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coverage: Treat the "merged node flow graph" as a plain data struct
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By removing all methods from this struct and treating it as a collection of
data fields, we make it easier for a future PR to store that data in a query
result, without having to move all of its methods into `rustc_middle`.
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@Zalathar
Zalathar committed Jan 24, 2025
1 parent 4b20a27 commit 2bdc67a
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Showing 3 changed files with 89 additions and 83 deletions.
12 changes: 7 additions & 5 deletions compiler/rustc_mir_transform/src/coverage/counters.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,9 @@ use rustc_middle::mir::coverage::{CounterId, CovTerm, Expression, ExpressionId,

use crate::coverage::counters::balanced_flow::BalancedFlowGraph;
use crate::coverage::counters::iter_nodes::IterNodes;
use crate::coverage::counters::node_flow::{CounterTerm, MergedNodeFlowGraph, NodeCounters};
use crate::coverage::counters::node_flow::{
CounterTerm, NodeCounters, make_node_counters, node_flow_data_for_balanced_graph,
};
use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph};

mod balanced_flow;
Expand All @@ -27,20 +29,20 @@ pub(super) fn make_bcb_counters(
) -> CoverageCounters {
// Create the derived graphs that are necessary for subsequent steps.
let balanced_graph = BalancedFlowGraph::for_graph(graph, |n| !graph[n].is_out_summable);
let merged_graph = MergedNodeFlowGraph::for_balanced_graph(&balanced_graph);
let node_flow_data = node_flow_data_for_balanced_graph(&balanced_graph);

// Use those graphs to determine which nodes get physical counters, and how
// to compute the execution counts of other nodes from those counters.
let nodes = make_node_counter_priority_list(graph, balanced_graph);
let node_counters = merged_graph.make_node_counters(&nodes);
let priority_list = make_node_flow_priority_list(graph, balanced_graph);
let node_counters = make_node_counters(&node_flow_data, &priority_list);

// Convert the counters into a form suitable for embedding into MIR.
transcribe_counters(&node_counters, bcb_needs_counter)
}

/// Arranges the nodes in `balanced_graph` into a list, such that earlier nodes
/// take priority in being given a counter expression instead of a physical counter.
fn make_node_counter_priority_list(
fn make_node_flow_priority_list(
graph: &CoverageGraph,
balanced_graph: BalancedFlowGraph<&CoverageGraph>,
) -> Vec<BasicCoverageBlock> {
Expand Down
148 changes: 75 additions & 73 deletions compiler/rustc_mir_transform/src/coverage/counters/node_flow.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,7 @@
use rustc_data_structures::graph;
use rustc_index::bit_set::DenseBitSet;
use rustc_index::{Idx, IndexVec};
use rustc_index::{Idx, IndexSlice, IndexVec};
use rustc_middle::mir::coverage::Op;

use crate::coverage::counters::iter_nodes::IterNodes;
Expand All @@ -17,8 +17,8 @@ use crate::coverage::counters::union_find::UnionFind;
#[cfg(test)]
mod tests;

/// View of some underlying graph, in which each node's successors have been
/// merged into a single "supernode".
/// Data representing a view of some underlying graph, in which each node's
/// successors have been merged into a single "supernode".
///
/// The resulting supernodes have no obvious meaning on their own.
/// However, merging successor nodes means that a node's out-edges can all
Expand All @@ -29,7 +29,7 @@ mod tests;
/// in the merged graph, it becomes possible to analyze the original node flows
/// using techniques for analyzing edge flows.
#[derive(Debug)]
pub(crate) struct MergedNodeFlowGraph<Node: Idx> {
pub(crate) struct NodeFlowData<Node: Idx> {
/// Maps each node to the supernode that contains it, indicated by some
/// arbitrary "root" node that is part of that supernode.
supernodes: IndexVec<Node, Node>,
Expand All @@ -41,66 +41,59 @@ pub(crate) struct MergedNodeFlowGraph<Node: Idx> {
succ_supernodes: IndexVec<Node, Node>,
}

impl<Node: Idx> MergedNodeFlowGraph<Node> {
/// Creates a "merged" view of an underlying graph.
///
/// The given graph is assumed to have [“balanced flow”](balanced-flow),
/// though it does not necessarily have to be a `BalancedFlowGraph`.
///
/// [balanced-flow]: `crate::coverage::counters::balanced_flow::BalancedFlowGraph`.
pub(crate) fn for_balanced_graph<G>(graph: G) -> Self
where
G: graph::DirectedGraph<Node = Node> + graph::Successors,
{
let mut supernodes = UnionFind::<G::Node>::new(graph.num_nodes());

