1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
use std::fmt::{self, Debug};

use rustc_data_structures::captures::Captures;
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::graph::DirectedGraph;
use rustc_index::IndexVec;
use rustc_middle::bug;
use rustc_middle::mir::coverage::{CounterId, CovTerm, Expression, ExpressionId, Op};

use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph, TraverseCoverageGraphWithLoops};

/// The coverage counter or counter expression associated with a particular
/// BCB node or BCB edge.
#[derive(Clone, Copy, PartialEq, Eq, Hash)]
pub(super) enum BcbCounter {
    Counter { id: CounterId },
    Expression { id: ExpressionId },
}

impl BcbCounter {
    pub(super) fn as_term(&self) -> CovTerm {
        match *self {
            BcbCounter::Counter { id, .. } => CovTerm::Counter(id),
            BcbCounter::Expression { id, .. } => CovTerm::Expression(id),
        }
    }
}

impl Debug for BcbCounter {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Counter { id, .. } => write!(fmt, "Counter({:?})", id.index()),
            Self::Expression { id } => write!(fmt, "Expression({:?})", id.index()),
        }
    }
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
struct BcbExpression {
    lhs: BcbCounter,
    op: Op,
    rhs: BcbCounter,
}

#[derive(Debug)]
pub(super) enum CounterIncrementSite {
    Node { bcb: BasicCoverageBlock },
    Edge { from_bcb: BasicCoverageBlock, to_bcb: BasicCoverageBlock },
}

/// Generates and stores coverage counter and coverage expression information
/// associated with nodes/edges in the BCB graph.
pub(super) struct CoverageCounters {
    /// List of places where a counter-increment statement should be injected
    /// into MIR, each with its corresponding counter ID.
    counter_increment_sites: IndexVec<CounterId, CounterIncrementSite>,

    /// Coverage counters/expressions that are associated with individual BCBs.
    bcb_counters: IndexVec<BasicCoverageBlock, Option<BcbCounter>>,
    /// Coverage counters/expressions that are associated with the control-flow
    /// edge between two BCBs.
    ///
    /// We currently don't iterate over this map, but if we do in the future,
    /// switch it back to `FxIndexMap` to avoid query stability hazards.
    bcb_edge_counters: FxHashMap<(BasicCoverageBlock, BasicCoverageBlock), BcbCounter>,

    /// Table of expression data, associating each expression ID with its
    /// corresponding operator (+ or -) and its LHS/RHS operands.
    expressions: IndexVec<ExpressionId, BcbExpression>,
    /// Remember expressions that have already been created (or simplified),
    /// so that we don't create unnecessary duplicates.
    expressions_memo: FxHashMap<BcbExpression, BcbCounter>,
}

impl CoverageCounters {
    /// Makes [`BcbCounter`] `Counter`s and `Expressions` for the `BasicCoverageBlock`s directly or
    /// indirectly associated with coverage spans, and accumulates additional `Expression`s
    /// representing intermediate values.
    pub(super) fn make_bcb_counters(
        basic_coverage_blocks: &CoverageGraph,
        bcb_has_coverage_spans: impl Fn(BasicCoverageBlock) -> bool,
    ) -> Self {
        let num_bcbs = basic_coverage_blocks.num_nodes();

        let mut this = Self {
            counter_increment_sites: IndexVec::new(),
            bcb_counters: IndexVec::from_elem_n(None, num_bcbs),
            bcb_edge_counters: FxHashMap::default(),
            expressions: IndexVec::new(),
            expressions_memo: FxHashMap::default(),
        };

