Generalize small dominators optimization

compiler/rustc_codegen_ssa/src/mir/analyze.rs

@@ -8,7 +8,7 @@ use rustc_index::bit_set::BitSet;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::mir::traversal;
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
-use rustc_middle::mir::{self, Location, TerminatorKind};
+use rustc_middle::mir::{self, DefLocation, Location, TerminatorKind};
 use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf};
 
 pub fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
@@ -67,21 +67,6 @@ enum LocalKind {
     SSA(DefLocation),
 }
 
-#[derive(Copy, Clone, PartialEq, Eq)]
-enum DefLocation {
-    Argument,
-    Body(Location),
-}
-
-impl DefLocation {
-    fn dominates(self, location: Location, dominators: &Dominators<mir::BasicBlock>) -> bool {
-        match self {
-            DefLocation::Argument => true,
-            DefLocation::Body(def) => def.successor_within_block().dominates(location, dominators),
-        }
-    }
-}
-
 struct LocalAnalyzer<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
     fx: &'mir FunctionCx<'a, 'tcx, Bx>,
     dominators: &'mir Dominators<mir::BasicBlock>,

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

@@ -26,7 +26,42 @@ rustc_index::newtype_index! {
     struct PreorderIndex {}
 }
 
-pub fn dominators<G: ControlFlowGraph>(graph: &G) -> Dominators<G::Node> {
+#[derive(Clone, Debug)]
+pub struct Dominators<Node: Idx> {
+    kind: Kind<Node>,
+}
+
+#[derive(Clone, Debug)]
+enum Kind<Node: Idx> {
+    /// A representation optimized for a small path graphs.
+    Path,
+    General(Inner<Node>),
+}
+
+pub fn dominators<G: ControlFlowGraph>(g: &G) -> Dominators<G::Node> {
+    // We often encounter MIR bodies with 1 or 2 basic blocks. Special case the dominators
+    // computation and representation for those cases.
+    if is_small_path_graph(g) {
+        Dominators { kind: Kind::Path }
+    } else {
+        Dominators { kind: Kind::General(dominators_impl(g)) }
+    }
+}
+
+fn is_small_path_graph<G: ControlFlowGraph>(g: &G) -> bool {
+    if g.start_node().index() != 0 {
+        return false;
+    }
+    if g.num_nodes() == 1 {
+        return true;
+    }
+    if g.num_nodes() == 2 {
+        return g.successors(g.start_node()).any(|n| n.index() == 1);
+    }
+    false
+}
+
+fn dominators_impl<G: ControlFlowGraph>(graph: &G) -> Inner<G::Node> {
     // compute the post order index (rank) for each node
     let mut post_order_rank = IndexVec::from_elem_n(0, graph.num_nodes());
 
@@ -245,7 +280,7 @@ pub fn dominators<G: ControlFlowGraph>(graph: &G) -> Dominators<G::Node> {
 
     let time = compute_access_time(start_node, &immediate_dominators);
 
-    Dominators { start_node, post_order_rank, immediate_dominators, time }
+    Inner { post_order_rank, immediate_dominators, time }
 }
 
 /// Evaluate the link-eval virtual forest, providing the currently minimum semi
@@ -310,8 +345,7 @@ fn compress(
 
 /// Tracks the list of dominators for each node.
 #[derive(Clone, Debug)]
-pub struct Dominators<N: Idx> {
-    start_node: N,
+struct Inner<N: Idx> {
     post_order_rank: IndexVec<N, usize>,
     // Even though we track only the immediate dominator of each node, it's
     // possible to get its full list of dominators by looking up the dominator
@@ -323,12 +357,24 @@ pub struct Dominators<N: Idx> {
 impl<Node: Idx> Dominators<Node> {
     /// Returns true if node is reachable from the start node.
     pub fn is_reachable(&self, node: Node) -> bool {
-        node == self.start_node || self.immediate_dominators[node].is_some()
+        match &self.kind {
+            Kind::Path => true,
+            Kind::General(g) => g.time[node].start != 0,
+        }
     }
 
     /// Returns the immediate dominator of node, if any.
     pub fn immediate_dominator(&self, node: Node) -> Option<Node> {
-        self.immediate_dominators[node]
+        match &self.kind {
+            Kind::Path => {
+                if 0 < node.index() {
+                    Some(Node::new(node.index() - 1))
+                } else {
+                    None
+                }
+            }
+            Kind::General(g) => g.immediate_dominators[node],
+        }
     }
 
