Merge from rustc

8 files changed, 161 insertions, 408 deletions

diff --git a/compiler/rustc_mir_transform/src/coverage/counters.rs b/compiler/rustc_mir_transform/src/coverage/counters.rs
index adb99a75a9e..5568d42ab8f 100644
--- a/compiler/rustc_mir_transform/src/coverage/counters.rs
+++ b/compiler/rustc_mir_transform/src/coverage/counters.rs
@@ -2,7 +2,6 @@ use std::cmp::Ordering;
 
 use either::Either;
 use itertools::Itertools;
-use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
 use rustc_data_structures::graph::DirectedGraph;
 use rustc_index::IndexVec;
@@ -11,31 +10,35 @@ use rustc_middle::mir::coverage::{CounterId, CovTerm, Expression, ExpressionId,
 
 use crate::coverage::counters::balanced_flow::BalancedFlowGraph;
 use crate::coverage::counters::node_flow::{
-    CounterTerm, NodeCounters, make_node_counters, node_flow_data_for_balanced_graph,
+    CounterTerm, NodeCounters, NodeFlowData, node_flow_data_for_balanced_graph,
 };
 use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph};
 
 mod balanced_flow;
-mod node_flow;
+pub(crate) mod node_flow;
 mod union_find;
 
-/// Ensures that each BCB node needing a counter has one, by creating physical
-/// counters or counter expressions for nodes as required.
-pub(super) fn make_bcb_counters(
-    graph: &CoverageGraph,
-    bcb_needs_counter: &DenseBitSet<BasicCoverageBlock>,
-) -> CoverageCounters {
+/// Struct containing the results of [`prepare_bcb_counters_data`].
+pub(crate) struct BcbCountersData {
+    pub(crate) node_flow_data: NodeFlowData<BasicCoverageBlock>,
+    pub(crate) priority_list: Vec<BasicCoverageBlock>,
+}
+
+/// Analyzes the coverage graph to create intermediate data structures that
+/// will later be used (during codegen) to create physical counters or counter
+/// expressions for each BCB node that needs one.
+pub(crate) fn prepare_bcb_counters_data(graph: &CoverageGraph) -> BcbCountersData {
     // Create the derived graphs that are necessary for subsequent steps.
     let balanced_graph = BalancedFlowGraph::for_graph(graph, |n| !graph[n].is_out_summable);
     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.
+    // Also create a "priority list" of coverage graph nodes, to help determine
+    // which ones get physical counters or counter expressions. This needs to
+    // be done now, because the later parts of the counter-creation process
+    // won't have access to the original coverage graph.
     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)
+    BcbCountersData { node_flow_data, priority_list }
 }
 
 /// Arranges the nodes in `balanced_graph` into a list, such that earlier nodes
@@ -47,7 +50,7 @@ fn make_node_flow_priority_list(
     // A "reloop" node has exactly one out-edge, which jumps back to the top
     // of an enclosing loop. Reloop nodes are typically visited more times
     // than loop-exit nodes, so try to avoid giving them physical counters.
-    let is_reloop_node = IndexVec::from_fn_n(
+    let is_reloop_node = IndexVec::<BasicCoverageBlock, _>::from_fn_n(
         |node| match graph.successors[node].as_slice() {
             &[succ] => graph.dominates(succ, node),
             _ => false,
@@ -74,31 +77,33 @@ fn make_node_flow_priority_list(
 }
 
 // Converts node counters into a form suitable for embedding into MIR.
-fn transcribe_counters(
+pub(crate) fn transcribe_counters(
     old: &NodeCounters<BasicCoverageBlock>,
     bcb_needs_counter: &DenseBitSet<BasicCoverageBlock>,
+    bcbs_seen: &DenseBitSet<BasicCoverageBlock>,
 ) -> CoverageCounters {
     let mut new = CoverageCounters::with_num_bcbs(bcb_needs_counter.domain_size());
 
     for bcb in bcb_needs_counter.iter() {
+        if !bcbs_seen.contains(bcb) {
+            // This BCB's code was removed by MIR opts, so its counter is always zero.
+            new.set_node_counter(bcb, CovTerm::Zero);
+            continue;
+        }
+
         // Our counter-creation algorithm doesn't guarantee that a node's list
         // of terms starts or ends with a positive term, so partition the
         // counters into "positive" and "negative" lists for easier handling.
-        let (mut pos, mut neg): (Vec<_>, Vec<_>) =
-            old.counter_terms[bcb].iter().partition_map(|&CounterTerm { node, op }| match op {
+        let (mut pos, mut neg): (Vec<_>, Vec<_>) = old.counter_terms[bcb]
+            .iter()
+            // Filter out any BCBs that were removed by MIR opts;
+            // this treats them as having an execution count of 0.
+            .filter(|term| bcbs_seen.contains(term.node))
+            .partition_map(|&CounterTerm { node, op }| match op {
                 Op::Add => Either::Left(node),
                 Op::Subtract => Either::Right(node),
             });
 