        MakeBcbCounters::new(&mut this, basic_coverage_blocks)
            .make_bcb_counters(bcb_has_coverage_spans);

        this
    }

    fn make_counter(&mut self, site: CounterIncrementSite) -> BcbCounter {
        let id = self.counter_increment_sites.push(site);
        BcbCounter::Counter { id }
    }

    fn make_expression(&mut self, lhs: BcbCounter, op: Op, rhs: BcbCounter) -> BcbCounter {
        let new_expr = BcbExpression { lhs, op, rhs };
        *self
            .expressions_memo
            .entry(new_expr)
            .or_insert_with(|| Self::make_expression_inner(&mut self.expressions, new_expr))
    }

    /// This is an associated function so that we can call it while borrowing
    /// `&mut self.expressions_memo`.
    fn make_expression_inner(
        expressions: &mut IndexVec<ExpressionId, BcbExpression>,
        new_expr: BcbExpression,
    ) -> BcbCounter {
        // Simplify expressions using basic algebra.
        //
        // Some of these cases might not actually occur in practice, depending
        // on the details of how the instrumentor builds expressions.
        let BcbExpression { lhs, op, rhs } = new_expr;

        if let BcbCounter::Expression { id } = lhs {
            let lhs_expr = &expressions[id];

            // Simplify `(a - b) + b` to `a`.
            if lhs_expr.op == Op::Subtract && op == Op::Add && lhs_expr.rhs == rhs {
                return lhs_expr.lhs;
            }
            // Simplify `(a + b) - b` to `a`.
            if lhs_expr.op == Op::Add && op == Op::Subtract && lhs_expr.rhs == rhs {
                return lhs_expr.lhs;
            }
            // Simplify `(a + b) - a` to `b`.
            if lhs_expr.op == Op::Add && op == Op::Subtract && lhs_expr.lhs == rhs {
                return lhs_expr.rhs;
            }
        }

        if let BcbCounter::Expression { id } = rhs {
            let rhs_expr = &expressions[id];

            // Simplify `a + (b - a)` to `b`.
            if op == Op::Add && rhs_expr.op == Op::Subtract && lhs == rhs_expr.rhs {
                return rhs_expr.lhs;
            }
            // Simplify `a - (a - b)` to `b`.
            if op == Op::Subtract && rhs_expr.op == Op::Subtract && lhs == rhs_expr.lhs {
                return rhs_expr.rhs;
            }
        }

        // Simplification failed, so actually create the new expression.
        let id = expressions.push(new_expr);
        BcbCounter::Expression { id }
    }

    /// Variant of `make_expression` that makes `lhs` optional and assumes [`Op::Add`].
    ///
    /// This is useful when using [`Iterator::fold`] to build an arbitrary-length sum.
    fn make_sum_expression(&mut self, lhs: Option<BcbCounter>, rhs: BcbCounter) -> BcbCounter {
        let Some(lhs) = lhs else { return rhs };
        self.make_expression(lhs, Op::Add, rhs)
    }

    pub(super) fn num_counters(&self) -> usize {
        self.counter_increment_sites.len()
    }

    #[cfg(test)]
    pub(super) fn num_expressions(&self) -> usize {
        self.expressions.len()
    }

    fn set_bcb_counter(&mut self, bcb: BasicCoverageBlock, counter_kind: BcbCounter) -> BcbCounter {
        if let Some(replaced) = self.bcb_counters[bcb].replace(counter_kind) {
            bug!(
                "attempt to set a BasicCoverageBlock coverage counter more than once; \
                {bcb:?} already had counter {replaced:?}",
            );
        } else {
            counter_kind
        }
    }

    fn set_bcb_edge_counter(
        &mut self,
        from_bcb: BasicCoverageBlock,
        to_bcb: BasicCoverageBlock,
        counter_kind: BcbCounter,
    ) -> BcbCounter {
        if let Some(replaced) = self.bcb_edge_counters.insert((from_bcb, to_bcb), counter_kind) {
            bug!(
                "attempt to set an edge counter more than once; from_bcb: \
                {from_bcb:?} already had counter {replaced:?}",
            );
        } else {
            counter_kind
        }
    }

    pub(super) fn bcb_counter(&self, bcb: BasicCoverageBlock) -> Option<BcbCounter> {
        self.bcb_counters[bcb]
    }