     /// Provides an iterator over each dominator up the CFG, for the given Node.
@@ -343,7 +389,10 @@ impl<Node: Idx> Dominators<Node> {
     /// 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 cmp_in_dominator_order(&self, lhs: Node, rhs: Node) -> Ordering {
-        self.post_order_rank[rhs].cmp(&self.post_order_rank[lhs])
+        match &self.kind {
+            Kind::Path => lhs.index().cmp(&rhs.index()),
+            Kind::General(g) => g.post_order_rank[rhs].cmp(&g.post_order_rank[lhs]),
+        }
     }
 
     /// Returns true if `a` dominates `b`.
@@ -352,10 +401,15 @@ impl<Node: Idx> Dominators<Node> {
     ///
     /// 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
+        match &self.kind {
+            Kind::Path => a.index() <= b.index(),
+            Kind::General(g) => {
+                let a = g.time[a];
+                let b = g.time[b];
+                assert!(b.start != 0, "node {b:?} is not reachable");
+                a.start <= b.start && b.finish <= a.finish
+            }
+        }
     }
 }
 

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

@@ -6,12 +6,11 @@ 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;
-    assert_eq!(immediate_dominators[0], None);
-    assert_eq!(immediate_dominators[1], Some(0));
-    assert_eq!(immediate_dominators[2], Some(0));
-    assert_eq!(immediate_dominators[3], Some(0));
+    let d = dominators(&graph);
+    assert_eq!(d.immediate_dominator(0), None);
+    assert_eq!(d.immediate_dominator(1), Some(0));
+    assert_eq!(d.immediate_dominator(2), Some(0));
+    assert_eq!(d.immediate_dominator(3), Some(0));
 }
 
 #[test]
@@ -22,15 +21,14 @@ 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;
-    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));
-    assert_eq!(immediate_dominators[3], Some(6));
-    assert_eq!(immediate_dominators[4], Some(6));
-    assert_eq!(immediate_dominators[5], Some(6));
-    assert_eq!(immediate_dominators[6], None);
+    let d = dominators(&graph);
+    assert_eq!(d.immediate_dominator(0), None); // <-- note that 0 is not in graph
+    assert_eq!(d.immediate_dominator(1), Some(6));
+    assert_eq!(d.immediate_dominator(2), Some(6));
+    assert_eq!(d.immediate_dominator(3), Some(6));
+    assert_eq!(d.immediate_dominator(4), Some(6));
+    assert_eq!(d.immediate_dominator(5), Some(6));
+    assert_eq!(d.immediate_dominator(6), None);
 }
 
 #[test]
@@ -47,11 +45,11 @@ fn paper_slt() {
 #[test]
 fn immediate_dominator() {
     let graph = TestGraph::new(1, &[(1, 2), (2, 3)]);
-    let dominators = dominators(&graph);
-    assert_eq!(dominators.immediate_dominator(0), None);
-    assert_eq!(dominators.immediate_dominator(1), None);
-    assert_eq!(dominators.immediate_dominator(2), Some(1));
-    assert_eq!(dominators.immediate_dominator(3), Some(2));
+    let d = dominators(&graph);
+    assert_eq!(d.immediate_dominator(0), None);
+    assert_eq!(d.immediate_dominator(1), None);
+    assert_eq!(d.immediate_dominator(2), Some(1));
+    assert_eq!(d.immediate_dominator(3), Some(2));
 }
 
 #[test]
@@ -75,8 +73,7 @@ fn transitive_dominator() {
         ],
     );
 
-    let dom_tree = dominators(&graph);
-    let immediate_dominators = &dom_tree.immediate_dominators;
-    assert_eq!(immediate_dominators[2], Some(0));
-    assert_eq!(immediate_dominators[3], Some(0)); // This used to return Some(1).
+    let d = dominators(&graph);
+    assert_eq!(d.immediate_dominator(2), Some(0));
+    assert_eq!(d.immediate_dominator(3), Some(0)); // This used to return Some(1).
 }

compiler/rustc_middle/src/mir/mod.rs

@@ -1590,6 +1590,23 @@ impl Location {
     }
 }
 
+/// `DefLocation` represents the location of a definition - either an argument or an assignment
+/// within MIR body.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum DefLocation {
+    Argument,
+    Body(Location),
+}
+
+impl DefLocation {
+    pub fn dominates(self, location: Location, dominators: &Dominators<BasicBlock>) -> bool {
+        match self {
+            DefLocation::Argument => true,
+            DefLocation::Body(def) => def.successor_within_block().dominates(location, dominators),
+        }
+    }
+}
+
 // Some nodes are used a lot. Make sure they don't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 mod size_asserts {