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
use rustc_index::bit_set::BitSet;

use super::*;

/// Preorder traversal of a graph.
///
/// Preorder traversal is when each node is visited after at least one of its predecessors. If you
/// are familiar with some basic graph theory, then this performs a depth first search and returns
/// nodes in order of discovery time.
///
/// ```text
///
///         A
///        / \
///       /   \
///      B     C
///       \   /
///        \ /
///         D
/// ```
///
/// A preorder traversal of this graph is either `A B D C` or `A C D B`
#[derive(Clone)]
pub struct Preorder<'a, 'tcx> {
    body: &'a Body<'tcx>,
    visited: BitSet<BasicBlock>,
    worklist: Vec<BasicBlock>,
    root_is_start_block: bool,
}

impl<'a, 'tcx> Preorder<'a, 'tcx> {
    pub fn new(body: &'a Body<'tcx>, root: BasicBlock) -> Preorder<'a, 'tcx> {
        let worklist = vec![root];

        Preorder {
            body,
            visited: BitSet::new_empty(body.basic_blocks.len()),
            worklist,
            root_is_start_block: root == START_BLOCK,
        }
    }
}

/// Preorder traversal of a graph.
///
/// This function creates an iterator over the `Body`'s basic blocks, that
/// returns basic blocks in a preorder.
///
/// See [`Preorder`]'s docs to learn what is preorder traversal.
pub fn preorder<'a, 'tcx>(body: &'a Body<'tcx>) -> Preorder<'a, 'tcx> {
    Preorder::new(body, START_BLOCK)
}

impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> {
    type Item = (BasicBlock, &'a BasicBlockData<'tcx>);

    fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
        while let Some(idx) = self.worklist.pop() {
            if !self.visited.insert(idx) {
                continue;
            }

            let data = &self.body[idx];

            if let Some(ref term) = data.terminator {
                self.worklist.extend(term.successors());
            }

            return Some((idx, data));
        }

        None
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        // All the blocks, minus the number of blocks we've visited.
        let upper = self.body.basic_blocks.len() - self.visited.count();

        let lower = if self.root_is_start_block {
            // We will visit all remaining blocks exactly once.
            upper
        } else {
            self.worklist.len()
        };

        (lower, Some(upper))
    }
}

/// Postorder traversal of a graph.
///
/// Postorder traversal is when each node is visited after all of its successors, except when the
/// successor is only reachable by a back-edge. If you are familiar with some basic graph theory,
/// then this performs a depth first search and returns nodes in order of completion time.
///
///
/// ```text
///
///         A
///        / \
///       /   \
///      B     C
///       \   /
///        \ /
///         D
/// ```
///
/// A Postorder traversal of this graph is `D B C A` or `D C B A`
pub struct Postorder<'a, 'tcx> {
    basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
    visited: BitSet<BasicBlock>,
    visit_stack: Vec<(BasicBlock, Successors<'a>)>,
    root_is_start_block: bool,
}

impl<'a, 'tcx> Postorder<'a, 'tcx> {
    pub fn new(
        basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
        root: BasicBlock,
    ) -> Postorder<'a, 'tcx> {
        let mut po = Postorder {
            basic_blocks,
            visited: BitSet::new_empty(basic_blocks.len()),
            visit_stack: Vec::new(),
            root_is_start_block: root == START_BLOCK,
        };

        let data = &po.basic_blocks[root];

        if let Some(ref term) = data.terminator {
            po.visited.insert(root);
            po.visit_stack.push((root, term.successors()));
            po.traverse_successor();
        }