    /// Returns an iterator over all the nodes/edges 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, &CounterIncrementSite)> {
        self.counter_increment_sites.iter_enumerated()
    }

    /// 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.bcb_counters.iter_enumerated().filter_map(|(bcb, &counter_kind)| match counter_kind {
            // Yield the BCB along with its associated expression ID.
            Some(BcbCounter::Expression { id }) => Some((bcb, id)),
            // This BCB is associated with a counter or nothing, so skip it.
            Some(BcbCounter::Counter { .. }) | None => None,
        })
    }

    pub(super) fn into_expressions(self) -> IndexVec<ExpressionId, Expression> {
        let old_len = self.expressions.len();
        let expressions = self
            .expressions
            .into_iter()
            .map(|BcbExpression { lhs, op, rhs }| Expression {
                lhs: lhs.as_term(),
                op,
                rhs: rhs.as_term(),
            })
            .collect::<IndexVec<ExpressionId, _>>();

        // Expression IDs are indexes into this vector, so make sure we didn't
        // accidentally invalidate them by changing its length.
        assert_eq!(old_len, expressions.len());
        expressions
    }
}

/// Traverse the `CoverageGraph` and add either a `Counter` or `Expression` to every BCB, to be
/// injected with coverage spans. `Expressions` have no runtime overhead, so if a viable expression
/// (adding or subtracting two other counters or expressions) can compute the same result as an
/// embedded counter, an `Expression` should be used.
struct MakeBcbCounters<'a> {
    coverage_counters: &'a mut CoverageCounters,
    basic_coverage_blocks: &'a CoverageGraph,
}

impl<'a> MakeBcbCounters<'a> {
    fn new(
        coverage_counters: &'a mut CoverageCounters,
        basic_coverage_blocks: &'a CoverageGraph,
    ) -> Self {
        Self { coverage_counters, basic_coverage_blocks }
    }

    /// If two `BasicCoverageBlock`s branch from another `BasicCoverageBlock`, one of the branches
    /// can be counted by `Expression` by subtracting the other branch from the branching
    /// block. Otherwise, the `BasicCoverageBlock` executed the least should have the `Counter`.
    /// One way to predict which branch executes the least is by considering loops. A loop is exited
    /// at a branch, so the branch that jumps to a `BasicCoverageBlock` outside the loop is almost
    /// always executed less than the branch that does not exit the loop.
    fn make_bcb_counters(&mut self, bcb_has_coverage_spans: impl Fn(BasicCoverageBlock) -> bool) {
        debug!("make_bcb_counters(): adding a counter or expression to each BasicCoverageBlock");

        // Walk the `CoverageGraph`. For each `BasicCoverageBlock` node with an associated
        // coverage span, add a counter. If the `BasicCoverageBlock` branches, add a counter or
        // expression to each branch `BasicCoverageBlock` (if the branch BCB has only one incoming
        // edge) or edge from the branching BCB to the branch BCB (if the branch BCB has multiple
        // incoming edges).
        //
        // The `TraverseCoverageGraphWithLoops` traversal ensures that, when a loop is encountered,
        // all `BasicCoverageBlock` nodes in the loop are visited before visiting any node outside
        // the loop. The `traversal` state includes a `context_stack`, providing a way to know if
        // the current BCB is in one or more nested loops or not.
        let mut traversal = TraverseCoverageGraphWithLoops::new(self.basic_coverage_blocks);
        while let Some(bcb) = traversal.next() {
            if bcb_has_coverage_spans(bcb) {
                debug!("{:?} has at least one coverage span. Get or make its counter", bcb);
                self.make_node_and_branch_counters(&traversal, bcb);
            } else {
                debug!(
                    "{:?} does not have any coverage spans. A counter will only be added if \
                    and when a covered BCB has an expression dependency.",
                    bcb,
                );
            }
        }

