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
//! Random access inspection of the results of a dataflow analysis.

use std::borrow::Borrow;
use std::cmp::Ordering;

use rustc_index::bit_set::BitSet;
use rustc_index::vec::Idx;
use rustc_middle::mir::{self, BasicBlock, Location};

use super::{Analysis, Direction, Effect, EffectIndex, Results};

/// A `ResultsCursor` that borrows the underlying `Results`.
pub type ResultsRefCursor<'a, 'mir, 'tcx, A> = ResultsCursor<'mir, 'tcx, A, &'a Results<'tcx, A>>;

/// Allows random access inspection of the results of a dataflow analysis.
///
/// This cursor only has linear performance within a basic block when its statements are visited in
/// the same order as the `DIRECTION` of the analysis. In the worst case—when statements are
/// visited in *reverse* order—performance will be quadratic in the number of statements in the
/// block. The order in which basic blocks are inspected has no impact on performance.
///
/// A `ResultsCursor` can either own (the default) or borrow the dataflow results it inspects. The
/// type of ownership is determined by `R` (see `ResultsRefCursor` above).
pub struct ResultsCursor<'mir, 'tcx, A, R = Results<'tcx, A>>
where
    A: Analysis<'tcx>,
{
    body: &'mir mir::Body<'tcx>,
    results: R,
    state: A::Domain,

    pos: CursorPosition,

    /// Indicates that `state` has been modified with a custom effect.
    ///
    /// When this flag is set, we need to reset to an entry set before doing a seek.
    state_needs_reset: bool,

    #[cfg(debug_assertions)]
    reachable_blocks: BitSet<BasicBlock>,
}

impl<'mir, 'tcx, A, R> ResultsCursor<'mir, 'tcx, A, R>
where
    A: Analysis<'tcx>,
    R: Borrow<Results<'tcx, A>>,
{
    /// Returns a new cursor that can inspect `results`.
    pub fn new(body: &'mir mir::Body<'tcx>, results: R) -> Self {
        let bottom_value = results.borrow().analysis.bottom_value(body);
        ResultsCursor {
            body,
            results,

            // Initialize to the `bottom_value` and set `state_needs_reset` to tell the cursor that
            // it needs to reset to block entry before the first seek. The cursor position is
            // immaterial.
            state_needs_reset: true,
            state: bottom_value,
            pos: CursorPosition::block_entry(mir::START_BLOCK),

            #[cfg(debug_assertions)]
            reachable_blocks: mir::traversal::reachable_as_bitset(body),
        }
    }

    pub fn body(&self) -> &'mir mir::Body<'tcx> {
        self.body
    }

    /// Returns the underlying `Results`.
    pub fn results(&self) -> &Results<'tcx, A> {
        &self.results.borrow()
    }

    /// Returns the `Analysis` used to generate the underlying `Results`.
    pub fn analysis(&self) -> &A {
        &self.results.borrow().analysis
    }

    /// Returns the dataflow state at the current location.
    pub fn get(&self) -> &A::Domain {
        &self.state
    }

    /// Resets the cursor to hold the entry set for the given basic block.
    ///
    /// For forward dataflow analyses, this is the dataflow state prior to the first statement.
    ///
    /// For backward dataflow analyses, this is the dataflow state after the terminator.
    pub(super) fn seek_to_block_entry(&mut self, block: BasicBlock) {
        #[cfg(debug_assertions)]
        assert!(self.reachable_blocks.contains(block));

        self.state.clone_from(&self.results.borrow().entry_set_for_block(block));
        self.pos = CursorPosition::block_entry(block);
        self.state_needs_reset = false;
    }

    /// Resets the cursor to hold the state prior to the first statement in a basic block.
    ///
    /// For forward analyses, this is the entry set for the given block.
    ///
    /// For backward analyses, this is the state that will be propagated to its
    /// predecessors (ignoring edge-specific effects).
    pub fn seek_to_block_start(&mut self, block: BasicBlock) {
        if A::Direction::is_forward() {
            self.seek_to_block_entry(block)
        } else {
            self.seek_after(Location { block, statement_index: 0 }, Effect::Primary)
        }
    }

