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 basic_blocks: &'a IndexVec<BasicBlock, BasicBlockData<'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> {
112 basic_blocks: &'a IndexVec<BasicBlock, BasicBlockData<'tcx>>,
114 ) -> Postorder<'a, 'tcx> {
115 let mut po = Postorder {
117 visited: BitSet::new_empty(basic_blocks.len()),
118 visit_stack: Vec::new(),
119 root_is_start_block: root == START_BLOCK,
122 let data = &po.basic_blocks[root];
124 if let Some(ref term) = data.terminator {
125 po.visited.insert(root);
126 po.visit_stack.push((root, term.successors()));
127 po.traverse_successor();
133 fn traverse_successor(&mut self) {
134 // This is quite a complex loop due to 1. the borrow checker not liking it much
135 // and 2. what exactly is going on is not clear
137 // It does the actual traversal of the graph, while the `next` method on the iterator
138 // just pops off of the stack. `visit_stack` is a stack containing pairs of nodes and
139 // iterators over the successors of those nodes. Each iteration attempts to get the next
140 // node from the top of the stack, then pushes that node and an iterator over the
141 // successors to the top of the stack. This loop only grows `visit_stack`, stopping when
142 // we reach a child that has no children that we haven't already visited.
144 // For a graph that looks like this:
157 // The state of the stack starts out with just the root node (`A` in this case);
160 // When the first call to `traverse_successor` happens, the following happens:
162 // [(B, [D]), // `B` taken from the successors of `A`, pushed to the
163 // // top of the stack along with the successors of `B`
166 // [(D, [E]), // `D` taken from successors of `B`, pushed to stack
170 // [(E, []), // `E` taken from successors of `D`, pushed to stack
175 // Now that the top of the stack has no successors we can traverse, each item will
176 // be popped off during iteration until we get back to `A`. This yields [E, D, B].
178 // When we yield `B` and call `traverse_successor`, we push `C` to the stack, but
179 // since we've already visited `E`, that child isn't added to the stack. The last
180 // two iterations yield `C` and finally `A` for a final traversal of [E, D, B, C, A]
182 let bb = if let Some(&mut (_, ref mut iter)) = self.visit_stack.last_mut() {
183 if let Some(bb) = iter.next() {
192 if self.visited.insert(bb) {
193 if let Some(term) = &self.basic_blocks[bb].terminator {
194 self.visit_stack.push((bb, term.successors()));
201 pub fn postorder<'a, 'tcx>(body: &'a Body<'tcx>) -> Postorder<'a, 'tcx> {
202 Postorder::new(&body.basic_blocks, START_BLOCK)
205 impl<'a, 'tcx> Iterator for Postorder<'a, 'tcx> {
206 type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
208 fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
209 let next = self.visit_stack.pop();
211 self.traverse_successor();
214 next.map(|(bb, _)| (bb, &self.basic_blocks[bb]))
217 fn size_hint(&self) -> (usize, Option<usize>) {
218 // All the blocks, minus the number of blocks we've visited.
219 let upper = self.basic_blocks.len() - self.visited.count();
221 let lower = if self.root_is_start_block {
222 // We will visit all remaining blocks exactly once.
225 self.visit_stack.len()
232 /// Reverse postorder traversal of a graph
234 /// Reverse postorder is the reverse order of a postorder traversal.
235 /// This is different to a preorder traversal and represents a natural
236 /// linearization of control-flow.
249 /// A reverse postorder traversal of this graph is either `A B C D` or `A C B D`
250 /// Note that for a graph containing no loops (i.e., A DAG), this is equivalent to
251 /// a topological sort.
253 /// Construction of a `ReversePostorder` traversal requires doing a full
254 /// postorder traversal of the graph, therefore this traversal should be
255 /// constructed as few times as possible. Use the `reset` method to be able
256 /// to re-use the traversal
258 pub struct ReversePostorder<'a, 'tcx> {
259 body: &'a Body<'tcx>,
260 blocks: Vec<BasicBlock>,
264 impl<'a, 'tcx> ReversePostorder<'a, 'tcx> {
265 pub fn new(body: &'a Body<'tcx>, root: BasicBlock) -> ReversePostorder<'a, 'tcx> {
266 let blocks: Vec<_> = Postorder::new(&body.basic_blocks, root).map(|(bb, _)| bb).collect();
267 let len = blocks.len();
268 ReversePostorder { body, blocks, idx: len }
272 impl<'a, 'tcx> Iterator for ReversePostorder<'a, 'tcx> {
273 type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
275 fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
281 self.blocks.get(self.idx).map(|&bb| (bb, &self.body[bb]))
284 fn size_hint(&self) -> (usize, Option<usize>) {
285 (self.idx, Some(self.idx))
289 impl<'a, 'tcx> ExactSizeIterator for ReversePostorder<'a, 'tcx> {}
291 /// Returns an iterator over all basic blocks reachable from the `START_BLOCK` in no particular
294 /// This is clearer than writing `preorder` in cases where the order doesn't matter.
295 pub fn reachable<'a, 'tcx>(
296 body: &'a Body<'tcx>,
297 ) -> impl 'a + Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> {
301 /// Returns a `BitSet` containing all basic blocks reachable from the `START_BLOCK`.
302 pub fn reachable_as_bitset(body: &Body<'_>) -> BitSet<BasicBlock> {
303 let mut iter = preorder(body);
304 (&mut iter).for_each(drop);
309 pub struct ReversePostorderIter<'a, 'tcx> {
310 body: &'a Body<'tcx>,
311 blocks: &'a [BasicBlock],
315 impl<'a, 'tcx> Iterator for ReversePostorderIter<'a, 'tcx> {
316 type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
318 fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
324 self.blocks.get(self.idx).map(|&bb| (bb, &self.body[bb]))
327 fn size_hint(&self) -> (usize, Option<usize>) {
328 (self.idx, Some(self.idx))
332 impl<'a, 'tcx> ExactSizeIterator for ReversePostorderIter<'a, 'tcx> {}
334 pub fn reverse_postorder<'a, 'tcx>(body: &'a Body<'tcx>) -> ReversePostorderIter<'a, 'tcx> {
335 let blocks = body.basic_blocks.postorder();
336 let len = blocks.len();
337 ReversePostorderIter { body, blocks, idx: len }