        assert!(
            traversal.is_complete(),
            "`TraverseCoverageGraphWithLoops` missed some `BasicCoverageBlock`s: {:?}",
            traversal.unvisited(),
        );
    }

    fn make_node_and_branch_counters(
        &mut self,
        traversal: &TraverseCoverageGraphWithLoops<'_>,
        from_bcb: BasicCoverageBlock,
    ) {
        // First, ensure that this node has a counter of some kind.
        // We might also use its term later to compute one of the branch counters.
        let from_bcb_operand = self.get_or_make_counter_operand(from_bcb);

        let branch_target_bcbs = self.basic_coverage_blocks.successors[from_bcb].as_slice();

        // If this node doesn't have multiple out-edges, or all of its out-edges
        // already have counters, then we don't need to create edge counters.
        let needs_branch_counters = branch_target_bcbs.len() > 1
            && branch_target_bcbs
                .iter()
                .any(|&to_bcb| self.branch_has_no_counter(from_bcb, to_bcb));
        if !needs_branch_counters {
            return;
        }

        debug!(
            "{from_bcb:?} has some branch(es) without counters:\n  {}",
            branch_target_bcbs
                .iter()
                .map(|&to_bcb| {
                    format!("{from_bcb:?}->{to_bcb:?}: {:?}", self.branch_counter(from_bcb, to_bcb))
                })
                .collect::<Vec<_>>()
                .join("\n  "),
        );

        // Of the branch edges that don't have counters yet, one can be given an expression
        // (computed from the other edges) instead of a dedicated counter.
        let expression_to_bcb = self.choose_preferred_expression_branch(traversal, from_bcb);

        // For each branch arm other than the one that was chosen to get an expression,
        // ensure that it has a counter (existing counter/expression or a new counter),
        // and accumulate the corresponding terms into a single sum term.
        let sum_of_all_other_branches: BcbCounter = {
            let _span = debug_span!("sum_of_all_other_branches", ?expression_to_bcb).entered();
            branch_target_bcbs
                .iter()
                .copied()
                // Skip the chosen branch, since we'll calculate it from the other branches.
                .filter(|&to_bcb| to_bcb != expression_to_bcb)
                .fold(None, |accum, to_bcb| {
                    let _span = debug_span!("to_bcb", ?accum, ?to_bcb).entered();
                    let branch_counter = self.get_or_make_edge_counter_operand(from_bcb, to_bcb);
                    Some(self.coverage_counters.make_sum_expression(accum, branch_counter))
                })
                .expect("there must be at least one other branch")
        };

        // For the branch that was chosen to get an expression, create that expression
        // by taking the count of the node we're branching from, and subtracting the
        // sum of all the other branches.
        debug!(
            "Making an expression for the selected expression_branch: \
            {expression_to_bcb:?} (expression_branch predecessors: {:?})",
            self.bcb_predecessors(expression_to_bcb),
        );
        let expression = self.coverage_counters.make_expression(
            from_bcb_operand,
            Op::Subtract,
            sum_of_all_other_branches,
        );
        debug!("{expression_to_bcb:?} gets an expression: {expression:?}");
        if self.basic_coverage_blocks.bcb_has_multiple_in_edges(expression_to_bcb) {
            self.coverage_counters.set_bcb_edge_counter(from_bcb, expression_to_bcb, expression);
        } else {
            self.coverage_counters.set_bcb_counter(expression_to_bcb, expression);
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn get_or_make_counter_operand(&mut self, bcb: BasicCoverageBlock) -> BcbCounter {
        // If the BCB already has a counter, return it.
        if let Some(counter_kind) = self.coverage_counters.bcb_counters[bcb] {
            debug!("{bcb:?} already has a counter: {counter_kind:?}");
            return counter_kind;
        }