-        if pos.is_empty() {
-            // If we somehow end up with no positive terms, fall back to
-            // creating a physical counter. There's no known way for this
-            // to happen, but we can avoid an ICE if it does.
-            debug_assert!(false, "{bcb:?} has no positive counter terms");
-            pos = vec![bcb];
-            neg = vec![];
-        }
-
         // These intermediate sorts are not strictly necessary, but were helpful
         // in reducing churn when switching to the current counter-creation scheme.
         // They also help to slightly decrease the overall size of the expression
@@ -109,14 +114,10 @@ fn transcribe_counters(
         let mut new_counters_for_sites = |sites: Vec<BasicCoverageBlock>| {
             sites.into_iter().map(|node| new.ensure_phys_counter(node)).collect::<Vec<_>>()
         };
-        let mut pos = new_counters_for_sites(pos);
-        let mut neg = new_counters_for_sites(neg);
-
-        // These sorts are also not strictly necessary; see above.
-        pos.sort();
-        neg.sort();
+        let pos = new_counters_for_sites(pos);
+        let neg = new_counters_for_sites(neg);
 
-        let pos_counter = new.make_sum(&pos).expect("`pos` should not be empty");
+        let pos_counter = new.make_sum(&pos).unwrap_or(CovTerm::Zero);
         let new_counter = new.make_subtracted_sum(pos_counter, &neg);
         new.set_node_counter(bcb, new_counter);
     }
@@ -129,15 +130,15 @@ fn transcribe_counters(
 pub(super) struct CoverageCounters {
     /// List of places where a counter-increment statement should be injected
     /// into MIR, each with its corresponding counter ID.
-    phys_counter_for_node: FxIndexMap<BasicCoverageBlock, CounterId>,
-    next_counter_id: CounterId,
+    pub(crate) phys_counter_for_node: FxIndexMap<BasicCoverageBlock, CounterId>,
+    pub(crate) next_counter_id: CounterId,
 
     /// Coverage counters/expressions that are associated with individual BCBs.
-    node_counters: IndexVec<BasicCoverageBlock, Option<CovTerm>>,
+    pub(crate) node_counters: IndexVec<BasicCoverageBlock, Option<CovTerm>>,
 
     /// Table of expression data, associating each expression ID with its
     /// corresponding operator (+ or -) and its LHS/RHS operands.
-    expressions: IndexVec<ExpressionId, Expression>,
+    pub(crate) expressions: IndexVec<ExpressionId, Expression>,
     /// Remember expressions that have already been created (or simplified),
     /// so that we don't create unnecessary duplicates.
     expressions_memo: FxHashMap<Expression, CovTerm>,
@@ -188,12 +189,6 @@ impl CoverageCounters {
         self.make_expression(lhs, Op::Subtract, rhs_sum)
     }
 
-    pub(super) fn num_counters(&self) -> usize {
-        let num_counters = self.phys_counter_for_node.len();
-        assert_eq!(num_counters, self.next_counter_id.as_usize());
-        num_counters
-    }
-
     fn set_node_counter(&mut self, bcb: BasicCoverageBlock, counter: CovTerm) -> CovTerm {
         let existing = self.node_counters[bcb].replace(counter);
         assert!(
@@ -202,34 +197,4 @@ impl CoverageCounters {
         );
         counter
     }
-
-    pub(super) fn term_for_bcb(&self, bcb: BasicCoverageBlock) -> Option<CovTerm> {
-        self.node_counters[bcb]
-    }
-
-    /// Returns an iterator over all the nodes in the coverage graph that
-    /// should have a counter-increment statement injected into MIR, along with
-    /// each site's corresponding counter ID.
-    pub(super) fn counter_increment_sites(
-        &self,
-    ) -> impl Iterator<Item = (CounterId, BasicCoverageBlock)> + Captures<'_> {
-        self.phys_counter_for_node.iter().map(|(&site, &id)| (id, site))
-    }
-
-    /// Returns an iterator over the subset of BCB nodes that have been associated
-    /// with a counter *expression*, along with the ID of that expression.
-    pub(super) fn bcb_nodes_with_coverage_expressions(
-        &self,
-    ) -> impl Iterator<Item = (BasicCoverageBlock, ExpressionId)> + Captures<'_> {
-        self.node_counters.iter_enumerated().filter_map(|(bcb, &counter)| match counter {
-            // Yield the BCB along with its associated expression ID.
-            Some(CovTerm::Expression(id)) => Some((bcb, id)),
-            // This BCB is associated with a counter or nothing, so skip it.
-            Some(CovTerm::Counter { .. } | CovTerm::Zero) | None => None,
-        })
-    }
-
-    pub(super) fn into_expressions(self) -> IndexVec<ExpressionId, Expression> {
-        self.expressions
-    }
 }
diff --git a/compiler/rustc_mir_transform/src/coverage/counters/node_flow.rs b/compiler/rustc_mir_transform/src/coverage/counters/node_flow.rs
index 9d80b3af42d..91ed54b8b59 100644
--- a/compiler/rustc_mir_transform/src/coverage/counters/node_flow.rs
+++ b/compiler/rustc_mir_transform/src/coverage/counters/node_flow.rs
@@ -9,6 +9,7 @@
 use rustc_data_structures::graph;
 use rustc_index::bit_set::DenseBitSet;
 use rustc_index::{Idx, IndexSlice, IndexVec};
+pub(crate) use rustc_middle::mir::coverage::NodeFlowData;
 use rustc_middle::mir::coverage::Op;
 
 use crate::coverage::counters::union_find::UnionFind;
@@ -16,30 +17,6 @@ use crate::coverage::counters::union_find::UnionFind;
 #[cfg(test)]
 mod tests;
 
