412 lines
13 KiB
Rust
412 lines
13 KiB
Rust
use super::*;
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/// Preorder traversal of a graph.
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///
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/// Preorder traversal is when each node is visited after at least one of its predecessors. If you
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/// are familiar with some basic graph theory, then this performs a depth first search and returns
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/// nodes in order of discovery time.
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///
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/// ```text
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///
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/// A
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/// / \
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/// / \
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/// B C
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/// \ /
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/// \ /
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/// D
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/// ```
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///
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/// A preorder traversal of this graph is either `A B D C` or `A C D B`
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#[derive(Clone)]
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pub struct Preorder<'a, 'tcx> {
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body: &'a Body<'tcx>,
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visited: BitSet<BasicBlock>,
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worklist: Vec<BasicBlock>,
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root_is_start_block: bool,
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}
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impl<'a, 'tcx> Preorder<'a, 'tcx> {
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pub fn new(body: &'a Body<'tcx>, root: BasicBlock) -> Preorder<'a, 'tcx> {
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let worklist = vec![root];
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Preorder {
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body,
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visited: BitSet::new_empty(body.basic_blocks.len()),
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worklist,
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root_is_start_block: root == START_BLOCK,
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}
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}
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}
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/// Preorder traversal of a graph.
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///
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/// This function creates an iterator over the `Body`'s basic blocks, that
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/// returns basic blocks in a preorder.
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///
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/// See [`Preorder`]'s docs to learn what is preorder traversal.
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pub fn preorder<'a, 'tcx>(body: &'a Body<'tcx>) -> Preorder<'a, 'tcx> {
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Preorder::new(body, START_BLOCK)
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}
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impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> {
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type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
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fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
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while let Some(idx) = self.worklist.pop() {
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if !self.visited.insert(idx) {
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continue;
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}
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let data = &self.body[idx];
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if let Some(ref term) = data.terminator {
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self.worklist.extend(term.successors());
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}
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return Some((idx, data));
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}
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None
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}
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fn size_hint(&self) -> (usize, Option<usize>) {
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// All the blocks, minus the number of blocks we've visited.
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let upper = self.body.basic_blocks.len() - self.visited.count();
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let lower = if self.root_is_start_block {
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// We will visit all remaining blocks exactly once.
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upper
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} else {
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self.worklist.len()
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};
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(lower, Some(upper))
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}
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}
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/// Postorder traversal of a graph.
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///
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/// Postorder traversal is when each node is visited after all of its successors, except when the
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/// successor is only reachable by a back-edge. If you are familiar with some basic graph theory,
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/// then this performs a depth first search and returns nodes in order of completion time.
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///
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///
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/// ```text
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///
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/// A
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/// / \
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/// / \
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/// B C
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/// \ /
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/// \ /
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/// D
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/// ```
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///
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/// A Postorder traversal of this graph is `D B C A` or `D C B A`
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pub struct Postorder<'a, 'tcx, C> {
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basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
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visited: BitSet<BasicBlock>,
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visit_stack: Vec<(BasicBlock, Successors<'a>)>,
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root_is_start_block: bool,
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extra: C,
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}
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impl<'a, 'tcx, C> Postorder<'a, 'tcx, C>
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where
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C: Customization<'tcx>,
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{
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pub fn new(
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basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
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root: BasicBlock,
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extra: C,
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) -> Postorder<'a, 'tcx, C> {
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let mut po = Postorder {
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basic_blocks,
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visited: BitSet::new_empty(basic_blocks.len()),
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visit_stack: Vec::new(),
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root_is_start_block: root == START_BLOCK,
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extra,
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};
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po.visit(root);
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po.traverse_successor();
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po
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}
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fn visit(&mut self, bb: BasicBlock) {
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if !self.visited.insert(bb) {
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return;
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}
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let data = &self.basic_blocks[bb];
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let successors = C::successors(data, self.extra);
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self.visit_stack.push((bb, successors));
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}
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fn traverse_successor(&mut self) {
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// This is quite a complex loop due to 1. the borrow checker not liking it much
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// and 2. what exactly is going on is not clear
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//
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// It does the actual traversal of the graph, while the `next` method on the iterator
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// just pops off of the stack. `visit_stack` is a stack containing pairs of nodes and
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// iterators over the successors of those nodes. Each iteration attempts to get the next
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// node from the top of the stack, then pushes that node and an iterator over the
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// successors to the top of the stack. This loop only grows `visit_stack`, stopping when
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// we reach a child that has no children that we haven't already visited.