        // A BCB with only one incoming edge gets a simple `Counter` (via `make_counter()`).
        // Also, a BCB that loops back to itself gets a simple `Counter`. This may indicate the
        // program results in a tight infinite loop, but it should still compile.
        let one_path_to_target = !self.basic_coverage_blocks.bcb_has_multiple_in_edges(bcb);
        if one_path_to_target || self.bcb_predecessors(bcb).contains(&bcb) {
            let counter_kind =
                self.coverage_counters.make_counter(CounterIncrementSite::Node { bcb });
            if one_path_to_target {
                debug!("{bcb:?} gets a new counter: {counter_kind:?}");
            } else {
                debug!(
                    "{bcb:?} has itself as its own predecessor. It can't be part of its own \
                    Expression sum, so it will get its own new counter: {counter_kind:?}. \
                    (Note, the compiled code will generate an infinite loop.)",
                );
            }
            return self.coverage_counters.set_bcb_counter(bcb, counter_kind);
        }

        // A BCB with multiple incoming edges can compute its count by ensuring that counters
        // exist for each of those edges, and then adding them up to get a total count.
        let sum_of_in_edges: BcbCounter = {
            let _span = debug_span!("sum_of_in_edges", ?bcb).entered();
            // We avoid calling `self.bcb_predecessors` here so that we can
            // call methods on `&mut self` inside the fold.
            self.basic_coverage_blocks.predecessors[bcb]
                .iter()
                .copied()
                .fold(None, |accum, from_bcb| {
                    let _span = debug_span!("from_bcb", ?accum, ?from_bcb).entered();
                    let edge_counter = self.get_or_make_edge_counter_operand(from_bcb, bcb);
                    Some(self.coverage_counters.make_sum_expression(accum, edge_counter))
                })
                .expect("there must be at least one in-edge")
        };

        debug!("{bcb:?} gets a new counter (sum of predecessor counters): {sum_of_in_edges:?}");
        self.coverage_counters.set_bcb_counter(bcb, sum_of_in_edges)
    }

    #[instrument(level = "debug", skip(self))]
    fn get_or_make_edge_counter_operand(
        &mut self,
        from_bcb: BasicCoverageBlock,
        to_bcb: BasicCoverageBlock,
    ) -> BcbCounter {
        // If the target BCB has only one in-edge (i.e. this one), then create
        // a node counter instead, since it will have the same value.
        if !self.basic_coverage_blocks.bcb_has_multiple_in_edges(to_bcb) {
            assert_eq!([from_bcb].as_slice(), self.basic_coverage_blocks.predecessors[to_bcb]);
            return self.get_or_make_counter_operand(to_bcb);
        }

        // If the source BCB has only one successor (assumed to be the given target), an edge
        // counter is unnecessary. Just get or make a counter for the source BCB.
        if self.bcb_successors(from_bcb).len() == 1 {
            return self.get_or_make_counter_operand(from_bcb);
        }

        // If the edge already has a counter, return it.
        if let Some(&counter_kind) =
            self.coverage_counters.bcb_edge_counters.get(&(from_bcb, to_bcb))
        {
            debug!("Edge {from_bcb:?}->{to_bcb:?} already has a counter: {counter_kind:?}");
            return counter_kind;
        }

        // Make a new counter to count this edge.
        let counter_kind =
            self.coverage_counters.make_counter(CounterIncrementSite::Edge { from_bcb, to_bcb });
        debug!("Edge {from_bcb:?}->{to_bcb:?} gets a new counter: {counter_kind:?}");
        self.coverage_counters.set_bcb_edge_counter(from_bcb, to_bcb, counter_kind)
    }

