compiler/rustc_middle/src/mir/traversal.rs

   1 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
   2 use rustc_data_structures::sync::OnceCell;
   3 use rustc_index::bit_set::BitSet;
   4 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
   5
   6 use super::*;
   7
   8 /// Preorder traversal of a graph.
   9 ///
  10 /// Preorder traversal is when each node is visited after at least one of its predecessors. If you
  11 /// are familiar with some basic graph theory, then this performs a depth first search and returns
  12 /// nodes in order of discovery time.
  13 ///
  14 /// ```text
  15 ///
  16 ///         A
  17 ///        / \
  18 ///       /   \
  19 ///      B     C
  20 ///       \   /
  21 ///        \ /
  22 ///         D
  23 /// ```
  24 ///
  25 /// A preorder traversal of this graph is either `A B D C` or `A C D B`
  26 #[derive(Clone)]
  27 pub struct Preorder<'a, 'tcx> {
  28     body: &'a Body<'tcx>,
  29     visited: BitSet<BasicBlock>,
  30     worklist: Vec<BasicBlock>,
  31     root_is_start_block: bool,
  32 }
  33
  34 impl<'a, 'tcx> Preorder<'a, 'tcx> {
  35     pub fn new(body: &'a Body<'tcx>, root: BasicBlock) -> Preorder<'a, 'tcx> {
  36         let worklist = vec![root];
  37
  38         Preorder {
  39             body,
  40             visited: BitSet::new_empty(body.basic_blocks().len()),
  41             worklist,
  42             root_is_start_block: root == START_BLOCK,
  43         }
  44     }
  45 }
  46
  47 pub fn preorder<'a, 'tcx>(body: &'a Body<'tcx>) -> Preorder<'a, 'tcx> {
  48     Preorder::new(body, START_BLOCK)
  49 }
  50
  51 impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> {
  52     type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
  53
  54     fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
  55         while let Some(idx) = self.worklist.pop() {
  56             if !self.visited.insert(idx) {
  57                 continue;
  58             }
  59
  60             let data = &self.body[idx];
  61
  62             if let Some(ref term) = data.terminator {
  63                 self.worklist.extend(term.successors());
  64             }
  65
  66             return Some((idx, data));
  67         }
  68
  69         None
  70     }
  71
  72     fn size_hint(&self) -> (usize, Option<usize>) {
  73         // All the blocks, minus the number of blocks we've visited.
  74         let upper = self.body.basic_blocks().len() - self.visited.count();
  75
  76         let lower = if self.root_is_start_block {
  77             // We will visit all remaining blocks exactly once.
  78             upper
  79         } else {
  80             self.worklist.len()
  81         };
  82
  83         (lower, Some(upper))
  84     }
  85 }
  86
  87 /// Postorder traversal of a graph.
  88 ///
  89 /// Postorder traversal is when each node is visited after all of its successors, except when the
  90 /// successor is only reachable by a back-edge. If you are familiar with some basic graph theory,
  91 /// then this performs a depth first search and returns nodes in order of completion time.
  92 ///
  93 ///
  94 /// ```text
  95 ///
  96 ///         A
  97 ///        / \
  98 ///       /   \
  99 ///      B     C
 100 ///       \   /
 101 ///        \ /
 102 ///         D
 103 /// ```
 104 ///
 105 /// A Postorder traversal of this graph is `D B C A` or `D C B A`
 106 pub struct Postorder<'a, 'tcx> {
 107     body: &'a Body<'tcx>,
 108     visited: BitSet<BasicBlock>,
 109     visit_stack: Vec<(BasicBlock, Successors<'a>)>,
 110     root_is_start_block: bool,
 111 }
 112
 113 impl<'a, 'tcx> Postorder<'a, 'tcx> {
 114     pub fn new(body: &'a Body<'tcx>, root: BasicBlock) -> Postorder<'a, 'tcx> {
 115         let mut po = Postorder {
 116             body,
 117             visited: BitSet::new_empty(body.basic_blocks().len()),
 118             visit_stack: Vec::new(),
 119             root_is_start_block: root == START_BLOCK,
 120         };
 121
 122         let data = &po.body[root];
 123
 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();
 128         }
 129
 130         po
 131     }
 132
 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
 136         //
 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.