// For each node, merge its successors into a single supernode, and
// arbitrarily choose one of those successors to represent all of them.
let successors = graph
.iter_nodes()
.map(|node| {
graph
.successors(node)
.reduce(|a, b| supernodes.unify(a, b))
.expect("each node in a balanced graph must have at least one out-edge")
})
.collect::<IndexVec<G::Node, G::Node>>();

// Now that unification is complete, take a snapshot of the supernode forest,
// and resolve each arbitrarily-chosen successor to its canonical root.
// (This avoids having to explicitly resolve them later.)
let supernodes = supernodes.snapshot();
let succ_supernodes = successors.into_iter().map(|succ| supernodes[succ]).collect();

Self { supernodes, succ_supernodes }
}

fn num_nodes(&self) -> usize {
self.succ_supernodes.len()
}
/// Creates a "merged" view of an underlying graph.
///
/// The given graph is assumed to have [“balanced flow”](balanced-flow),
/// though it does not necessarily have to be a `BalancedFlowGraph`.
///
/// [balanced-flow]: `crate::coverage::counters::balanced_flow::BalancedFlowGraph`.
pub(crate) fn node_flow_data_for_balanced_graph<G>(graph: G) -> NodeFlowData<G::Node>
where
G: graph::Successors,
{
let mut supernodes = UnionFind::<G::Node>::new(graph.num_nodes());

// For each node, merge its successors into a single supernode, and
// arbitrarily choose one of those successors to represent all of them.
let successors = graph
.iter_nodes()
.map(|node| {
graph
.successors(node)
.reduce(|a, b| supernodes.unify(a, b))
.expect("each node in a balanced graph must have at least one out-edge")
})
.collect::<IndexVec<G::Node, G::Node>>();

// Now that unification is complete, take a snapshot of the supernode forest,
// and resolve each arbitrarily-chosen successor to its canonical root.
// (This avoids having to explicitly resolve them later.)
let supernodes = supernodes.snapshot();
let succ_supernodes = successors.into_iter().map(|succ| supernodes[succ]).collect();

NodeFlowData { supernodes, succ_supernodes }
}

fn is_supernode(&self, node: Node) -> bool {
self.supernodes[node] == node
/// Uses the graph information in `node_flow_data`, together with a given
/// permutation of all nodes in the graph, to create physical counters and
/// counter expressions for each node in the underlying graph.
///
/// The given list must contain exactly one copy of each node in the
/// underlying balanced-flow graph. The order of nodes is used as a hint to
/// influence counter allocation:
/// - Earlier nodes are more likely to receive counter expressions.
/// - Later nodes are more likely to receive physical counters.
pub(crate) fn make_node_counters<Node: Idx>(
node_flow_data: &NodeFlowData<Node>,
priority_list: &[Node],
) -> NodeCounters<Node> {
let mut builder = SpantreeBuilder::new(node_flow_data);

for &node in priority_list {
builder.visit_node(node);
}

/// Using the information in this merged graph, together with a given
/// permutation of all nodes in the graph, to create physical counters and
/// counter expressions for each node in the underlying graph.
///
/// The given list must contain exactly one copy of each node in the
/// underlying balanced-flow graph. The order of nodes is used as a hint to
/// influence counter allocation:
/// - Earlier nodes are more likely to receive counter expressions.
/// - Later nodes are more likely to receive physical counters.
pub(crate) fn make_node_counters(&self, all_nodes_permutation: &[Node]) -> NodeCounters<Node> {
let mut builder = SpantreeBuilder::new(self);

for &node in all_nodes_permutation {
builder.visit_node(node);
}

NodeCounters { counter_terms: builder.finish() }
}
NodeCounters { counter_terms: builder.finish() }
}

/// End result of allocating physical counters and counter expressions for the
Expand Down Expand Up @@ -141,7 +134,9 @@ pub(crate) struct CounterTerm<Node> {

#[derive(Debug)]
struct SpantreeBuilder<'a, Node: Idx> {
graph: &'a MergedNodeFlowGraph<Node>,
supernodes: &'a IndexSlice<Node, Node>,
succ_supernodes: &'a IndexSlice<Node, Node>,

is_unvisited: DenseBitSet<Node>,
/// Links supernodes to each other, gradually forming a spanning tree of
/// the merged-flow graph.
Expand All @@ -157,22 +152,28 @@ struct SpantreeBuilder<'a, Node: Idx> {
}

