1 use rustc_index::bit_set::BitSet;
5 /// Preorder traversal of a graph.
7 /// Preorder traversal is when each node is visited after at least one of its predecessors. If you
8 /// are familiar with some basic graph theory, then this performs a depth first search and returns
9 /// nodes in order of discovery time.
22 /// A preorder traversal of this graph is either `A B D C` or `A C D B`
24 pub struct Preorder<'a, 'tcx> {
26 visited: BitSet<BasicBlock>,
27 worklist: Vec<BasicBlock>,
28 root_is_start_block: bool,
31 impl<'a, 'tcx> Preorder<'a, 'tcx> {
32 pub fn new(body: &'a Body<'tcx>, root: BasicBlock) -> Preorder<'a, 'tcx> {
33 let worklist = vec![root];
37 visited: BitSet::new_empty(body.basic_blocks().len()),
39 root_is_start_block: root == START_BLOCK,
44 pub fn preorder<'a, 'tcx>(body: &'a Body<'tcx>) -> Preorder<'a, 'tcx> {
45 Preorder::new(body, START_BLOCK)
48 impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> {
49 type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
51 fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
52 while let Some(idx) = self.worklist.pop() {
53 if !self.visited.insert(idx) {
57 let data = &self.body[idx];
59 if let Some(ref term) = data.terminator {
60 self.worklist.extend(term.successors());
63 return Some((idx, data));
69 fn size_hint(&self) -> (usize, Option<usize>) {
70 // All the blocks, minus the number of blocks we've visited.
71 let upper = self.body.basic_blocks().len() - self.visited.count();
73 let lower = if self.root_is_start_block {
74 // We will visit all remaining blocks exactly once.
84 /// Postorder traversal of a graph.
86 /// Postorder traversal is when each node is visited after all of its successors, except when the
87 /// successor is only reachable by a back-edge. If you are familiar with some basic graph theory,
88 /// then this performs a depth first search and returns nodes in order of completion time.
102 /// A Postorder traversal of this graph is `D B C A` or `D C B A`
103 pub struct Postorder<'a, 'tcx> {
104 body: &'a Body<'tcx>,
105 visited: BitSet<BasicBlock>,
106 visit_stack: Vec<(BasicBlock, Successors<'a>)>,
107 root_is_start_block: bool,
110 impl<'a, 'tcx> Postorder<'a, 'tcx> {
111 pub fn new(body: &'a Body<'tcx>, root: BasicBlock) -> Postorder<'a, 'tcx> {
112 let mut po = Postorder {
114 visited: BitSet::new_empty(body.basic_blocks().len()),
115 visit_stack: Vec::new(),
116 root_is_start_block: root == START_BLOCK,
119 let data = &po.body[root];
121 if let Some(ref term) = data.terminator {
122 po.visited.insert(root);
123 po.visit_stack.push((root, term.successors()));
124 po.traverse_successor();
130 fn traverse_successor(&mut self) {
131 // This is quite a complex loop due to 1. the borrow checker not liking it much
132 // and 2. what exactly is going on is not clear
134 // It does the actual traversal of the graph, while the `next` method on the iterator
135 // just pops off of the stack. `visit_stack` is a stack containing pairs of nodes and
136 // iterators over the successors of those nodes. Each iteration attempts to get the next
137 // node from the top of the stack, then pushes that node and an iterator over the
138 // successors to the top of the stack. This loop only grows `visit_stack`, stopping when
139 // we reach a child that has no children that we haven't already visited.
141 // For a graph that looks like this:
154 // The state of the stack starts out with just the root node (`A` in this case);
157 // When the first call to `traverse_successor` happens, the following happens:
159 // [(B, [D]), // `B` taken from the successors of `A`, pushed to the
160 // // top of the stack along with the successors of `B`
163 // [(D, [E]), // `D` taken from successors of `B`, pushed to stack
167 // [(E, []), // `E` taken from successors of `D`, pushed to stack
172 // Now that the top of the stack has no successors we can traverse, each item will
173 // be popped off during iteration until we get back to `A`. This yields [E, D, B].