-/// 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
-/// be combined into a single out-edge, whose flow is the same as the flow
-/// (execution count) of its corresponding node in the original graph.
-///
-/// With all node flows now in the original graph now represented as edge flows
-/// in the merged graph, it becomes possible to analyze the original node flows
-/// using techniques for analyzing edge flows.
-#[derive(Debug)]
-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>,
-    /// For each node, stores the single supernode that all of its successors
-    /// have been merged into.
-    ///
-    /// (Note that each node in a supernode can potentially have a _different_
-    /// successor supernode from its peers.)
-    succ_supernodes: IndexVec<Node, Node>,
-}
-
 /// Creates a "merged" view of an underlying graph.
 ///
 /// The given graph is assumed to have [“balanced flow”](balanced-flow),
diff --git a/compiler/rustc_mir_transform/src/coverage/graph.rs b/compiler/rustc_mir_transform/src/coverage/graph.rs
index 392b54c8d81..dcc7c5b91d7 100644
--- a/compiler/rustc_mir_transform/src/coverage/graph.rs
+++ b/compiler/rustc_mir_transform/src/coverage/graph.rs
@@ -2,12 +2,12 @@ use std::cmp::Ordering;
 use std::ops::{Index, IndexMut};
 use std::{mem, slice};
 
-use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_data_structures::graph::{self, DirectedGraph, StartNode};
 use rustc_index::IndexVec;
 use rustc_index::bit_set::DenseBitSet;
+pub(crate) use rustc_middle::mir::coverage::{BasicCoverageBlock, START_BCB};
 use rustc_middle::mir::{self, BasicBlock, Terminator, TerminatorKind};
 use tracing::debug;
 
@@ -42,7 +42,7 @@ impl CoverageGraph {
         // `SwitchInt` to have multiple targets to the same destination `BasicBlock`, so
         // de-duplication is required. This is done without reordering the successors.
 
-        let successors = IndexVec::from_fn_n(
+        let successors = IndexVec::<BasicCoverageBlock, _>::from_fn_n(
             |bcb| {
                 let mut seen_bcbs = FxHashSet::default();
                 let terminator = mir_body[bcbs[bcb].last_bb()].terminator();
@@ -217,7 +217,7 @@ impl CoverageGraph {
     pub(crate) fn reloop_predecessors(
         &self,
         to_bcb: BasicCoverageBlock,
-    ) -> impl Iterator<Item = BasicCoverageBlock> + Captures<'_> {
+    ) -> impl Iterator<Item = BasicCoverageBlock> {
         self.predecessors[to_bcb].iter().copied().filter(move |&pred| self.dominates(to_bcb, pred))
     }
 }
@@ -269,15 +269,6 @@ impl graph::Predecessors for CoverageGraph {
     }
 }
 
-rustc_index::newtype_index! {
-    /// A node in the control-flow graph of CoverageGraph.
-    #[orderable]
-    #[debug_format = "bcb{}"]
-    pub(crate) struct BasicCoverageBlock {
-        const START_BCB = 0;
-    }
-}
-
 /// `BasicCoverageBlockData` holds the data indexed by a `BasicCoverageBlock`.
 ///
 /// A `BasicCoverageBlock` (BCB) represents the maximal-length sequence of MIR `BasicBlock`s without
diff --git a/compiler/rustc_mir_transform/src/coverage/mappings.rs b/compiler/rustc_mir_transform/src/coverage/mappings.rs
index 8d0d92dc367..d83c0d40a7e 100644
--- a/compiler/rustc_mir_transform/src/coverage/mappings.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mappings.rs
@@ -1,9 +1,7 @@
 use std::collections::BTreeSet;
 
 use rustc_data_structures::fx::FxIndexMap;
-use rustc_data_structures::graph::DirectedGraph;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::coverage::{
     BlockMarkerId, BranchSpan, ConditionId, ConditionInfo, CoverageInfoHi, CoverageKind,
 };
@@ -63,10 +61,6 @@ const MCDC_MAX_BITMAP_SIZE: usize = i32::MAX as usize;
 
 #[derive(Default)]
 pub(super) struct ExtractedMappings {
-    /// Store our own copy of [`CoverageGraph::num_nodes`], so that we don't
-    /// need access to the whole graph when allocating per-BCB data. This is
-    /// only public so that other code can still use exhaustive destructuring.
-    pub(super) num_bcbs: usize,
     pub(super) code_mappings: Vec<CodeMapping>,
     pub(super) branch_pairs: Vec<BranchPair>,
     pub(super) mcdc_bitmap_bits: usize,
@@ -118,7 +112,6 @@ pub(super) fn extract_all_mapping_info_from_mir<'tcx>(
     );
 
     ExtractedMappings {
-        num_bcbs: graph.num_nodes(),
         code_mappings,
         branch_pairs,
         mcdc_bitmap_bits,
@@ -127,60 +120,6 @@ pub(super) fn extract_all_mapping_info_from_mir<'tcx>(
     }
 }
 
