introduce a VecGraph abstraction that cheaply stores graphs

This is perhaps better than the linked list approach I was using
before. Lower memory overhead, Theta(N+E) storage. Does require a
sort. =)

src/librustc_data_structures/graph/mod.rs

@@ -5,6 +5,7 @@
 pub mod iterate;
 mod reference;
 pub mod scc;
+pub mod vec_graph;
 
 #[cfg(test)]
 mod test;
@@ -17,6 +18,10 @@ pub trait WithNumNodes: DirectedGraph {
     fn num_nodes(&self) -> usize;
 }
 
+pub trait WithNumEdges: DirectedGraph {
+    fn num_edges(&self) -> usize;
+}
+
 pub trait WithSuccessors: DirectedGraph
 where
     Self: for<'graph> GraphSuccessors<'graph, Item = <Self as DirectedGraph>::Node>,

src/librustc_data_structures/graph/scc/mod.rs

@@ -4,7 +4,8 @@
 //! O(n) time.
 
 use crate::fx::FxHashSet;
-use crate::graph::{DirectedGraph, WithNumNodes, WithSuccessors, GraphSuccessors};
+use crate::graph::{DirectedGraph, WithNumNodes, WithNumEdges, WithSuccessors, GraphSuccessors};
+use crate::graph::vec_graph::VecGraph;
 use crate::indexed_vec::{Idx, IndexVec};
 use std::ops::Range;
 
@@ -58,6 +59,18 @@ pub fn scc(&self, r: N) -> S {
     pub fn successors(&self, scc: S) -> &[S] {
         self.scc_data.successors(scc)
     }
+
+    /// Construct the reverse graph of the SCC graph.
+    pub fn reverse(&self) -> VecGraph<S> {
+        VecGraph::new(
+            self.num_sccs(),
+            self.all_sccs()
+                .flat_map(|source| self.successors(source).iter().map(move |&target| {
+                    (target, source)
+                }))
+                .collect(),
+        )
+    }
 }
 
 impl<N: Idx, S: Idx> DirectedGraph for Sccs<N, S> {
@@ -70,6 +83,12 @@ fn num_nodes(&self) -> usize {
     }
 }
 
+impl<N: Idx, S: Idx> WithNumEdges for Sccs<N, S> {
+    fn num_edges(&self) -> usize {
+        self.scc_data.all_successors.len()
+    }
+}
+
 impl<N: Idx, S: Idx> GraphSuccessors<'graph> for Sccs<N, S> {
     type Item = S;
 

