path: root/src/librustc/dep_graph/thread.rs

// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Manages the communication between the compiler's main thread and
//! the thread that constructs the dependency graph. The basic idea is
//! to use double buffering to lower the cost of producing a message.
//! In the compiler thread, we accumulate messages in a vector until
//! the vector is full, or until we want to query the graph, and then
//! we send that vector over to the depgraph thread. At the same time,
//! we receive an empty vector from the depgraph thread that we can use
//! to accumulate more messages. This way we only ever have two vectors
//! allocated (and both have a fairly large capacity).

use hir::def_id::DefId;
use rustc_data_structures::veccell::VecCell;
use std::cell::Cell;
use std::sync::mpsc::{self, Sender, Receiver};
use std::thread;

use super::DepGraphQuery;
use super::DepNode;
use super::edges::DepGraphEdges;

#[derive(Debug)]
pub enum DepMessage {
    Read(DepNode<DefId>),
    Write(DepNode<DefId>),
    PushTask(DepNode<DefId>),
    PopTask(DepNode<DefId>),
    PushIgnore,
    PopIgnore,
    Query,
}

pub struct DepGraphThreadData {
    enabled: bool,

    // Local counter that just tracks how many tasks are pushed onto the
    // stack, so that we still get an error in the case where one is
    // missing. If dep-graph construction is enabled, we'd get the same
    // error when processing tasks later on, but that's annoying because
    // it lacks precision about the source of the error.
    tasks_pushed: Cell<usize>,

    // current buffer, where we accumulate messages
    messages: VecCell<DepMessage>,

    // whence to receive new buffer when full
    swap_in: Receiver<Vec<DepMessage>>,

    // where to send buffer when full
    swap_out: Sender<Vec<DepMessage>>,

    // where to receive query results
    query_in: Receiver<DepGraphQuery<DefId>>,
}

const INITIAL_CAPACITY: usize = 2048;

impl DepGraphThreadData {
    pub fn new(enabled: bool) -> DepGraphThreadData {
        let (tx1, rx1) = mpsc::channel();
        let (tx2, rx2) = mpsc::channel();
        let (txq, rxq) = mpsc::channel();

        if enabled {
            thread::spawn(move || main(rx1, tx2, txq));
        }

        DepGraphThreadData {
            enabled: enabled,
            tasks_pushed: Cell::new(0),
            messages: VecCell::with_capacity(INITIAL_CAPACITY),
            swap_in: rx2,
            swap_out: tx1,
            query_in: rxq,
        }
    }

    #[inline]
    pub fn enabled(&self) -> bool {
        self.enabled
    }

    /// Sends the current batch of messages to the thread. Installs a
    /// new vector of messages.
    fn swap(&self) {
        assert!(self.enabled, "should never swap if not enabled");

        // should be a buffer waiting for us (though of course we may
        // have to wait for depgraph thread to finish processing the
        // old messages)
        let new_messages = self.swap_in.recv().unwrap();
        assert!(new_messages.is_empty());

        // swap in the empty buffer and extract the full one
        let old_messages = self.messages.swap(new_messages);

        // send full buffer to depgraph thread to be processed
        self.swap_out.send(old_messages).unwrap();
    }

    pub fn query(&self) -> DepGraphQuery<DefId> {
        assert!(self.enabled, "cannot query if dep graph construction not enabled");
        self.enqueue(DepMessage::Query);
        self.swap();
        self.query_in.recv().unwrap()
    }

    /// Enqueue a message to be sent when things are next swapped. (If
    /// the buffer is full, this may swap.)
    #[inline]
    pub fn enqueue(&self, message: DepMessage) {
        debug!("enqueue: {:?} tasks_pushed={}", message, self.tasks_pushed.get());

        // Regardless of whether dep graph construction is enabled, we
        // still want to check that we always have a valid task on the
        // stack when a read/write/etc event occurs.
        match message {
            DepMessage::Read(_) | DepMessage::Write(_) =>
                if self.tasks_pushed.get() == 0 {
                    self.invalid_message("read/write but no current task")
                },
            DepMessage::PushTask(_) | DepMessage::PushIgnore =>
                self.tasks_pushed.set(self.tasks_pushed.get() + 1),
            DepMessage::PopTask(_) | DepMessage::PopIgnore =>
                self.tasks_pushed.set(self.tasks_pushed.get() - 1),
            DepMessage::Query =>
                (),
        }

        if self.enabled {
            self.enqueue_enabled(message);
        }
    }

    // Outline this fn since I expect it may want to be inlined
    // separately.
    fn enqueue_enabled(&self, message: DepMessage) {
        let len = self.messages.push(message);
        if len == INITIAL_CAPACITY {
            self.swap();
        }
    }

    // Outline this too.
    fn invalid_message(&self, string: &str) {
        bug!("{}; see src/librustc/dep_graph/README.md for more information", string)
    }
}

/// Definition of the depgraph thread.
pub fn main(swap_in: Receiver<Vec<DepMessage>>,
            swap_out: Sender<Vec<DepMessage>>,
            query_out: Sender<DepGraphQuery<DefId>>) {
    let mut edges = DepGraphEdges::new();

    // the compiler thread always expects a fresh buffer to be
    // waiting, so queue one up
    swap_out.send(Vec::with_capacity(INITIAL_CAPACITY)).unwrap();

    // process the buffers from compiler thread as we receive them
    for mut messages in swap_in {
        for msg in messages.drain(..) {
            match msg {
                DepMessage::Read(node) => edges.read(node),
                DepMessage::Write(node) => edges.write(node),
                DepMessage::PushTask(node) => edges.push_task(node),
                DepMessage::PopTask(node) => edges.pop_task(node),
                DepMessage::PushIgnore => edges.push_ignore(),
                DepMessage::PopIgnore => edges.pop_ignore(),
                DepMessage::Query => query_out.send(edges.query()).unwrap(),
            }
        }
        if let Err(_) = swap_out.send(messages) {
            // the receiver must have been dropped already
            break;
        }
    }
}