open-vaf 0.4.2

/*

 * ******************************************************************************************
 * Copyright (c) 2019 Pascal Kuthe. This file is part of the OpenVAF project.
 * It is subject to the license terms in the LICENSE file found in the top-level directory
 *  of this distribution and at  https://gitlab.com/DSPOM/OpenVAF/blob/master/LICENSE.
 *  No part of OpenVAF, including this file, may be copied, modified, propagated, or
 *  distributed except according to the terms contained in the LICENSE file.
 * *****************************************************************************************

 Adapted from https://github.com/rust-lang/rust src/librustc_middle/mir/traversal.rs under MIT-License

    Permission is hereby granted, free of charge, to any person obtaining a copy
    of this software and associated documentation files (the "Software"), to deal
    in the Software without restriction, including without limitation the rights
    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
    copies of the Software, and to permit persons to whom the Software is
    furnished to do so, subject to the following conditions:

    The above copyright notice and this permission notice shall be included in all
    copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
    ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
    TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
    PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
    SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
    CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
    OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
    IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
    DEALINGS IN THE SOFTWARE.
*/

use crate::data_structures::BitSet;
use crate::ir::cfg::{BasicBlock, BasicBlockId, ControlFlowGraph, Successors};
/// 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
///
///
/// ```text
///
///         A
///        / \
///       /   \
///      B     C
///       \   /
///        \ /
///         D
/// ```
///
/// A Postorder traversal of this graph is `D B C A` or `D C B A`
///
pub struct Postorder {
    visited: BitSet<BasicBlockId>,
    visit_stack: Vec<(BasicBlockId, Successors)>,
    root_is_start_block: bool,
}

impl<'lt> Postorder {
    pub fn new(cfg: &ControlFlowGraph, root: BasicBlockId) -> Postorder {
        let mut po = Postorder {
            visited: BitSet::new_empty(cfg.blocks.len_idx()),
            visit_stack: Vec::new(),
            root_is_start_block: root == cfg.start(),
        };

        po.visited.insert(root);
        po.visit_stack.push((root, cfg.successors(root)));
        po.traverse_successor(cfg);

        po
    }

    fn traverse_successor(&mut self, cfg: &ControlFlowGraph) {
        // 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:
        //
        //     [(B, [D]),  // `B` taken from the successors of `A`, pushed to the
        //                 // top of the stack along with the successors of `B`
        //      (A, [C])]
        //
        //     [(D, [E]),  // `D` taken from successors of `B`, pushed to stack
        //      (B, []),
        //      (A, [C])]
        //
        //     [(E, []),   // `E` taken from successors of `D`, pushed to stack
        //      (D, []),
        //      (B, []),
        //      (A, [C])]
        //
        // 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, B].
        //
        // When we yield `B` and call `traverse_successor`, we push `C` to the stack, but
        // since we've already visited `E`, that child isn't added to the stack. The last
        // two iterations yield `C` and finally `A` for a final traversal of [E, D, B, C, A]
        while let Some(bb) = self
            .visit_stack
            .last_mut()
            .and_then(|(_, iter)| iter.next())
        {
            if !self.visited.put(bb) {
                self.visit_stack.push((bb, cfg[bb].terminator.successors()));
            }
        }
    }
}

pub struct PostorderIter<'lt> {
    pub base: Postorder,
    pub cfg: &'lt ControlFlowGraph,
}

impl<'lt> PostorderIter<'lt> {
    pub fn new(cfg: &'lt ControlFlowGraph, root: BasicBlockId) -> Self {
        Self {
            base: Postorder::new(cfg, root),
            cfg,
        }
    }
}

impl<'lt> Iterator for PostorderIter<'lt> {
    type Item = (BasicBlockId, &'lt BasicBlock);

    fn next(&mut self) -> Option<(BasicBlockId, &'lt BasicBlock)> {
        let next = self.base.visit_stack.pop();
        if next.is_some() {
            self.base.traverse_successor(self.cfg);
        }

        next.map(|(bb, _)| (bb, &self.cfg[bb]))
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        // All the blocks, minus the number of blocks we've visited.
        let upper = self.cfg.blocks.len() - self.base.visited.ones().count();

        let lower = if self.base.root_is_start_block {
            // We will visit all remaining blocks exactly once.
            upper
        } else {
            self.base.visit_stack.len()
        };