impl<'a, Node: Idx> SpantreeBuilder<'a, Node> {
fn new(graph: &'a MergedNodeFlowGraph<Node>) -> Self {
let num_nodes = graph.num_nodes();
fn new(node_flow_data: &'a NodeFlowData<Node>) -> Self {
let NodeFlowData { supernodes, succ_supernodes } = node_flow_data;
let num_nodes = supernodes.len();
Self {
graph,
supernodes,
succ_supernodes,
is_unvisited: DenseBitSet::new_filled(num_nodes),
span_edges: IndexVec::from_fn_n(|_| None, num_nodes),
yank_buffer: vec![],
counter_terms: IndexVec::from_fn_n(|_| vec![], num_nodes),
}
}

fn is_supernode(&self, node: Node) -> bool {
self.supernodes[node] == node
}

/// Given a supernode, finds the supernode that is the "root" of its
/// spantree component. Two nodes that have the same spantree root are
/// connected in the spantree.
fn spantree_root(&self, this: Node) -> Node {
debug_assert!(self.graph.is_supernode(this));
debug_assert!(self.is_supernode(this));

match self.span_edges[this] {
None => this,
Expand All @@ -183,7 +184,7 @@ impl<'a, Node: Idx> SpantreeBuilder<'a, Node> {
/// Rotates edges in the spantree so that `this` is the root of its
/// spantree component.
fn yank_to_spantree_root(&mut self, this: Node) {
debug_assert!(self.graph.is_supernode(this));
debug_assert!(self.is_supernode(this));

// The rotation is done iteratively, by first traversing from `this` to
// its root and storing the path in a buffer, and then traversing the
Expand Down Expand Up @@ -225,12 +226,12 @@ impl<'a, Node: Idx> SpantreeBuilder<'a, Node> {

// Get the supernode containing `this`, and make it the root of its
// component of the spantree.
let this_supernode = self.graph.supernodes[this];
let this_supernode = self.supernodes[this];
self.yank_to_spantree_root(this_supernode);

// Get the supernode containing all of this's successors.
let succ_supernode = self.graph.succ_supernodes[this];
debug_assert!(self.graph.is_supernode(succ_supernode));
let succ_supernode = self.succ_supernodes[this];
debug_assert!(self.is_supernode(succ_supernode));

// If two supernodes are already connected in the spantree, they will
// have the same spantree root. (Each supernode is connected to itself.)
Expand Down Expand Up @@ -279,18 +280,19 @@ impl<'a, Node: Idx> SpantreeBuilder<'a, Node> {
/// Asserts that all nodes have been visited, and returns the computed
/// counter expressions (made up of physical counters) for each node.
fn finish(self) -> IndexVec<Node, Vec<CounterTerm<Node>>> {
let Self { graph, is_unvisited, span_edges, yank_buffer: _, counter_terms } = self;
let Self { ref span_edges, ref is_unvisited, ref counter_terms, .. } = self;
assert!(is_unvisited.is_empty(), "some nodes were never visited: {is_unvisited:?}");
debug_assert!(
span_edges
.iter_enumerated()
.all(|(node, span_edge)| { span_edge.is_some() <= graph.is_supernode(node) }),
.all(|(node, span_edge)| { span_edge.is_some() <= self.is_supernode(node) }),
"only supernodes can have a span edge",
);
debug_assert!(
counter_terms.iter().all(|terms| !terms.is_empty()),
"after visiting all nodes, every node should have at least one term",
);
counter_terms

self.counter_terms
}
}
12 changes: 7 additions & 5 deletions compiler/rustc_mir_transform/src/coverage/counters/node_flow/tests.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4,10 +4,12 @@ use rustc_data_structures::graph::vec_graph::VecGraph;
use rustc_index::Idx;
use rustc_middle::mir::coverage::Op;

use super::{CounterTerm, MergedNodeFlowGraph, NodeCounters};
use crate::coverage::counters::node_flow::{
CounterTerm, NodeCounters, NodeFlowData, make_node_counters, node_flow_data_for_balanced_graph,
};

fn merged_node_flow_graph<G: graph::Successors>(graph: G) -> MergedNodeFlowGraph<G::Node> {
MergedNodeFlowGraph::for_balanced_graph(graph)
fn node_flow_data<G: graph::Successors>(graph: G) -> NodeFlowData<G::Node> {
node_flow_data_for_balanced_graph(graph)
}

fn make_graph<Node: Idx + Ord>(num_nodes: usize, edge_pairs: Vec<(Node, Node)>) -> VecGraph<Node> {
Expand All @@ -30,8 +32,8 @@ fn example_driver() {
(4, 0),
]);

let merged = merged_node_flow_graph(&graph);
let counters = merged.make_node_counters(&[3, 1, 2, 0, 4]);
let node_flow_data = node_flow_data(&graph);
let counters = make_node_counters(&node_flow_data, &[3, 1, 2, 0, 4]);

assert_eq!(format_counter_expressions(&counters), &[
// (comment to force vertical formatting for clarity)
Expand Down

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