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//
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// For a graph that looks like this:
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//
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// A
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// / \
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// / \
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// B C
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// | |
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// | |
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// | D
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// \ /
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// \ /
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// E
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//
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// The state of the stack starts out with just the root node (`A` in this case);
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// [(A, [B, C])]
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//
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// When the first call to `traverse_successor` happens, the following happens:
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//
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// [(C, [D]), // `C` taken from the successors of `A`, pushed to the
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// // top of the stack along with the successors of `C`
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// (A, [B])]
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//
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// [(D, [E]), // `D` taken from successors of `C`, pushed to stack
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// (C, []),
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// (A, [B])]
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//
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// [(E, []), // `E` taken from successors of `D`, pushed to stack
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// (D, []),
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// (C, []),
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// (A, [B])]
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//
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// Now that the top of the stack has no successors we can traverse, each item will
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// be popped off during iteration until we get back to `A`. This yields [E, D, C].
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//
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// When we yield `C` and call `traverse_successor`, we push `B` to the stack, but
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// since we've already visited `E`, that child isn't added to the stack. The last
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// two iterations yield `B` and finally `A` for a final traversal of [E, D, C, B, A]
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while let Some(bb) = self.visit_stack.last_mut().and_then(|(_, iter)| iter.next_back()) {
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self.visit(bb);
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}
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}
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}
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impl<'tcx, C> Iterator for Postorder<'_, 'tcx, C>
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where
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C: Customization<'tcx>,
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{
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type Item = BasicBlock;
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fn next(&mut self) -> Option<BasicBlock> {
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let (bb, _) = self.visit_stack.pop()?;
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self.traverse_successor();
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Some(bb)
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}
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fn size_hint(&self) -> (usize, Option<usize>) {
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// All the blocks, minus the number of blocks we've visited.
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let upper = self.basic_blocks.len() - self.visited.count();
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let lower = if self.root_is_start_block {
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// We will visit all remaining blocks exactly once.
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upper
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} else {
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self.visit_stack.len()
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};
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(lower, Some(upper))
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}
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}
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/// Postorder traversal of a graph.
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///
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/// This function creates an iterator over the `Body`'s basic blocks, that:
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/// - returns basic blocks in a postorder,
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/// - traverses the `BasicBlocks` CFG cache's reverse postorder backwards, and does not cache the
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/// postorder itself.
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///
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/// See [`Postorder`]'s docs to learn what is postorder traversal.
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pub fn postorder<'a, 'tcx>(
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body: &'a Body<'tcx>,
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) -> impl Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> + ExactSizeIterator + DoubleEndedIterator
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{
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reverse_postorder(body).rev()
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}
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/// Lets us plug in some additional logic and data into a Postorder traversal. Or not.
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pub trait Customization<'tcx>: Copy {
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fn successors<'a>(_: &'a BasicBlockData<'tcx>, _: Self) -> Successors<'a>;
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}
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impl<'tcx> Customization<'tcx> for () {
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fn successors<'a>(data: &'a BasicBlockData<'tcx>, _: ()) -> Successors<'a> {
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data.terminator().successors()
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}
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}
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impl<'tcx> Customization<'tcx> for (TyCtxt<'tcx>, Instance<'tcx>) {
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fn successors<'a>(
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data: &'a BasicBlockData<'tcx>,
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(tcx, instance): (TyCtxt<'tcx>, Instance<'tcx>),
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) -> Successors<'a> {
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data.mono_successors(tcx, instance)
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}
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}
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pub fn mono_reachable_reverse_postorder<'a, 'tcx>(
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body: &'a Body<'tcx>,
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tcx: TyCtxt<'tcx>,
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instance: Instance<'tcx>,
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) -> Vec<BasicBlock> {
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let mut iter = Postorder::new(&body.basic_blocks, START_BLOCK, (tcx, instance));
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let mut items = Vec::with_capacity(body.basic_blocks.len());
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while let Some(block) = iter.next() {
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items.push(block);
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}
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items.reverse();
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items
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}
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/// Returns an iterator over all basic blocks reachable from the `START_BLOCK` in no particular
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/// order.
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///
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/// This is clearer than writing `preorder` in cases where the order doesn't matter.
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pub fn reachable<'a, 'tcx>(
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body: &'a Body<'tcx>,
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) -> impl 'a + Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> {
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preorder(body)
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}
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/// Returns a `BitSet` containing all basic blocks reachable from the `START_BLOCK`.
