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]))
}