-impl ExtractedMappings {
-    pub(super) fn all_bcbs_with_counter_mappings(&self) -> DenseBitSet<BasicCoverageBlock> {
-        // Fully destructure self to make sure we don't miss any fields that have mappings.
-        let Self {
-            num_bcbs,
-            code_mappings,
-            branch_pairs,
-            mcdc_bitmap_bits: _,
-            mcdc_degraded_branches,
-            mcdc_mappings,
-        } = self;
-
-        // Identify which BCBs have one or more mappings.
-        let mut bcbs_with_counter_mappings = DenseBitSet::new_empty(*num_bcbs);
-        let mut insert = |bcb| {
-            bcbs_with_counter_mappings.insert(bcb);
-        };
-
-        for &CodeMapping { span: _, bcb } in code_mappings {
-            insert(bcb);
-        }
-        for &BranchPair { true_bcb, false_bcb, .. } in branch_pairs {
-            insert(true_bcb);
-            insert(false_bcb);
-        }
-        for &MCDCBranch { true_bcb, false_bcb, .. } in mcdc_degraded_branches
-            .iter()
-            .chain(mcdc_mappings.iter().map(|(_, branches)| branches.into_iter()).flatten())
-        {
-            insert(true_bcb);
-            insert(false_bcb);
-        }
-
-        // MC/DC decisions refer to BCBs, but don't require those BCBs to have counters.
-        if bcbs_with_counter_mappings.is_empty() {
-            debug_assert!(
-                mcdc_mappings.is_empty(),
-                "A function with no counter mappings shouldn't have any decisions: {mcdc_mappings:?}",
-            );
-        }
-
-        bcbs_with_counter_mappings
-    }
-
-    /// Returns the set of BCBs that have one or more `Code` mappings.
-    pub(super) fn bcbs_with_ordinary_code_mappings(&self) -> DenseBitSet<BasicCoverageBlock> {
-        let mut bcbs = DenseBitSet::new_empty(self.num_bcbs);
-        for &CodeMapping { span: _, bcb } in &self.code_mappings {
-            bcbs.insert(bcb);
-        }
-        bcbs
-    }
-}
-
 fn resolve_block_markers(
     coverage_info_hi: &CoverageInfoHi,
     mir_body: &mir::Body<'_>,
diff --git a/compiler/rustc_mir_transform/src/coverage/mod.rs b/compiler/rustc_mir_transform/src/coverage/mod.rs
index 15487d05a30..1ccae0fd7fe 100644
--- a/compiler/rustc_mir_transform/src/coverage/mod.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mod.rs
@@ -10,7 +10,6 @@ mod unexpand;
 
 use rustc_hir as hir;
 use rustc_hir::intravisit::{Visitor, walk_expr};
-use rustc_middle::hir::map::Map;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::mir::coverage::{
     CoverageKind, DecisionInfo, FunctionCoverageInfo, Mapping, MappingKind,
@@ -21,7 +20,7 @@ use rustc_span::Span;
 use rustc_span::def_id::LocalDefId;
 use tracing::{debug, debug_span, trace};
 
-use crate::coverage::counters::CoverageCounters;
+use crate::coverage::counters::BcbCountersData;
 use crate::coverage::graph::CoverageGraph;
 use crate::coverage::mappings::ExtractedMappings;
 
@@ -82,28 +81,21 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
     let extracted_mappings =
         mappings::extract_all_mapping_info_from_mir(tcx, mir_body, &hir_info, &graph);
 
-    ////////////////////////////////////////////////////
-    // Create an optimized mix of `Counter`s and `Expression`s for the `CoverageGraph`. Ensure
-    // every coverage span has a `Counter` or `Expression` assigned to its `BasicCoverageBlock`
-    // and all `Expression` dependencies (operands) are also generated, for any other
-    // `BasicCoverageBlock`s not already associated with a coverage span.
-    let bcbs_with_counter_mappings = extracted_mappings.all_bcbs_with_counter_mappings();
-    if bcbs_with_counter_mappings.is_empty() {
-        // No relevant spans were found in MIR, so skip instrumenting this function.
-        return;
-    }
-
-    let coverage_counters = counters::make_bcb_counters(&graph, &bcbs_with_counter_mappings);
-
-    let mappings = create_mappings(&extracted_mappings, &coverage_counters);
+    let mappings = create_mappings(&extracted_mappings);
     if mappings.is_empty() {
         // No spans could be converted into valid mappings, so skip this function.
         debug!("no spans could be converted into valid mappings; skipping");
         return;
     }
 
-    inject_coverage_statements(mir_body, &graph, &extracted_mappings, &coverage_counters);
+    // Use the coverage graph to prepare intermediate data that will eventually
+    // be used to assign physical counters and counter expressions to points in
+    // the control-flow graph
+    let BcbCountersData { node_flow_data, priority_list } =
+        counters::prepare_bcb_counters_data(&graph);
 
+    // Inject coverage statements into MIR.
+    inject_coverage_statements(mir_body, &graph);
     inject_mcdc_statements(mir_body, &graph, &extracted_mappings);
 
     let mcdc_num_condition_bitmaps = extracted_mappings
@@ -116,29 +108,25 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
     mir_body.function_coverage_info = Some(Box::new(FunctionCoverageInfo {
         function_source_hash: hir_info.function_source_hash,
         body_span: hir_info.body_span,
-        num_counters: coverage_counters.num_counters(),
-        mcdc_bitmap_bits: extracted_mappings.mcdc_bitmap_bits,
-        expressions: coverage_counters.into_expressions(),
+
+        node_flow_data,
+        priority_list,
+
         mappings,
+
+        mcdc_bitmap_bits: extracted_mappings.mcdc_bitmap_bits,
         mcdc_num_condition_bitmaps,
     }));
 }
 