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pub fn reachable_as_bitset(body: &Body<'_>) -> BitSet<BasicBlock> {
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let mut iter = preorder(body);
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while let Some(_) = iter.next() {}
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iter.visited
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}
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/// Reverse postorder traversal of a graph.
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///
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/// This function creates an iterator over the `Body`'s basic blocks, that:
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/// - returns basic blocks in a reverse postorder,
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/// - makes use of the `BasicBlocks` CFG cache's reverse postorder.
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///
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/// Reverse postorder is the reverse order of a postorder traversal.
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/// This is different to a preorder traversal and represents a natural
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/// linearization of control-flow.
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///
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/// ```text
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///
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/// A
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/// / \
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/// / \
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/// B C
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/// \ /
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/// \ /
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/// D
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/// ```
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///
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/// A reverse postorder traversal of this graph is either `A B C D` or `A C B D`
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/// Note that for a graph containing no loops (i.e., A DAG), this is equivalent to
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/// a topological sort.
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pub fn reverse_postorder<'a, 'tcx>(
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body: &'a Body<'tcx>,
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) -> impl Iterator<Item = (BasicBlock, &'a BasicBlockData<'tcx>)> + ExactSizeIterator + DoubleEndedIterator
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{
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body.basic_blocks.reverse_postorder().iter().map(|&bb| (bb, &body.basic_blocks[bb]))
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}
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/// Traversal of a [`Body`] that tries to avoid unreachable blocks in a monomorphized [`Instance`].
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///
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/// This is allowed to have false positives; blocks may be visited even if they are not actually
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/// reachable.
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///
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/// Such a traversal is mostly useful because it lets us skip lowering the `false` side
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/// of `if <T as Trait>::CONST`, as well as [`NullOp::UbChecks`].
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///
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/// [`NullOp::UbChecks`]: rustc_middle::mir::NullOp::UbChecks
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pub fn mono_reachable<'a, 'tcx>(
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body: &'a Body<'tcx>,
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tcx: TyCtxt<'tcx>,
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instance: Instance<'tcx>,
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) -> MonoReachable<'a, 'tcx> {
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MonoReachable::new(body, tcx, instance)
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}
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/// [`MonoReachable`] internally accumulates a [`BitSet`] of visited blocks. This is just a
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/// convenience function to run that traversal then extract its set of reached blocks.
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pub fn mono_reachable_as_bitset<'a, 'tcx>(
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body: &'a Body<'tcx>,
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tcx: TyCtxt<'tcx>,
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instance: Instance<'tcx>,
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) -> BitSet<BasicBlock> {
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let mut iter = mono_reachable(body, tcx, instance);
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while let Some(_) = iter.next() {}
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iter.visited
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}
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pub struct MonoReachable<'a, 'tcx> {
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body: &'a Body<'tcx>,
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tcx: TyCtxt<'tcx>,
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instance: Instance<'tcx>,
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visited: BitSet<BasicBlock>,
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// Other traversers track their worklist in a Vec. But we don't care about order, so we can
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// store ours in a BitSet and thus save allocations because BitSet has a small size
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// optimization.
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worklist: BitSet<BasicBlock>,
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}
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impl<'a, 'tcx> MonoReachable<'a, 'tcx> {
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pub fn new(
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body: &'a Body<'tcx>,
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tcx: TyCtxt<'tcx>,
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instance: Instance<'tcx>,
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) -> MonoReachable<'a, 'tcx> {
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let mut worklist = BitSet::new_empty(body.basic_blocks.len());
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worklist.insert(START_BLOCK);
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MonoReachable {
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body,
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tcx,
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instance,
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visited: BitSet::new_empty(body.basic_blocks.len()),
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worklist,
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}
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}
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fn add_work(&mut self, blocks: impl IntoIterator<Item = BasicBlock>) {
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for block in blocks.into_iter() {
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if !self.visited.contains(block) {
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self.worklist.insert(block);
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}
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}
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}
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}
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impl<'a, 'tcx> Iterator for MonoReachable<'a, 'tcx> {
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type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
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fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
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while let Some(idx) = self.worklist.iter().next() {
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self.worklist.remove(idx);
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if !self.visited.insert(idx) {
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continue;
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}
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let data = &self.body[idx];
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let targets = data.mono_successors(self.tcx, self.instance);
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self.add_work(targets);
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return Some((idx, data));
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}
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None
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}
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}
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