1 files changed, 191 insertions, 0 deletions
diff --git a/servo/components/style/driver.rs b/servo/components/style/driver.rs
new file mode 100644
index 0000000000..be884e1c0b
--- /dev/null
+++ b/servo/components/style/driver.rs
@@ -0,0 +1,191 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+//! Implements traversal over the DOM tree. The traversal starts in sequential
+//! mode, and optionally parallelizes as it discovers work.
+
+#![deny(missing_docs)]
+
+use crate::context::{PerThreadTraversalStatistics, StyleContext};
+use crate::context::{ThreadLocalStyleContext, TraversalStatistics};
+use crate::dom::{SendNode, TElement, TNode};
+use crate::parallel;
+use crate::parallel::{DispatchMode, WORK_UNIT_MAX};
+use crate::scoped_tls::ScopedTLS;
+use crate::traversal::{DomTraversal, PerLevelTraversalData, PreTraverseToken};
+use rayon;
+use std::collections::VecDeque;
+use std::mem;
+use time;
+
+#[cfg(feature = "servo")]
+fn should_report_statistics() -> bool {
+    false
+}
+
+#[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;
+}
+
+/// 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
+where
+    E: TElement,
+    D: DomTraversal<E>,
+{
+    let root = token
+        .traversal_root()
+        .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 {
+        Some(time::precise_time_s())
+    } else {
+        None
+    };
+
+    // 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 tls_slots = None;
+    let mut tlc = ThreadLocalStyleContext::new();
+    let mut context = StyleContext {
+        shared: traversal.shared_context(),
+        thread_local: &mut tlc,
+    };
+
+    // Process the nodes breadth-first, just like the parallel traversal does.
+    // This helps keep similar traversal characteristics for the style sharing
+    // cache.
+    let mut discovered = VecDeque::<SendNode<E::ConcreteNode>>::with_capacity(WORK_UNIT_MAX * 2);
+    let mut depth = root.depth();
+    let mut nodes_remaining_at_current_depth = 1;
+    discovered.push_back(unsafe { SendNode::new(root.as_node()) });
+    while let Some(node) = discovered.pop_front() {
+        let mut children_to_process = 0isize;
+        let traversal_data = PerLevelTraversalData {
+            current_dom_depth: depth,
+        };
+        traversal.process_preorder(&traversal_data, &mut context, *node, |n| {
+            children_to_process += 1;
+            discovered.push_back(unsafe { SendNode::new(n) });
+        });
+
+        traversal.handle_postorder_traversal(
+            &mut context,
+            root.as_node().opaque(),
+            *node,
+            children_to_process,
+        );
+
+        nodes_remaining_at_current_depth -= 1;
+        if nodes_remaining_at_current_depth == 0 {
+            depth += 1;
+            // If there is enough work to parallelize over, and the caller allows
+            // parallelism, switch to the parallel driver. We do this only when
+            // moving to the next level in the dom so that we can pass the same
+            // depth for all the children.
+            if pool.is_some() && discovered.len() > WORK_UNIT_MAX {
+                let pool = pool.unwrap();
+                let tls = ScopedTLS::<ThreadLocalStyleContext<E>>::new(pool);
+                let root_opaque = root.as_node().opaque();
+                let drain = discovered.drain(..);
+                pool.scope_fifo(|scope| {
+                    // Enable a breadth-first rayon traversal. This causes the work
+                    // queue to be always FIFO, rather than FIFO for stealers and
+                    // FILO for the owner (which is what rayon does by default). This
+                    // ensures that we process all the elements at a given depth before
+                    // proceeding to the next depth, which is important for style sharing.
+                    gecko_profiler_label!(Layout, StyleComputation);
+                    parallel::traverse_nodes(
+                        drain,
+                        DispatchMode::TailCall,
+                        /* recursion_ok = */ true,
+                        root_opaque,
+                        PerLevelTraversalData {
+                            current_dom_depth: depth,
+                        },
+                        scope,
+                        pool,
+                        traversal,
+                        &tls,
+                    );
+                });
+
+                tls_slots = Some(tls.into_slots());
+                break;
+            }
+            nodes_remaining_at_current_depth = discovered.len();
+        }
+    }
+
+    // Collect statistics from thread-locals if requested.
+    if dump_stats || report_stats {
+        let mut aggregate = mem::replace(&mut context.thread_local.statistics, Default::default());
+        let parallel = tls_slots.is_some();
+        if let Some(ref mut tls) = tls_slots {
+            for slot in tls.iter_mut() {
+                if let Some(cx) = slot.get_mut() {
+                    aggregate += cx.statistics.clone();
+                }
+            }
+        }
+
+        if report_stats {
+            report_statistics(&aggregate);
+        }
+        // dump statistics to stdout if requested
+        if dump_stats {
+            let stats =
+                TraversalStatistics::new(aggregate, traversal, parallel, start_time.unwrap());
+            if stats.is_large {
+                println!("{}", stats);
+            }
+        }
+    }
+
+    root
+}