-/// For each coverage span extracted from MIR, create a corresponding
-/// mapping.
+/// For each coverage span extracted from MIR, create a corresponding mapping.
 ///
-/// Precondition: All BCBs corresponding to those spans have been given
-/// coverage counters.
-fn create_mappings(
-    extracted_mappings: &ExtractedMappings,
-    coverage_counters: &CoverageCounters,
-) -> Vec<Mapping> {
-    let term_for_bcb =
-        |bcb| coverage_counters.term_for_bcb(bcb).expect("all BCBs with spans were given counters");
-
+/// FIXME(Zalathar): This used to be where BCBs in the extracted mappings were
+/// resolved to a `CovTerm`. But that is now handled elsewhere, so this
+/// function can potentially be simplified even further.
+fn create_mappings(extracted_mappings: &ExtractedMappings) -> Vec<Mapping> {
     // Fully destructure the mappings struct to make sure we don't miss any kinds.
     let ExtractedMappings {
-        num_bcbs: _,
         code_mappings,
         branch_pairs,
         mcdc_bitmap_bits: _,
@@ -150,23 +138,18 @@ fn create_mappings(
     mappings.extend(code_mappings.iter().map(
         // Ordinary code mappings are the simplest kind.
         |&mappings::CodeMapping { span, bcb }| {
-            let kind = MappingKind::Code(term_for_bcb(bcb));
+            let kind = MappingKind::Code { bcb };
             Mapping { kind, span }
         },
     ));
 
     mappings.extend(branch_pairs.iter().map(
         |&mappings::BranchPair { span, true_bcb, false_bcb }| {
-            let true_term = term_for_bcb(true_bcb);
-            let false_term = term_for_bcb(false_bcb);
-            let kind = MappingKind::Branch { true_term, false_term };
+            let kind = MappingKind::Branch { true_bcb, false_bcb };
             Mapping { kind, span }
         },
     ));
 
-    let term_for_bcb =
-        |bcb| coverage_counters.term_for_bcb(bcb).expect("all BCBs with spans were given counters");
-
     // MCDC branch mappings are appended with their decisions in case decisions were ignored.
     mappings.extend(mcdc_degraded_branches.iter().map(
         |&mappings::MCDCBranch {
@@ -176,11 +159,7 @@ fn create_mappings(
              condition_info: _,
              true_index: _,
              false_index: _,
-         }| {
-            let true_term = term_for_bcb(true_bcb);
-            let false_term = term_for_bcb(false_bcb);
-            Mapping { kind: MappingKind::Branch { true_term, false_term }, span }
-        },
+         }| { Mapping { kind: MappingKind::Branch { true_bcb, false_bcb }, span } },
     ));
 
     for (decision, branches) in mcdc_mappings {
@@ -201,12 +180,10 @@ fn create_mappings(
                      true_index: _,
                      false_index: _,
                  }| {
-                    let true_term = term_for_bcb(true_bcb);
-                    let false_term = term_for_bcb(false_bcb);
                     Mapping {
                         kind: MappingKind::MCDCBranch {
-                            true_term,
-                            false_term,
+                            true_bcb,
+                            false_bcb,
                             mcdc_params: condition_info,
                         },
                         span,
@@ -227,41 +204,11 @@ fn create_mappings(
     mappings
 }
 
-/// For each BCB node or BCB edge that has an associated coverage counter,
-/// inject any necessary coverage statements into MIR.
-fn inject_coverage_statements<'tcx>(
-    mir_body: &mut mir::Body<'tcx>,
-    graph: &CoverageGraph,
-    extracted_mappings: &ExtractedMappings,
-    coverage_counters: &CoverageCounters,
-) {
-    // Inject counter-increment statements into MIR.
-    for (id, bcb) in coverage_counters.counter_increment_sites() {
-        let target_bb = graph[bcb].leader_bb();
-        inject_statement(mir_body, CoverageKind::CounterIncrement { id }, target_bb);
-    }
-
-    // For each counter expression that is directly associated with at least one
-    // span, we inject an "expression-used" statement, so that coverage codegen
-    // can check whether the injected statement survived MIR optimization.
-    // (BCB edges can't have spans, so we only need to process BCB nodes here.)
-    //
-    // We only do this for ordinary `Code` mappings, because branch and MC/DC
-    // mappings might have expressions that don't correspond to any single
-    // point in the control-flow graph.
-    //
-    // See the code in `rustc_codegen_llvm::coverageinfo::map_data` that deals
-    // with "expressions seen" and "zero terms".
-    let eligible_bcbs = extracted_mappings.bcbs_with_ordinary_code_mappings();
-    for (bcb, expression_id) in coverage_counters
-        .bcb_nodes_with_coverage_expressions()
-        .filter(|&(bcb, _)| eligible_bcbs.contains(bcb))
-    {
-        inject_statement(
-            mir_body,
-            CoverageKind::ExpressionUsed { id: expression_id },
-            graph[bcb].leader_bb(),
-        );
+/// Inject any necessary coverage statements into MIR, so that they influence codegen.
+fn inject_coverage_statements<'tcx>(mir_body: &mut mir::Body<'tcx>, graph: &CoverageGraph) {
+    for (bcb, data) in graph.iter_enumerated() {
+        let target_bb = data.leader_bb();
+        inject_statement(mir_body, CoverageKind::VirtualCounter { bcb }, target_bb);
     }
 }
 