 143         //
 144         // For a graph that looks like this:
 145         //
 146         //         A
 147         //        / \
 148         //       /   \
 149         //      B     C
 150         //      |     |
 151         //      |     |
 152         //      D     |
 153         //       \   /
 154         //        \ /
 155         //         E
 156         //
 157         // The state of the stack starts out with just the root node (`A` in this case);
 158         //     [(A, [B, C])]
 159         //
 160         // When the first call to `traverse_successor` happens, the following happens:
 161         //
 162         //     [(B, [D]),  // `B` taken from the successors of `A`, pushed to the
 163         //                 // top of the stack along with the successors of `B`
 164         //      (A, [C])]
 165         //
 166         //     [(D, [E]),  // `D` taken from successors of `B`, pushed to stack
 167         //      (B, []),
 168         //      (A, [C])]
 169         //
 170         //     [(E, []),   // `E` taken from successors of `D`, pushed to stack
 171         //      (D, []),
 172         //      (B, []),
 173         //      (A, [C])]
 174         //
 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].
 177         //
 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]
 181         loop {
 182             let bb = if let Some(&mut (_, ref mut iter)) = self.visit_stack.last_mut() {
 183                 if let Some(bb) = iter.next() {
 184                     bb
 185                 } else {
 186                     break;
 187                 }
 188             } else {
 189                 break;
 190             };
 191
 192             if self.visited.insert(bb) {
 193                 if let Some(term) = &self.body[bb].terminator {
 194                     self.visit_stack.push((bb, term.successors()));
 195                 }
 196             }
 197         }
 198     }
 199 }
 200
 201 pub fn postorder<'a, 'tcx>(body: &'a Body<'tcx>) -> Postorder<'a, 'tcx> {
 202     Postorder::new(body, START_BLOCK)
 203 }
 204
 205 impl<'a, 'tcx> Iterator for Postorder<'a, 'tcx> {
 206     type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
 207
 208     fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
 209         let next = self.visit_stack.pop();
 210         if next.is_some() {
 211             self.traverse_successor();
 212         }
 213
 214         next.map(|(bb, _)| (bb, &self.body[bb]))
 215     }
 216
 217     fn size_hint(&self) -> (usize, Option<usize>) {
 218         // All the blocks, minus the number of blocks we've visited.
 219         let upper = self.body.basic_blocks().len() - self.visited.count();
 220
 221         let lower = if self.root_is_start_block {
 222             // We will visit all remaining blocks exactly once.
 223             upper
 224         } else {
 225             self.visit_stack.len()
 226         };
 227
 228         (lower, Some(upper))
 229     }
 230 }
 231
 232 /// Reverse postorder traversal of a graph
 233 ///
 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.
 237 ///
 238 /// ```text
 239 ///
 240 ///         A
 241 ///        / \
 242 ///       /   \
 243 ///      B     C
 244 ///       \   /
 245 ///        \ /
 246 ///         D
 247 /// ```
 248 ///
 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.
 252 ///
 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
 257 #[derive(Clone)]
 258 pub struct ReversePostorder<'a, 'tcx> {
 259     body: &'a Body<'tcx>,
 260     blocks: Vec<BasicBlock>,
 261     idx: usize,
 262 }
 263
 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, root).map(|(bb, _)| bb).collect();
 267
 268         let len = blocks.len();
 269
 270         ReversePostorder { body, blocks, idx: len }
 271     }
 272 }
 273
 274 impl<'a, 'tcx> Iterator for ReversePostorder<'a, 'tcx> {
 275     type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
 276
 277     fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
 278         if self.idx == 0 {
 279             return None;
 280         }
 281         self.idx -= 1;
 282
 283         self.blocks.get(self.idx).map(|&bb| (bb, &self.body[bb]))
 284     }
 285
 286     fn size_hint(&self) -> (usize, Option<usize>) {
 287         (self.idx, Some(self.idx))
 288     }
 289 }
 290
 291 impl<'a, 'tcx> ExactSizeIterator for ReversePostorder<'a, 'tcx> {}
 292
 293 /// Returns an iterator over all basic blocks reachable from the `START_BLOCK` in no particular
 294 /// order.
