rustc_codegen_llvm/coverageinfo/
map_data.rs

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
use rustc_data_structures::captures::Captures;
use rustc_data_structures::fx::FxIndexSet;
use rustc_index::bit_set::BitSet;
use rustc_middle::mir::coverage::{
    CounterId, CovTerm, Expression, ExpressionId, FunctionCoverageInfo, Mapping, MappingKind, Op,
    SourceRegion,
};
use rustc_middle::ty::Instance;
use rustc_span::Symbol;
use tracing::{debug, instrument};

use crate::coverageinfo::ffi::{Counter, CounterExpression, ExprKind};

/// Holds all of the coverage mapping data associated with a function instance,
/// collected during traversal of `Coverage` statements in the function's MIR.
#[derive(Debug)]
pub(crate) struct FunctionCoverageCollector<'tcx> {
    /// Coverage info that was attached to this function by the instrumentor.
    function_coverage_info: &'tcx FunctionCoverageInfo,
    is_used: bool,

    /// Tracks which counters have been seen, so that we can identify mappings
    /// to counters that were optimized out, and set them to zero.
    counters_seen: BitSet<CounterId>,
    /// Contains all expression IDs that have been seen in an `ExpressionUsed`
    /// coverage statement, plus all expression IDs that aren't directly used
    /// by any mappings (and therefore do not have expression-used statements).
    /// After MIR traversal is finished, we can conclude that any IDs missing
    /// from this set must have had their statements deleted by MIR opts.
    expressions_seen: BitSet<ExpressionId>,
}

impl<'tcx> FunctionCoverageCollector<'tcx> {
    /// Creates a new set of coverage data for a used (called) function.
    pub(crate) fn new(
        instance: Instance<'tcx>,
        function_coverage_info: &'tcx FunctionCoverageInfo,
    ) -> Self {
        Self::create(instance, function_coverage_info, true)
    }

    /// Creates a new set of coverage data for an unused (never called) function.
    pub(crate) fn unused(
        instance: Instance<'tcx>,
        function_coverage_info: &'tcx FunctionCoverageInfo,
    ) -> Self {
        Self::create(instance, function_coverage_info, false)
    }

    fn create(
        instance: Instance<'tcx>,
        function_coverage_info: &'tcx FunctionCoverageInfo,
        is_used: bool,
    ) -> Self {
        let num_counters = function_coverage_info.num_counters;
        let num_expressions = function_coverage_info.expressions.len();
        debug!(
            "FunctionCoverage::create(instance={instance:?}) has \
            num_counters={num_counters}, num_expressions={num_expressions}, is_used={is_used}"
        );

        // Create a filled set of expression IDs, so that expressions not
        // directly used by mappings will be treated as "seen".
        // (If they end up being unused, LLVM will delete them for us.)
        let mut expressions_seen = BitSet::new_filled(num_expressions);
        // 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 function_coverage_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(term) = mapping.kind
                && let CovTerm::Expression(id) = term
            {
                expressions_seen.remove(id);
            }
        }

        Self {
            function_coverage_info,
            is_used,
            counters_seen: BitSet::new_empty(num_counters),
            expressions_seen,
        }
    }

    /// Marks a counter ID as having been seen in a counter-increment statement.
    #[instrument(level = "debug", skip(self))]
    pub(crate) fn mark_counter_id_seen(&mut self, id: CounterId) {
        self.counters_seen.insert(id);
    }

    /// Marks an expression ID as having been seen in an expression-used statement.
    #[instrument(level = "debug", skip(self))]
    pub(crate) fn mark_expression_id_seen(&mut self, id: ExpressionId) {
        self.expressions_seen.insert(id);
    }

    /// Identify expressions that will always have a value of zero, and note
    /// their IDs in [`ZeroExpressions`]. Mappings that refer to a zero expression
    /// can instead become mappings to a constant zero value.
    ///
    /// This method 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(&self) -> ZeroExpressions {
        // 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 = ZeroExpressions::default();