@@ -343,7 +290,7 @@ fn extract_hir_info<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> ExtractedHir
 
     let hir_node = tcx.hir_node_by_def_id(def_id);
     let fn_body_id = hir_node.body_id().expect("HIR node is a function with body");
-    let hir_body = tcx.hir().body(fn_body_id);
+    let hir_body = tcx.hir_body(fn_body_id);
 
     let maybe_fn_sig = hir_node.fn_sig();
     let is_async_fn = maybe_fn_sig.is_some_and(|fn_sig| fn_sig.header.is_async());
@@ -399,35 +346,37 @@ fn extract_hole_spans_from_hir<'tcx>(
     body_span: Span, // Usually `hir_body.value.span`, but not always
     hir_body: &hir::Body<'tcx>,
 ) -> Vec<Span> {
-    struct HolesVisitor<'hir, F> {
-        hir: Map<'hir>,
-        visit_hole_span: F,
+    struct HolesVisitor<'tcx> {
+        tcx: TyCtxt<'tcx>,
+        body_span: Span,
+        hole_spans: Vec<Span>,
     }
 
-    impl<'hir, F: FnMut(Span)> Visitor<'hir> for HolesVisitor<'hir, F> {
-        /// - We need `NestedFilter::INTRA = true` so that `visit_item` will be called.
-        /// - Bodies of nested items don't actually get visited, because of the
-        ///   `visit_item` override.
-        /// - For nested bodies that are not part of an item, we do want to visit any
-        ///   items contained within them.
-        type NestedFilter = nested_filter::All;
+    impl<'tcx> Visitor<'tcx> for HolesVisitor<'tcx> {
+        /// We have special handling for nested items, but we still want to
+        /// traverse into nested bodies of things that are not considered items,
+        /// such as "anon consts" (e.g. array lengths).
+        type NestedFilter = nested_filter::OnlyBodies;
 
-        fn nested_visit_map(&mut self) -> Self::Map {
-            self.hir
+        fn maybe_tcx(&mut self) -> TyCtxt<'tcx> {
+            self.tcx
         }
 
-        fn visit_item(&mut self, item: &'hir hir::Item<'hir>) {
-            (self.visit_hole_span)(item.span);
+        /// We override `visit_nested_item` instead of `visit_item` because we
+        /// only need the item's span, not the item itself.
+        fn visit_nested_item(&mut self, id: hir::ItemId) -> Self::Result {
+            let span = self.tcx.def_span(id.owner_id.def_id);
+            self.visit_hole_span(span);
             // Having visited this item, we don't care about its children,
             // so don't call `walk_item`.
         }
 
         // We override `visit_expr` instead of the more specific expression
         // visitors, so that we have direct access to the expression span.
-        fn visit_expr(&mut self, expr: &'hir hir::Expr<'hir>) {
+        fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
             match expr.kind {
                 hir::ExprKind::Closure(_) | hir::ExprKind::ConstBlock(_) => {
-                    (self.visit_hole_span)(expr.span);
+                    self.visit_hole_span(expr.span);
                     // Having visited this expression, we don't care about its
                     // children, so don't call `walk_expr`.
                 }
@@ -437,18 +386,17 @@ fn extract_hole_spans_from_hir<'tcx>(
             }
         }
     }
-
-    let mut hole_spans = vec![];
-    let mut visitor = HolesVisitor {
-        hir: tcx.hir(),
-        visit_hole_span: |hole_span| {
+    impl HolesVisitor<'_> {
+        fn visit_hole_span(&mut self, hole_span: Span) {
             // Discard any holes that aren't directly visible within the body span.
-            if body_span.contains(hole_span) && body_span.eq_ctxt(hole_span) {
-                hole_spans.push(hole_span);
+            if self.body_span.contains(hole_span) && self.body_span.eq_ctxt(hole_span) {
+                self.hole_spans.push(hole_span);
             }
-        },
-    };
+        }
+    }
+
+    let mut visitor = HolesVisitor { tcx, body_span, hole_spans: vec![] };
 
     visitor.visit_body(hir_body);
-    hole_spans
+    visitor.hole_spans
 }
diff --git a/compiler/rustc_mir_transform/src/coverage/query.rs b/compiler/rustc_mir_transform/src/coverage/query.rs
index a849ed4c3e2..ccf76dc7108 100644
--- a/compiler/rustc_mir_transform/src/coverage/query.rs
+++ b/compiler/rustc_mir_transform/src/coverage/query.rs
@@ -1,10 +1,6 @@
-use rustc_data_structures::captures::Captures;
 use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
-use rustc_middle::mir::coverage::{
-    CounterId, CovTerm, CoverageIdsInfo, CoverageKind, Expression, ExpressionId,
-    FunctionCoverageInfo, MappingKind, Op,
-};
+use rustc_middle::mir::coverage::{BasicCoverageBlock, CoverageIdsInfo, CoverageKind, MappingKind};
 use rustc_middle::mir::{Body, Statement, StatementKind};
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_middle::util::Providers;
@@ -12,6 +8,9 @@ use rustc_span::def_id::LocalDefId;
 use rustc_span::sym;
 use tracing::trace;
 
+use crate::coverage::counters::node_flow::make_node_counters;
+use crate::coverage::counters::{CoverageCounters, transcribe_counters};
+
 /// Registers query/hook implementations related to coverage.
 pub(crate) fn provide(providers: &mut Providers) {
     providers.hooks.is_eligible_for_coverage = is_eligible_for_coverage;
@@ -66,7 +65,7 @@ fn coverage_attr_on(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
             Some(_) | None => {
                 // Other possibilities should have been rejected by `rustc_parse::validate_attr`.
                 // Use `span_delayed_bug` to avoid an ICE in failing builds (#127880).
-                tcx.dcx().span_delayed_bug(attr.span, "unexpected value of coverage attribute");
+                tcx.dcx().span_delayed_bug(attr.span(), "unexpected value of coverage attribute");
             }
         }
     }
@@ -89,44 +88,71 @@ fn coverage_ids_info<'tcx>(
     let mir_body = tcx.instance_mir(instance_def);
     let fn_cov_info = mir_body.function_coverage_info.as_deref()?;
 
