waffle/src/cfg/structured.rs

//! Recovery of structured control flow information. Loop nest
//! computation, block order linearization and loop/block region
//! generation.

use fxhash::{FxHashMap, FxHashSet};

use crate::{cfg::CFGInfo, BlockId};

#[derive(Clone, Debug)]
pub enum Node {
    Leaf(BlockId),
    Loop(BlockId, Vec<Node>),
}

impl Node {
    pub fn header(&self) -> BlockId {
        match self {
            &Node::Leaf(block) => block,
            &Node::Loop(block, ..) => block,
        }
    }
    pub fn is_loop(&self) -> bool {
        match self {
            &Node::Loop(..) => true,
            _ => false,
        }
    }
    pub fn is_leaf(&self) -> bool {
        match self {
            &Node::Leaf(..) => true,
            _ => false,
        }
    }
}

pub struct LoopNest {
    nodes: Vec<Node>,
}

impl LoopNest {
    pub fn compute(cfg: &CFGInfo) -> LoopNest {
        // Find loop backedges: any successor edge from a higher- to
        // lower-numbered block in RPO.
        let mut backedges: Vec<(BlockId, BlockId)> = vec![];
        for (block_rpo, &block) in cfg.postorder.iter().rev().enumerate() {
            for &succ in &cfg.block_succs[block] {
                let succ_po = cfg.postorder_pos[succ]
                    .expect("Edge from reachable to unreachable block is impossible");
                let succ_rpo = cfg.postorder.len() - 1 - succ_po;
                if succ_rpo <= block_rpo {
                    log::trace!("LoopNest compute: backedge from {} to {}", block, succ);
                    backedges.push((block, succ));
                }
            }
        }

        // For each backedge, find the backedge's natural loop and
        // accumulate those blocks into the set of blocks in each loop
        // body.
        let mut loop_bodies: FxHashMap<BlockId, FxHashSet<BlockId>> = FxHashMap::default();
        for &(from, to) in &backedges {
            assert!(
                cfg.dominates(to, from),
                "Irreducible CFG edge from {} to {}",
                from,
                to
            );
            let body = loop_bodies
                .entry(to)
                .or_insert_with(|| FxHashSet::default());
            Self::collect_loop_body(body, to, cfg);
            log::trace!("loop body for header {}: {:?}", to, body);
        }

        // Now build the loop nest.
        let mut nodes = vec![];
        let mut visited = FxHashSet::default();
        for &block in cfg.postorder.iter().rev() {
            if visited.contains(&block) {
                continue;
            }
            if loop_bodies.contains_key(&block) {
                nodes.push(Self::loop_node(cfg, block, &loop_bodies, &mut visited));
            } else {
                nodes.push(Node::Leaf(block));
                visited.insert(block);
            }
        }

        log::trace!("loop nest nodes: {:?}", nodes);
        LoopNest { nodes }
    }

    fn collect_loop_body(blocks: &mut FxHashSet<BlockId>, header: BlockId, cfg: &CFGInfo) {
        let mut workset = vec![header];
        while let Some(block) = workset.pop() {
            for &pred in &cfg.block_preds[block] {
                if blocks.contains(&pred) {
                    continue;
                }
                if cfg.dominates(header, pred) {
                    blocks.insert(pred);
                    workset.push(pred);
                }
            }
        }
    }

    fn loop_node(
        cfg: &CFGInfo,
        header: BlockId,
        loops: &FxHashMap<BlockId, FxHashSet<BlockId>>,
        visited: &mut FxHashSet<BlockId>,
    ) -> Node {
        let mut body_blocks = loops
            .get(&header)
            .unwrap()
            .iter()
            .cloned()
            .collect::<Vec<_>>();
        body_blocks.sort_by_key(|&block| -(cfg.postorder_pos[block].unwrap() as isize));

        let mut body_nodes = vec![];
        for block in body_blocks {
            if visited.contains(&block) {
                continue;
            }
            if block != header && loops.contains_key(&block) {
                body_nodes.push(Self::loop_node(cfg, block, loops, visited));
            } else {
                body_nodes.push(Node::Leaf(block));
                visited.insert(block);
            }
        }

        Node::Loop(header, body_nodes)
    }
}

fn compute_forward_edge_targets(cfg: &CFGInfo) -> FxHashSet<BlockId> {
    let mut ret = FxHashSet::default();
    for (block_rpo, &block) in cfg.postorder.iter().rev().enumerate() {
        for &succ in &cfg.block_succs[block] {
            let succ_po = cfg.postorder_pos[succ].unwrap();
            let succ_rpo = cfg.postorder.len() - 1 - succ_po;
            if succ_rpo > block_rpo {
                ret.insert(succ);
            }
        }
    }
    ret
}

#[derive(Clone, Debug)]
pub enum WasmRegion {
    /// Block starting at the first `BlockId`, with a fallthrough/exit
    /// label at the second `BlockId`.
    Block(BlockId, Option<BlockId>, Vec<WasmRegion>),
    /// Loop with a header at the given `BlockId`.
    Loop(BlockId, Vec<WasmRegion>),
    /// An individual basic block, just included inline (with no
    /// Wasm-level structure).
    Leaf(BlockId),
}

impl WasmRegion {
    pub fn header(&self) -> BlockId {
        match self {
            &WasmRegion::Block(block, ..) => block,
            &WasmRegion::Loop(block, ..) => block,
            &WasmRegion::Leaf(block) => block,
        }
    }

    pub fn compute(cfg: &CFGInfo, loop_nest: &LoopNest) -> WasmRegion {
        assert!(!loop_nest.nodes.is_empty());
        assert!(loop_nest.nodes[0].header() == 0);

        let forward_targets = compute_forward_edge_targets(cfg);
        log::trace!(
            "WasmRegion::compute: forward_targets = {:?}",
            forward_targets
        );

        let top = WasmRegion::Block(
            0,
            None,
            loop_nest
                .nodes
                .iter()
                .map(|node| Self::compute_for_node(cfg, &forward_targets, node))
                .collect::<Vec<_>>(),
        );

        log::trace!("Wasm region: {:?}", top);
        top
    }

    fn compute_for_node(
        cfg: &CFGInfo,
        forward_targets: &FxHashSet<BlockId>,
        node: &Node,
    ) -> WasmRegion {
        log::trace!("WasmRegion::compute_for_node: node {:?}", node);
        match node {
            &Node::Leaf(block) => {
                log::trace!(" -> leaf {}", block);
                WasmRegion::Leaf(block)
            }
            &Node::Loop(block, ref subnodes) => {
                // Scan subnodes and find forward-edge targets that
                // are at this level of the loop nest.
                let block_targets = subnodes
                    .iter()
                    .map(|n| n.header())
                    .filter(|n| forward_targets.contains(&n))
                    .collect::<FxHashSet<_>>();
                log::trace!(" -> block targets are {:?}", block_targets,);

                let mut subregions: Vec<WasmRegion> = vec![];
                for subnode in subnodes {
                    if subnode.header() != block && block_targets.contains(&subnode.header()) {
                        let subsubregions = std::mem::take(&mut subregions);
                        assert!(!subsubregions.is_empty());
                        let first = subsubregions[0].header();
                        let enclosing_block =
                            WasmRegion::Block(first, Some(subnode.header()), subsubregions);
                        subregions.push(enclosing_block);
                    }

                    let subregion = Self::compute_for_node(cfg, forward_targets, subnode);
                    subregions.push(subregion);
                }

                log::trace!(" -> loop header {} subregions {:?}", block, subregions);
                WasmRegion::Loop(block, subregions)
            }
        }
    }
}