        (lower, Some(upper))
    }
}

pub struct PostorderIterMut<'lt> {
    base: Postorder,
    cfg: &'lt mut ControlFlowGraph,
}

impl<'lt> PostorderIterMut<'lt> {
    pub fn new(cfg: &'lt mut ControlFlowGraph, root: BasicBlockId) -> Self {
        Self {
            base: Postorder::new(cfg, root),
            cfg,
        }
    }
}

impl<'lt> Iterator for PostorderIterMut<'lt> {
    type Item = (BasicBlockId, &'lt mut BasicBlock);

    fn next(&mut self) -> Option<(BasicBlockId, &'lt mut BasicBlock)> {
        let next = self.base.visit_stack.pop();
        if next.is_some() {
            self.base.traverse_successor(self.cfg);
        }

        /*
        This transmute  is here to transmut `&'_ mut BasicBlock` to `&'lt mut BasicBlock`
        This is save since the iterator already has an exclusive borrow of the ControlFlow Graph/Basic Blocks for 'lt
        and only yields elements exactly once so not two mutable refences to the same block can be handed out.
        Furthermore the mutable cfg reference is private so it can not be used from elsewhere to access the cfg.
        Internally this iterator does read from the cfg. However it only ever reads from blocks that have not been visited yet.
        As such this is also save since after hadning out a &'lt mut to a block it is never read internally either
        */

        next.map(|(bb, _)| (bb, unsafe { std::mem::transmute(&mut self.cfg[bb]) }))
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        // All the blocks, minus the number of blocks we've visited.
        let upper = self.cfg.blocks.len() - self.base.visited.ones().count();

        let lower = if self.base.root_is_start_block {
            // We will visit all remaining blocks exactly once.
            upper
        } else {
            self.base.visit_stack.len()
        };

        (lower, Some(upper))
    }
}

/// Reverse postorder traversal of a graph
///
/// 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.
///
/// Construction of a `ReversePostorder` traversal requires doing a full
/// postorder traversal of the graph, therefore this traversal should be
/// constructed as few times as possible. Use the `reset` method to be able
/// to re-use the traversal
#[derive(Clone, Debug)]
pub struct ReversePostorder {
    blocks: Vec<BasicBlockId>,
    idx: usize,
}

impl ReversePostorder {
    pub fn new(cfg: &ControlFlowGraph, root: BasicBlockId) -> Self {
        let blocks: Vec<_> = PostorderIter::new(cfg, root).map(|(bb, _)| bb).collect();

        let len = blocks.len();

        ReversePostorder { blocks, idx: len }
    }

    pub fn reset(&mut self) {
        self.idx = self.blocks.len();
    }
}

impl Iterator for ReversePostorder {
    type Item = BasicBlockId;

    fn next(&mut self) -> Option<BasicBlockId> {
        if self.idx == 0 {
            return None;
        }
        self.idx -= 1;

        self.blocks.get(self.idx).copied()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.idx, Some(self.idx))
    }
}

impl ExactSizeIterator for ReversePostorder {}

pub struct ReversePostorderIter<'lt> {
    cfg: &'lt ControlFlowGraph,
    base: ReversePostorder,
}

impl<'lt> ReversePostorderIter<'lt> {
    pub fn new(cfg: &'lt ControlFlowGraph, root: BasicBlockId) -> Self {
        Self {
            base: ReversePostorder::new(cfg, root),
            cfg,
        }
    }
}

impl<'lt> Iterator for ReversePostorderIter<'lt> {
    type Item = (BasicBlockId, &'lt BasicBlock);

    fn next(&mut self) -> Option<(BasicBlockId, &'lt BasicBlock)> {
        self.base.next().map(|bb| (bb, &self.cfg[bb]))
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.base.size_hint()
    }
}

pub struct ReversePostorderIterMut<'lt> {
    cfg: &'lt mut ControlFlowGraph,
    base: ReversePostorder,
}

impl<'lt> ReversePostorderIterMut<'lt> {
    pub fn new(cfg: &'lt mut ControlFlowGraph, root: BasicBlockId) -> Self {
        Self {
            base: ReversePostorder::new(cfg, root),
            cfg,
        }
    }
}

impl<'lt> Iterator for ReversePostorderIterMut<'lt> {
    type Item = (BasicBlockId, &'lt mut BasicBlock);

    fn next(&mut self) -> Option<(BasicBlockId, &'lt mut BasicBlock)> {
        /*
        This transmute  is here to transmut &'_ mut BasicBlock to &'lt mut BasicBlock
        This is save since the iterator already has an exclusive borrow of the ControlFlow Graph/Basic Blocks for 'lt
        and only yields elements exactly once so not two mutable refences to the same block can be handed out.
        Furthermore the mutable cfg reference is private so it can not be used from elsewhere to access the cfg.
        Internally this iterator never accesses the cfg (this is just a conviniance struct) so this is fine here
        */
        self.base
            .next()
            .map(|bb| (bb, unsafe { std::mem::transmute(&mut self.cfg[bb]) }))
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.base.size_hint()
    }
}