-    let mut counters_seen = DenseBitSet::new_empty(fn_cov_info.num_counters);
-    let mut expressions_seen = DenseBitSet::new_filled(fn_cov_info.expressions.len());
-
-    // For each expression ID that is directly used by one or more mappings,
-    // mark it as not-yet-seen. This indicates that we expect to see a
-    // corresponding `ExpressionUsed` statement during MIR traversal.
-    for mapping in fn_cov_info.mappings.iter() {
-        // Currently we only worry about ordinary code mappings.
-        // For branch and MC/DC mappings, expressions might not correspond
-        // to any particular point in the control-flow graph.
-        // (Keep this in sync with the injection of `ExpressionUsed`
-        // statements in the `InstrumentCoverage` MIR pass.)
-        if let MappingKind::Code(CovTerm::Expression(id)) = mapping.kind {
-            expressions_seen.remove(id);
-        }
-    }
-
+    // Scan through the final MIR to see which BCBs survived MIR opts.
+    // Any BCB not in this set was optimized away.
+    let mut bcbs_seen = DenseBitSet::new_empty(fn_cov_info.priority_list.len());
     for kind in all_coverage_in_mir_body(mir_body) {
         match *kind {
-            CoverageKind::CounterIncrement { id } => {
-                counters_seen.insert(id);
-            }
-            CoverageKind::ExpressionUsed { id } => {
-                expressions_seen.insert(id);
+            CoverageKind::VirtualCounter { bcb } => {
+                bcbs_seen.insert(bcb);
             }
             _ => {}
         }
     }
 
-    let zero_expressions =
-        identify_zero_expressions(fn_cov_info, &counters_seen, &expressions_seen);
+    // Determine the set of BCBs that are referred to by mappings, and therefore
+    // need a counter. Any node not in this set will only get a counter if it
+    // is part of the counter expression for a node that is in the set.
+    let mut bcb_needs_counter =
+        DenseBitSet::<BasicCoverageBlock>::new_empty(fn_cov_info.priority_list.len());
+    for mapping in &fn_cov_info.mappings {
+        match mapping.kind {
+            MappingKind::Code { bcb } => {
+                bcb_needs_counter.insert(bcb);
+            }
+            MappingKind::Branch { true_bcb, false_bcb } => {
+                bcb_needs_counter.insert(true_bcb);
+                bcb_needs_counter.insert(false_bcb);
+            }
+            MappingKind::MCDCBranch { true_bcb, false_bcb, mcdc_params: _ } => {
+                bcb_needs_counter.insert(true_bcb);
+                bcb_needs_counter.insert(false_bcb);
+            }
+            MappingKind::MCDCDecision(_) => {}
+        }
+    }
 
-    Some(CoverageIdsInfo { counters_seen, zero_expressions })
+    // Clone the priority list so that we can re-sort it.
+    let mut priority_list = fn_cov_info.priority_list.clone();
+    // The first ID in the priority list represents the synthetic "sink" node,
+    // and must remain first so that it _never_ gets a physical counter.
+    debug_assert_eq!(priority_list[0], priority_list.iter().copied().max().unwrap());
+    assert!(!bcbs_seen.contains(priority_list[0]));
+    // Partition the priority list, so that unreachable nodes (removed by MIR opts)
+    // are sorted later and therefore are _more_ likely to get a physical counter.
+    // This is counter-intuitive, but it means that `transcribe_counters` can
+    // easily skip those unused physical counters and replace them with zero.
+    // (The original ordering remains in effect within both partitions.)
+    priority_list[1..].sort_by_key(|&bcb| !bcbs_seen.contains(bcb));
+
+    let node_counters = make_node_counters(&fn_cov_info.node_flow_data, &priority_list);
+    let coverage_counters = transcribe_counters(&node_counters, &bcb_needs_counter, &bcbs_seen);
+
+    let CoverageCounters {
+        phys_counter_for_node, next_counter_id, node_counters, expressions, ..
+    } = coverage_counters;
+
+    Some(CoverageIdsInfo {
+        num_counters: next_counter_id.as_u32(),
+        phys_counter_for_node,
+        term_for_bcb: node_counters,
+        expressions,
+    })
 }
 