175 // When we yield `B` and call `traverse_successor`, we push `C` to the stack, but
176 // since we've already visited `E`, that child isn't added to the stack. The last
177 // two iterations yield `C` and finally `A` for a final traversal of [E, D, B, C, A]
179 let bb = if let Some(&mut (_, ref mut iter)) = self.visit_stack.last_mut() {
180 if let Some(&bb) = iter.next() {
189 if self.visited.insert(bb) {
190 if let Some(term) = &self.body[bb].terminator {
191 self.visit_stack.push((bb, term.successors()));
198 pub fn postorder<'a, 'tcx>(body: &'a Body<'tcx>) -> Postorder<'a, 'tcx> {
199 Postorder::new(body, START_BLOCK)
202 impl<'a, 'tcx> Iterator for Postorder<'a, 'tcx> {
203 type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
205 fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
206 let next = self.visit_stack.pop();
208 self.traverse_successor();
211 next.map(|(bb, _)| (bb, &self.body[bb]))
214 fn size_hint(&self) -> (usize, Option<usize>) {
215 // All the blocks, minus the number of blocks we've visited.
216 let upper = self.body.basic_blocks().len() - self.visited.count();
218 let lower = if self.root_is_start_block {
219 // We will visit all remaining blocks exactly once.
222 self.visit_stack.len()
229 /// Reverse postorder traversal of a graph
231 /// Reverse postorder is the reverse order of a postorder traversal.
232 /// This is different to a preorder traversal and represents a natural
233 /// linearization of control-flow.
246 /// A reverse postorder traversal of this graph is either `A B C D` or `A C B D`
247 /// Note that for a graph containing no loops (i.e., A DAG), this is equivalent to
248 /// a topological sort.
250 /// Construction of a `ReversePostorder` traversal requires doing a full
251 /// postorder traversal of the graph, therefore this traversal should be
252 /// constructed as few times as possible. Use the `reset` method to be able
253 /// to re-use the traversal
255 pub struct ReversePostorder<'a, 'tcx> {
256 body: &'a Body<'tcx>,
257 blocks: Vec<BasicBlock>,
261 impl<'a, 'tcx> ReversePostorder<'a, 'tcx> {
262 pub fn new(body: &'a Body<'tcx>, root: BasicBlock) -> ReversePostorder<'a, 'tcx> {
263 let blocks: Vec<_> = Postorder::new(body, root).map(|(bb, _)| bb).collect();
265 let len = blocks.len();
267 ReversePostorder { body, blocks, idx: len }
271 pub fn reverse_postorder<'a, 'tcx>(body: &'a Body<'tcx>) -> ReversePostorder<'a, 'tcx> {
272 ReversePostorder::new(body, START_BLOCK)
275 impl<'a, 'tcx> Iterator for ReversePostorder<'a, 'tcx> {
276 type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
278 fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
284 self.blocks.get(self.idx).map(|&bb| (bb, &self.body[bb]))
287 fn size_hint(&self) -> (usize, Option<usize>) {
288 (self.idx, Some(self.idx))
292 impl<'a, 'tcx> ExactSizeIterator for ReversePostorder<'a, 'tcx> {}
294 /// Returns an iterator over all basic blocks reachable from the `START_BLOCK` in no particular
297 /// This is clearer than writing `preorder` in cases where the order doesn't matter.
298 pub fn reachable<'a, 'tcx>(
299 body: &'a Body<'tcx>,
300 ) -> impl 'a + Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> {
304 /// Returns a `BitSet` containing all basic blocks reachable from the `START_BLOCK`.
305 pub fn reachable_as_bitset<'tcx>(body: &Body<'tcx>) -> BitSet<BasicBlock> {
306 let mut iter = preorder(body);
307 (&mut iter).for_each(drop);