        po
    }

    fn traverse_successor(&mut self) {
        // This is quite a complex loop due to 1. the borrow checker not liking it much
        // and 2. what exactly is going on is not clear
        //
        // It does the actual traversal of the graph, while the `next` method on the iterator
        // just pops off of the stack. `visit_stack` is a stack containing pairs of nodes and
        // iterators over the successors of those nodes. Each iteration attempts to get the next
        // node from the top of the stack, then pushes that node and an iterator over the
        // successors to the top of the stack. This loop only grows `visit_stack`, stopping when
        // we reach a child that has no children that we haven't already visited.
        //
        // For a graph that looks like this:
        //
        //         A
        //        / \
        //       /   \
        //      B     C
        //      |     |
        //      |     |
        //      |     D
        //       \   /
        //        \ /
        //         E
        //
        // The state of the stack starts out with just the root node (`A` in this case);
        //     [(A, [B, C])]
        //
        // When the first call to `traverse_successor` happens, the following happens:
        //
        //     [(C, [D]),  // `C` taken from the successors of `A`, pushed to the
        //                 // top of the stack along with the successors of `C`
        //      (A, [B])]
        //
        //     [(D, [E]),  // `D` taken from successors of `C`, pushed to stack
        //      (C, []),
        //      (A, [B])]
        //
        //     [(E, []),   // `E` taken from successors of `D`, pushed to stack
        //      (D, []),
        //      (C, []),
        //      (A, [B])]
        //
        // Now that the top of the stack has no successors we can traverse, each item will
        // be popped off during iteration until we get back to `A`. This yields [E, D, C].
        //
        // When we yield `C` and call `traverse_successor`, we push `B` to the stack, but
        // since we've already visited `E`, that child isn't added to the stack. The last
        // two iterations yield `B` and finally `A` for a final traversal of [E, D, C, B, A]
        while let Some(bb) = self.visit_stack.last_mut().and_then(|(_, iter)| iter.next_back()) {
            if self.visited.insert(bb) {
                if let Some(term) = &self.basic_blocks[bb].terminator {
                    self.visit_stack.push((bb, term.successors()));
                }
            }
        }
    }
}

impl<'tcx> Iterator for Postorder<'_, 'tcx> {
    type Item = BasicBlock;

    fn next(&mut self) -> Option<BasicBlock> {
        let (bb, _) = self.visit_stack.pop()?;
        self.traverse_successor();

        Some(bb)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        // All the blocks, minus the number of blocks we've visited.
        let upper = self.basic_blocks.len() - self.visited.count();

        let lower = if self.root_is_start_block {
            // We will visit all remaining blocks exactly once.
            upper
        } else {
            self.visit_stack.len()
        };

        (lower, Some(upper))
    }
}

/// Postorder traversal of a graph.
///
/// This function creates an iterator over the `Body`'s basic blocks, that:
/// - returns basic blocks in a postorder,
/// - traverses the `BasicBlocks` CFG cache's reverse postorder backwards, and does not cache the
///   postorder itself.
///
/// See [`Postorder`]'s docs to learn what is postorder traversal.
pub fn postorder<'a, 'tcx>(
    body: &'a Body<'tcx>,
) -> impl Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> + ExactSizeIterator + DoubleEndedIterator
{
    reverse_postorder(body).rev()
}

/// Returns an iterator over all basic blocks reachable from the `START_BLOCK` in no particular
/// order.
///
/// This is clearer than writing `preorder` in cases where the order doesn't matter.
pub fn reachable<'a, 'tcx>(
    body: &'a Body<'tcx>,
) -> impl 'a + Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> {
    preorder(body)
}

/// Returns a `BitSet` containing all basic blocks reachable from the `START_BLOCK`.
pub fn reachable_as_bitset(body: &Body<'_>) -> BitSet<BasicBlock> {
    let mut iter = preorder(body);
    iter.by_ref().for_each(drop);
    iter.visited
}

/// Reverse postorder traversal of a graph.
///
/// This function creates an iterator over the `Body`'s basic blocks, that:
/// - returns basic blocks in a reverse postorder,
/// - makes use of the `BasicBlocks` CFG cache's reverse postorder.
///
/// Reverse postorder is the reverse order of a postorder traversal.
/// This is different to a preorder traversal and represents a natural
/// linearization of control-flow.
///
/// ```text
///
///         A
///        / \
///       /   \
///      B     C
///       \   /
///        \ /
///         D
/// ```
///
/// A reverse postorder traversal of this graph is either `A B C D` or `A C B D`
/// Note that for a graph containing no loops (i.e., A DAG), this is equivalent to
/// a topological sort.
pub fn reverse_postorder<'a, 'tcx>(
    body: &'a Body<'tcx>,
) -> impl Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> + ExactSizeIterator + DoubleEndedIterator
{
    body.basic_blocks.reverse_postorder().iter().map(|&bb| (bb, &body.basic_blocks[bb]))
}