src/librustc_data_structures/graph/vec_graph/mod.rs

@@ -0,0 +1,113 @@
+use crate::indexed_vec::{Idx, IndexVec};
+use crate::graph::{DirectedGraph, WithNumNodes, WithNumEdges, WithSuccessors, GraphSuccessors};
+
+mod test;
+
+pub struct VecGraph<N: Idx> {
+    /// Maps from a given node to an index where the set of successors
+    /// for that node starts. The index indexes into the `edges`
+    /// vector. To find the range for a given node, we look up the
+    /// start for that node and then the start for the next node
+    /// (i.e., with an index 1 higher) and get the range between the
+    /// two. This vector always has an extra entry so that this works
+    /// even for the max element.
+    node_starts: IndexVec<N, usize>,
+
+    edge_targets: Vec<N>,
+}
+
+impl<N: Idx> VecGraph<N> {
+    pub fn new(
+        num_nodes: usize,
+        mut edge_pairs: Vec<(N, N)>,
+    ) -> Self {
+        // Sort the edges by the source -- this is important.
+        edge_pairs.sort();
+
+        let num_edges = edge_pairs.len();
+
+        // Store the *target* of each edge into `edge_targets`
+        let edge_targets: Vec<N> = edge_pairs.iter().map(|&(_, target)| target).collect();
+
+        // Create the *edge starts* array. We are iterating over over
+        // the (sorted) edge pairs. We maintain the invariant that the
+        // length of the `node_starts` arary is enough to store the
+        // current source node -- so when we see that the source node
+        // for an edge is greater than the current length, we grow the
+        // edge-starts array by just enough.
+        let mut node_starts = IndexVec::with_capacity(num_edges);
+        for (index, &(source, _)) in edge_pairs.iter().enumerate() {
+            // If we have a list like `[(0, x), (2, y)]`:
+            //
+            // - Start out with `node_starts` of `[]`
+            // - Iterate to `(0, x)` at index 0:
+            //   - Push one entry because `node_starts.len()` (0) is <= the source (0)
+            //   - Leaving us with `node_starts` of `[0]`
+            // - Iterate to `(2, y)` at index 1:
+            //   - Push one entry because `node_starts.len()` (1) is <= the source (2)
+            //   - Push one entry because `node_starts.len()` (2) is <= the source (2)
+            //   - Leaving us with `node_starts` of `[0, 1, 1]`
+            // - Loop terminates
+            while node_starts.len() <= source.index() {
+                node_starts.push(index);
+            }
+        }
+
+        // Pad out the `node_starts` array so that it has `num_nodes +
+        // 1` entries. Continuing our example above, if `num_nodes` is
+        // be `3`, we would push one more index: `[0, 1, 1, 2]`.
+        //
+        // Interpretation of that vector:
+        //
+        // [0, 1, 1, 2]
+        //        ---- range for N=2
+        //     ---- range for N=1
+        //  ---- range for N=0
+        while node_starts.len() <= num_nodes {
+            node_starts.push(edge_targets.len());
+        }
+
+        assert_eq!(node_starts.len(), num_nodes + 1);
+
+        Self { node_starts, edge_targets }
+    }
+
+    /// Gets the successors for `source` as a slice.
+    pub fn successors(&self, source: N) -> &[N] {
+        let start_index = self.node_starts[source];
+        let end_index = self.node_starts[source.plus(1)];
+        &self.edge_targets[start_index..end_index]
+    }
+}
+
+impl<N: Idx> DirectedGraph for VecGraph<N> {
+    type Node = N;
+}
+
+impl<N: Idx> WithNumNodes for VecGraph<N> {
+    fn num_nodes(&self) -> usize {
+        self.node_starts.len() - 1
+    }
+}
+
+impl<N: Idx> WithNumEdges for VecGraph<N> {
+    fn num_edges(&self) -> usize {
+        self.edge_targets.len()
+    }
+}
+
+impl<N: Idx> GraphSuccessors<'graph> for VecGraph<N> {
+    type Item = N;
+
+    type Iter = std::iter::Cloned<std::slice::Iter<'graph, N>>;
+}
+
+impl<N: Idx> WithSuccessors for VecGraph<N> {
+    fn successors<'graph>(
+        &'graph self,
+        node: N
+    ) -> <Self as GraphSuccessors<'graph>>::Iter {
+        self.successors(node).iter().cloned()
+    }
+}
+

src/librustc_data_structures/graph/vec_graph/test.rs

@@ -0,0 +1,46 @@
+#![cfg(test)]
+
+use super::*;
+
+fn create_graph() -> VecGraph<usize> {
+    // Create a simple graph
+    //
+    //          5
+    //          |
+    //          V
+    //    0 --> 1 --> 2
+    //          |
+    //          v
+    //          3 --> 4
+    //
+    //    6
+
+    VecGraph::new(
+        7,
+        vec![
+            (0, 1),
+            (1, 2),
+            (1, 3),
+            (3, 4),
+            (5, 1),
+        ],
+    )
+}
+
+#[test]
+fn num_nodes() {
+    let graph = create_graph();
+    assert_eq!(graph.num_nodes(), 7);
+}
+
+#[test]
+fn succesors() {
+    let graph = create_graph();
+    assert_eq!(graph.successors(0), &[1]);
+    assert_eq!(graph.successors(1), &[2, 3]);
+    assert_eq!(graph.successors(2), &[]);
+    assert_eq!(graph.successors(3), &[4]);
+    assert_eq!(graph.successors(4), &[]);
+    assert_eq!(graph.successors(5), &[1]);
+    assert_eq!(graph.successors(6), &[]);
+}

src/librustc_data_structures/indexed_vec.rs

@@ -19,8 +19,11 @@ pub trait Idx: Copy + 'static + Ord + Debug + Hash {
     fn index(self) -> usize;
 
     fn increment_by(&mut self, amount: usize) {
-        let v = self.index() + amount;
-        *self = Self::new(v);
+        *self = self.plus(amount);
+    }
+
+    fn plus(self, amount: usize) -> Self {
+        Self::new(self.index() + amount)
     }
 }
 
@@ -167,6 +170,14 @@ impl $type {
             }
         }
 
+        impl std::ops::Add<usize> for $type {
+            type Output = Self;
+
+            fn add(self, other: usize) -> Self {
+                Self::new(self.index() + other)
+            }
+        }
+
         impl Idx for $type {
             #[inline]
             fn new(value: usize) -> Self {