        // 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 self.function_coverage_info.expressions.iter_enumerated() {
            if !self.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(&self.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
    }

    pub(crate) fn into_finished(self) -> FunctionCoverage<'tcx> {
        let zero_expressions = self.identify_zero_expressions();
        let FunctionCoverageCollector { function_coverage_info, is_used, counters_seen, .. } = self;

        FunctionCoverage { function_coverage_info, is_used, counters_seen, zero_expressions }
    }
}

pub(crate) struct FunctionCoverage<'tcx> {
    function_coverage_info: &'tcx FunctionCoverageInfo,
    is_used: bool,

    counters_seen: BitSet<CounterId>,
    zero_expressions: ZeroExpressions,
}

impl<'tcx> FunctionCoverage<'tcx> {
    /// Returns true for a used (called) function, and false for an unused function.
    pub(crate) fn is_used(&self) -> bool {
        self.is_used
    }

    /// Return the source hash, generated from the HIR node structure, and used to indicate whether
    /// or not the source code structure changed between different compilations.
    pub(crate) fn source_hash(&self) -> u64 {
        if self.is_used { self.function_coverage_info.function_source_hash } else { 0 }
    }

    /// Returns an iterator over all filenames used by this function's mappings.
    pub(crate) fn all_file_names(&self) -> impl Iterator<Item = Symbol> + Captures<'_> {
        self.function_coverage_info.mappings.iter().map(|mapping| mapping.source_region.file_name)
    }

    /// Convert this function's coverage expression data into a form that can be
    /// passed through FFI to LLVM.
    pub(crate) fn counter_expressions(
        &self,
    ) -> impl Iterator<Item = CounterExpression> + ExactSizeIterator + Captures<'_> {
        // We know that LLVM will optimize out any unused expressions before
        // producing the final coverage map, so there's no need to do the same
        // thing on the Rust side unless we're confident we can do much better.
        // (See `CounterExpressionsMinimizer` in `CoverageMappingWriter.cpp`.)

        self.function_coverage_info.expressions.iter().map(move |&Expression { lhs, op, rhs }| {
            CounterExpression {
                lhs: self.counter_for_term(lhs),
                kind: match op {
                    Op::Add => ExprKind::Add,
                    Op::Subtract => ExprKind::Subtract,
                },
                rhs: self.counter_for_term(rhs),
            }
        })
    }

    /// Converts this function's coverage mappings into an intermediate form
    /// that will be used by `mapgen` when preparing for FFI.
    pub(crate) fn counter_regions(
        &self,
    ) -> impl Iterator<Item = (MappingKind, &SourceRegion)> + ExactSizeIterator {
        self.function_coverage_info.mappings.iter().map(move |mapping| {
            let Mapping { kind, source_region } = mapping;
            let kind =
                kind.map_terms(|term| if self.is_zero_term(term) { CovTerm::Zero } else { term });
            (kind, source_region)
        })
    }

    fn counter_for_term(&self, term: CovTerm) -> Counter {
        if self.is_zero_term(term) { Counter::ZERO } else { Counter::from_term(term) }
    }

    fn is_zero_term(&self, term: CovTerm) -> bool {
        is_zero_term(&self.counters_seen, &self.zero_expressions, term)
    }
}

/// Set of expression IDs that are known to always evaluate to zero.
/// Any mapping or expression operand that refers to these expressions can have
/// that reference replaced with a constant zero value.
#[derive(Default)]
struct ZeroExpressions(FxIndexSet<ExpressionId>);

impl ZeroExpressions {
    fn insert(&mut self, id: ExpressionId) {
        self.0.insert(id);
    }

    fn contains(&self, id: ExpressionId) -> bool {
        self.0.contains(&id)
    }
}

/// 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: &BitSet<CounterId>,
    zero_expressions: &ZeroExpressions,
    term: CovTerm,
) -> bool {
    match term {
        CovTerm::Zero => true,
        CovTerm::Counter(id) => !counters_seen.contains(id),
        CovTerm::Expression(id) => zero_expressions.contains(id),
    }
}