     for (bcb, _) in basic_coverage_blocks.iter_enumerated() {
         for &successor in &basic_coverage_blocks.successors[bcb] {
             if basic_coverage_blocks.dominates(successor, bcb) {