    /// Resets the cursor to hold the state after the terminator in a basic block.
    ///
    /// For backward analyses, this is the entry set for the given block.
    ///
    /// For forward analyses, this is the state that will be propagated to its
    /// successors (ignoring edge-specific effects).
    pub fn seek_to_block_end(&mut self, block: BasicBlock) {
        if A::Direction::is_backward() {
            self.seek_to_block_entry(block)
        } else {
            self.seek_after(self.body.terminator_loc(block), Effect::Primary)
        }
    }

    /// Advances the cursor to hold the dataflow state at `target` before its "primary" effect is
    /// applied.
    ///
    /// The "before" effect at the target location *will be* applied.
    pub fn seek_before_primary_effect(&mut self, target: Location) {
        self.seek_after(target, Effect::Before)
    }

    /// Advances the cursor to hold the dataflow state at `target` after its "primary" effect is
    /// applied.
    ///
    /// The "before" effect at the target location will be applied as well.
    pub fn seek_after_primary_effect(&mut self, target: Location) {
        self.seek_after(target, Effect::Primary)
    }

    fn seek_after(&mut self, target: Location, effect: Effect) {
        assert!(target <= self.body.terminator_loc(target.block));

        // Reset to the entry of the target block if any of the following are true:
        //   - A custom effect has been applied to the cursor state.
        //   - We are in a different block than the target.
        //   - We are in the same block but have advanced past the target effect.
        if self.state_needs_reset || self.pos.block != target.block {
            self.seek_to_block_entry(target.block);
        } else if let Some(curr_effect) = self.pos.curr_effect_index {
            let mut ord = curr_effect.statement_index.cmp(&target.statement_index);
            if A::Direction::is_backward() {
                ord = ord.reverse()
            }

            match ord.then_with(|| curr_effect.effect.cmp(&effect)) {
                Ordering::Equal => return,
                Ordering::Greater => self.seek_to_block_entry(target.block),
                Ordering::Less => {}
            }
        }

        // At this point, the cursor is in the same block as the target location at an earlier
        // statement.
        debug_assert_eq!(target.block, self.pos.block);

        let block_data = &self.body[target.block];
        let next_effect = if A::Direction::is_forward() {
            #[rustfmt::skip]
            self.pos.curr_effect_index.map_or_else(
                || Effect::Before.at_index(0),
                EffectIndex::next_in_forward_order,
            )
        } else {
            self.pos.curr_effect_index.map_or_else(
                || Effect::Before.at_index(block_data.statements.len()),
                EffectIndex::next_in_backward_order,
            )
        };

        let analysis = &self.results.borrow().analysis;
        let target_effect_index = effect.at_index(target.statement_index);

        A::Direction::apply_effects_in_range(
            analysis,
            &mut self.state,
            target.block,
            block_data,
            next_effect..=target_effect_index,
        );

        self.pos =
            CursorPosition { block: target.block, curr_effect_index: Some(target_effect_index) };
    }

    /// Applies `f` to the cursor's internal state.
    ///
    /// This can be used, e.g., to apply the call return effect directly to the cursor without
    /// creating an extra copy of the dataflow state.
    pub fn apply_custom_effect(&mut self, f: impl FnOnce(&A, &mut A::Domain)) {
        f(&self.results.borrow().analysis, &mut self.state);
        self.state_needs_reset = true;
    }
}

impl<'mir, 'tcx, A, R, T> ResultsCursor<'mir, 'tcx, A, R>
where
    A: Analysis<'tcx, Domain = BitSet<T>>,
    T: Idx,
    R: Borrow<Results<'tcx, A>>,
{
    pub fn contains(&self, elem: T) -> bool {
        self.get().contains(elem)
    }
}

#[derive(Clone, Copy, Debug)]
struct CursorPosition {
    block: BasicBlock,
    curr_effect_index: Option<EffectIndex>,
}

impl CursorPosition {
    fn block_entry(block: BasicBlock) -> CursorPosition {
        CursorPosition { block, curr_effect_index: None }
    }
}