 295 ///
 296 /// This is clearer than writing `preorder` in cases where the order doesn't matter.
 297 pub fn reachable<'a, 'tcx>(
 298     body: &'a Body<'tcx>,
 299 ) -> impl 'a + Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> {
 300     preorder(body)
 301 }
 302
 303 /// Returns a `BitSet` containing all basic blocks reachable from the `START_BLOCK`.
 304 pub fn reachable_as_bitset<'tcx>(body: &Body<'tcx>) -> BitSet<BasicBlock> {
 305     let mut iter = preorder(body);
 306     (&mut iter).for_each(drop);
 307     iter.visited
 308 }
 309
 310 #[derive(Clone)]
 311 pub struct ReversePostorderIter<'a, 'tcx> {
 312     body: &'a Body<'tcx>,
 313     blocks: &'a [BasicBlock],
 314     idx: usize,
 315 }
 316
 317 impl<'a, 'tcx> Iterator for ReversePostorderIter<'a, 'tcx> {
 318     type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
 319
 320     fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
 321         if self.idx == 0 {
 322             return None;
 323         }
 324         self.idx -= 1;
 325
 326         self.blocks.get(self.idx).map(|&bb| (bb, &self.body[bb]))
 327     }
 328
 329     fn size_hint(&self) -> (usize, Option<usize>) {
 330         (self.idx, Some(self.idx))
 331     }
 332 }
 333
 334 impl<'a, 'tcx> ExactSizeIterator for ReversePostorderIter<'a, 'tcx> {}
 335
 336 pub fn reverse_postorder<'a, 'tcx>(body: &'a Body<'tcx>) -> ReversePostorderIter<'a, 'tcx> {
 337     let blocks = body.postorder_cache.compute(body);
 338
 339     let len = blocks.len();
 340
 341     ReversePostorderIter { body, blocks, idx: len }
 342 }
 343
 344 #[derive(Clone, Debug)]
 345 pub(super) struct PostorderCache {
 346     cache: OnceCell<Vec<BasicBlock>>,
 347 }
 348
 349 impl PostorderCache {
 350     #[inline]
 351     pub(super) fn new() -> Self {
 352         PostorderCache { cache: OnceCell::new() }
 353     }
 354
 355     /// Invalidates the postorder cache.
 356     #[inline]
 357     pub(super) fn invalidate(&mut self) {
 358         self.cache = OnceCell::new();
 359     }
 360
 361     /// Returns the `&[BasicBlocks]` represents the postorder graph for this MIR.
 362     #[inline]
 363     pub(super) fn compute(&self, body: &Body<'_>) -> &[BasicBlock] {
 364         self.cache.get_or_init(|| Postorder::new(body, START_BLOCK).map(|(bb, _)| bb).collect())
 365     }
 366 }
 367
 368 impl<S: Encoder> Encodable<S> for PostorderCache {
 369     #[inline]
 370     fn encode(&self, _s: &mut S) {}
 371 }
 372
 373 impl<D: Decoder> Decodable<D> for PostorderCache {
 374     #[inline]
 375     fn decode(_: &mut D) -> Self {
 376         Self::new()
 377     }
 378 }
 379
 380 impl<CTX> HashStable<CTX> for PostorderCache {
 381     #[inline]
 382     fn hash_stable(&self, _: &mut CTX, _: &mut StableHasher) {
 383         // do nothing
 384     }
 385 }
 386
 387 TrivialTypeFoldableAndLiftImpls! {
 388     PostorderCache,
 389 }