 fn all_coverage_in_mir_body<'a, 'tcx>(
     body: &'a Body<'tcx>,
-) -> impl Iterator<Item = &'a CoverageKind> + Captures<'tcx> {
+) -> impl Iterator<Item = &'a CoverageKind> {
     body.basic_blocks.iter().flat_map(|bb_data| &bb_data.statements).filter_map(|statement| {
         match statement.kind {
             StatementKind::Coverage(ref kind) if !is_inlined(body, statement) => Some(kind),
@@ -139,94 +165,3 @@ fn is_inlined(body: &Body<'_>, statement: &Statement<'_>) -> bool {
     let scope_data = &body.source_scopes[statement.source_info.scope];
     scope_data.inlined.is_some() || scope_data.inlined_parent_scope.is_some()
 }
-
-/// Identify expressions that will always have a value of zero, and note their
-/// IDs in a `DenseBitSet`. Mappings that refer to a zero expression can instead
-/// become mappings to a constant zero value.
-///
-/// This function mainly exists to preserve the simplifications that were
-/// already being performed by the Rust-side expression renumbering, so that
-/// the resulting coverage mappings don't get worse.
-fn identify_zero_expressions(
-    fn_cov_info: &FunctionCoverageInfo,
-    counters_seen: &DenseBitSet<CounterId>,
-    expressions_seen: &DenseBitSet<ExpressionId>,
-) -> DenseBitSet<ExpressionId> {
-    // The set of expressions that either were optimized out entirely, or
-    // have zero as both of their operands, and will therefore always have
-    // a value of zero. Other expressions that refer to these as operands
-    // can have those operands replaced with `CovTerm::Zero`.
-    let mut zero_expressions = DenseBitSet::new_empty(fn_cov_info.expressions.len());
-
-    // Simplify a copy of each expression based on lower-numbered expressions,
-    // and then update the set of always-zero expressions if necessary.
-    // (By construction, expressions can only refer to other expressions
-    // that have lower IDs, so one pass is sufficient.)
-    for (id, expression) in fn_cov_info.expressions.iter_enumerated() {
-        if !expressions_seen.contains(id) {
-            // If an expression was not seen, it must have been optimized away,
-            // so any operand that refers to it can be replaced with zero.
-            zero_expressions.insert(id);
-            continue;
-        }
-
-        // We don't need to simplify the actual expression data in the
-        // expressions list; we can just simplify a temporary copy and then
-        // use that to update the set of always-zero expressions.
-        let Expression { mut lhs, op, mut rhs } = *expression;
-
-        // If an expression has an operand that is also an expression, the
-        // operand's ID must be strictly lower. This is what lets us find
-        // all zero expressions in one pass.
-        let assert_operand_expression_is_lower = |operand_id: ExpressionId| {
-            assert!(
-                operand_id < id,
-                "Operand {operand_id:?} should be less than {id:?} in {expression:?}",
-            )
-        };
-
-        // If an operand refers to a counter or expression that is always
-        // zero, then that operand can be replaced with `CovTerm::Zero`.
-        let maybe_set_operand_to_zero = |operand: &mut CovTerm| {
-            if let CovTerm::Expression(id) = *operand {
-                assert_operand_expression_is_lower(id);
-            }
-
-            if is_zero_term(&counters_seen, &zero_expressions, *operand) {
-                *operand = CovTerm::Zero;
-            }
-        };
-        maybe_set_operand_to_zero(&mut lhs);
-        maybe_set_operand_to_zero(&mut rhs);
-
-        // Coverage counter values cannot be negative, so if an expression
-        // involves subtraction from zero, assume that its RHS must also be zero.
-        // (Do this after simplifications that could set the LHS to zero.)
-        if lhs == CovTerm::Zero && op == Op::Subtract {
-            rhs = CovTerm::Zero;
-        }
-
-        // After the above simplifications, if both operands are zero, then
-        // we know that this expression is always zero too.
-        if lhs == CovTerm::Zero && rhs == CovTerm::Zero {
-            zero_expressions.insert(id);
-        }
-    }
-
-    zero_expressions
-}
-
-/// Returns `true` if the given term is known to have a value of zero, taking
-/// into account knowledge of which counters are unused and which expressions
-/// are always zero.
-fn is_zero_term(
-    counters_seen: &DenseBitSet<CounterId>,
-    zero_expressions: &DenseBitSet<ExpressionId>,
-    term: CovTerm,
-) -> bool {
-    match term {
-        CovTerm::Zero => true,
-        CovTerm::Counter(id) => !counters_seen.contains(id),
-        CovTerm::Expression(id) => zero_expressions.contains(id),
-    }
-}
diff --git a/compiler/rustc_mir_transform/src/coverage/spans.rs b/compiler/rustc_mir_transform/src/coverage/spans.rs
index 314a86ea52f..b9ed6984ddb 100644
--- a/compiler/rustc_mir_transform/src/coverage/spans.rs
+++ b/compiler/rustc_mir_transform/src/coverage/spans.rs
@@ -1,6 +1,5 @@
 use std::collections::VecDeque;
 
-use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_middle::mir;
 use rustc_span::{DesugaringKind, ExpnKind, MacroKind, Span};
@@ -182,7 +181,7 @@ fn divide_spans_into_buckets(input_covspans: Vec<Covspan>, holes: &[Hole]) -> Ve
 fn drain_front_while<'a, T>(
     queue: &'a mut VecDeque<T>,
     mut pred_fn: impl FnMut(&T) -> bool,
-) -> impl Iterator<Item = T> + Captures<'a> {
+) -> impl Iterator<Item = T> {
     std::iter::from_fn(move || if pred_fn(queue.front()?) { queue.pop_front() } else { None })
 }
 
diff --git a/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs b/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
index 26ce743be36..73b68d7155c 100644
--- a/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
+++ b/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
@@ -137,8 +137,7 @@ fn filtered_statement_span(statement: &Statement<'_>) -> Option<Span> {
 
         // These coverage statements should not exist prior to coverage instrumentation.
         StatementKind::Coverage(
-            CoverageKind::CounterIncrement { .. }
-            | CoverageKind::ExpressionUsed { .. }
+            CoverageKind::VirtualCounter { .. }
             | CoverageKind::CondBitmapUpdate { .. }
             | CoverageKind::TestVectorBitmapUpdate { .. },
         ) => bug!(