    /// Select a branch for the expression, either the recommended `reloop_branch`, or if none was
    /// found, select any branch.
    fn choose_preferred_expression_branch(
        &self,
        traversal: &TraverseCoverageGraphWithLoops<'_>,
        from_bcb: BasicCoverageBlock,
    ) -> BasicCoverageBlock {
        let good_reloop_branch = self.find_good_reloop_branch(traversal, from_bcb);
        if let Some(reloop_target) = good_reloop_branch {
            assert!(self.branch_has_no_counter(from_bcb, reloop_target));
            debug!("Selecting reloop target {reloop_target:?} to get an expression");
            reloop_target
        } else {
            let &branch_without_counter = self
                .bcb_successors(from_bcb)
                .iter()
                .find(|&&to_bcb| self.branch_has_no_counter(from_bcb, to_bcb))
                .expect(
                    "needs_branch_counters was `true` so there should be at least one \
                    branch",
                );
            debug!(
                "Selecting any branch={:?} that still needs a counter, to get the \
                `Expression` because there was no `reloop_branch`, or it already had a \
                counter",
                branch_without_counter
            );
            branch_without_counter
        }
    }

    /// Tries to find a branch that leads back to the top of a loop, and that
    /// doesn't already have a counter. Such branches are good candidates to
    /// be given an expression (instead of a physical counter), because they
    /// will tend to be executed more times than a loop-exit branch.
    fn find_good_reloop_branch(
        &self,
        traversal: &TraverseCoverageGraphWithLoops<'_>,
        from_bcb: BasicCoverageBlock,
    ) -> Option<BasicCoverageBlock> {
        let branch_target_bcbs = self.bcb_successors(from_bcb);

        // Consider each loop on the current traversal context stack, top-down.
        for reloop_bcbs in traversal.reloop_bcbs_per_loop() {
            let mut all_branches_exit_this_loop = true;

            // Try to find a branch that doesn't exit this loop and doesn't
            // already have a counter.
            for &branch_target_bcb in branch_target_bcbs {
                // A branch is a reloop branch if it dominates any BCB that has
                // an edge back to the loop header. (Other branches are exits.)
                let is_reloop_branch = reloop_bcbs.iter().any(|&reloop_bcb| {
                    self.basic_coverage_blocks.dominates(branch_target_bcb, reloop_bcb)
                });

                if is_reloop_branch {
                    all_branches_exit_this_loop = false;
                    if self.branch_has_no_counter(from_bcb, branch_target_bcb) {
                        // We found a good branch to be given an expression.
                        return Some(branch_target_bcb);
                    }
                    // Keep looking for another reloop branch without a counter.
                } else {
                    // This branch exits the loop.
                }
            }

            if !all_branches_exit_this_loop {
                // We found one or more reloop branches, but all of them already
                // have counters. Let the caller choose one of the exit branches.
                debug!("All reloop branches had counters; skip checking the other loops");
                return None;
            }

            // All of the branches exit this loop, so keep looking for a good
            // reloop branch for one of the outer loops.
        }

        None
    }

    #[inline]
    fn bcb_predecessors(&self, bcb: BasicCoverageBlock) -> &[BasicCoverageBlock] {
        &self.basic_coverage_blocks.predecessors[bcb]
    }

    #[inline]
    fn bcb_successors(&self, bcb: BasicCoverageBlock) -> &[BasicCoverageBlock] {
        &self.basic_coverage_blocks.successors[bcb]
    }

    #[inline]
    fn branch_has_no_counter(
        &self,
        from_bcb: BasicCoverageBlock,
        to_bcb: BasicCoverageBlock,
    ) -> bool {
        self.branch_counter(from_bcb, to_bcb).is_none()
    }

    fn branch_counter(
        &self,
        from_bcb: BasicCoverageBlock,
        to_bcb: BasicCoverageBlock,
    ) -> Option<&BcbCounter> {
        if self.basic_coverage_blocks.bcb_has_multiple_in_edges(to_bcb) {
            self.coverage_counters.bcb_edge_counters.get(&(from_bcb, to_bcb))
        } else {
            self.coverage_counters.bcb_counters[to_bcb].as_ref()
        }
    }
}