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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
//! Implements traversal over the DOM tree. The traversal starts in sequential
//! mode, and optionally parallelizes as it discovers work.
use crate::context::{PerThreadTraversalStatistics, StyleContext};
use crate::context::{ThreadLocalStyleContext, TraversalStatistics};
use crate::dom::{SendNode, TElement, TNode};
use crate::parallel;
use crate::scoped_tls::ScopedTLS;
use crate::traversal::{DomTraversal, PerLevelTraversalData, PreTraverseToken};
use rayon;
use std::collections::VecDeque;
use time;
#[cfg(feature = "servo")]
fn should_report_statistics() -> bool {
#[cfg(feature = "gecko")]
fn should_report_statistics() -> bool {
unsafe { crate::gecko_bindings::structs::ServoTraversalStatistics_sActive }
#[cfg(feature = "servo")]
fn report_statistics(_stats: &PerThreadTraversalStatistics) {
unreachable!("Servo never report stats");
#[cfg(feature = "gecko")]
fn report_statistics(stats: &PerThreadTraversalStatistics) {
// This should only be called in the main thread, or it may be racy
// to update the statistics in a global variable.
debug_assert!(unsafe { crate::gecko_bindings::bindings::Gecko_IsMainThread() });
let gecko_stats =
unsafe { &mut crate::gecko_bindings::structs::ServoTraversalStatistics_sSingleton };
gecko_stats.mElementsTraversed += stats.elements_traversed;
gecko_stats.mElementsStyled += stats.elements_styled;
gecko_stats.mElementsMatched += stats.elements_matched;
gecko_stats.mStylesShared += stats.styles_shared;
gecko_stats.mStylesReused += stats.styles_reused;
fn with_pool_in_place_scope<'scope, R>(
work_unit_max: usize,
pool: Option<&rayon::ThreadPool>,
closure: impl FnOnce(Option<&rayon::ScopeFifo<'scope>>) -> R,
) -> R {
if work_unit_max == 0 || pool.is_none() {
} else {
.in_place_scope_fifo(|scope| closure(Some(scope)))
/// See documentation of the pref for performance characteristics.
fn work_unit_max() -> usize {
static_prefs::pref!("layout.css.stylo-work-unit-size") as usize
/// Do a DOM traversal for top-down and (optionally) bottom-up processing, generic over `D`.
/// We use an adaptive traversal strategy. We start out with simple sequential processing, until we
/// arrive at a wide enough level in the DOM that the parallel traversal would parallelize it.
/// If a thread pool is provided, we then transfer control over to the parallel traversal.
/// Returns true if the traversal was parallel, and also returns the statistics object containing
/// information on nodes traversed (on nightly only). Not all of its fields will be initialized
/// since we don't call finish().
pub fn traverse_dom<E, D>(
traversal: &D,
token: PreTraverseToken<E>,
pool: Option<&rayon::ThreadPool>,
) -> E
E: TElement,
D: DomTraversal<E>,
let root = token
.expect("Should've ensured we needed to traverse");
let report_stats = should_report_statistics();
let dump_stats = traversal.shared_context().options.dump_style_statistics;
let start_time = if dump_stats {
} else {
// Declare the main-thread context, as well as the worker-thread contexts,
// which we may or may not instantiate. It's important to declare the worker-
// thread contexts first, so that they get dropped second. This matters because:
// * ThreadLocalContexts borrow AtomicRefCells in TLS.
// * Dropping a ThreadLocalContext can run SequentialTasks.
// * Sequential tasks may call into functions like
// Servo_StyleSet_GetBaseComputedValuesForElement, which instantiate a
// ThreadLocalStyleContext on the main thread. If the main thread
// ThreadLocalStyleContext has not released its TLS borrow by that point,
// we'll panic on double-borrow.
let mut scoped_tls = ScopedTLS::<ThreadLocalStyleContext<E>>::new(pool);
// Process the nodes breadth-first. This helps keep similar traversal characteristics for the
// style sharing cache.
let work_unit_max = work_unit_max();
with_pool_in_place_scope(work_unit_max, pool, |maybe_scope| {
let mut tlc = scoped_tls.ensure(parallel::create_thread_local_context);
let mut context = StyleContext {
shared: traversal.shared_context(),
thread_local: &mut tlc,
"Main thread should be the first thread"
let mut discovered = VecDeque::with_capacity(work_unit_max * 2);
discovered.push_back(unsafe { SendNode::new(root.as_node()) });
&mut context,
PerLevelTraversalData {
current_dom_depth: root.depth(),
// Collect statistics from thread-locals if requested.
if dump_stats || report_stats {
let mut aggregate = PerThreadTraversalStatistics::default();
for slot in scoped_tls.slots() {
if let Some(cx) = slot.get_mut() {
aggregate += cx.statistics.clone();
if report_stats {
// dump statistics to stdout if requested
if dump_stats {
let parallel = pool.is_some();
let stats =
TraversalStatistics::new(aggregate, traversal, parallel, start_time.unwrap());
if stats.is_large {
println!("{}", stats);