geckolib: use a global thread pool for styling

By having a single thread pool, rather than one per document, we use
less memory.  This addresses
https://bugzilla.mozilla.org/show_bug.cgi?id=1324250.

ports/geckolib/Cargo.toml

@@ -23,6 +23,7 @@ libc = "0.2"
 log = {version = "0.3.5", features = ["release_max_level_info"]}
 num_cpus = "1.1.0"
 parking_lot = "0.3"
+rayon = "0.6"
 selectors = {path = "../../components/selectors"}
 servo_url = {path = "../../components/url"}
 style = {path = "../../components/style", features = ["gecko"]}

ports/geckolib/glue.rs

@@ -9,6 +9,7 @@ use cssparser::ToCss as ParserToCss;
 use env_logger::LogBuilder;
 use euclid::Size2D;
 use parking_lot::RwLock;
+use rayon;
 use selectors::Element;
 use servo_url::ServoUrl;
 use std::borrow::Cow;
@@ -90,6 +91,20 @@ use stylesheet_loader::StylesheetLoader;
  * depend on but good enough for our purposes.
  */
 
+lazy_static! {
+    static ref STYLE_THREAD_POOL: Option<rayon::ThreadPool> = {
+        let num_threads = *NUM_THREADS;
+        if num_threads <= 1 {
+            return None;
+        }
+
+        let configuration =
+            rayon::Configuration::new().set_num_threads(num_threads);
+        let pool = rayon::ThreadPool::new(configuration).ok();
+        pool
+    };
+}
+
 #[no_mangle]
 pub extern "C" fn Servo_Initialize() -> () {
     // Initialize logging.
@@ -148,7 +163,7 @@ fn traverse_subtree(element: GeckoElement, raw_data: RawServoStyleSetBorrowed,
         }
     }
 
-    let mut per_doc_data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
+    let per_doc_data = PerDocumentStyleData::from_ffi(raw_data).borrow();
 
     let token = RecalcStyleOnly::pre_traverse(element, &per_doc_data.stylist, unstyled_children_only);
     if !token.should_traverse() {
@@ -159,7 +174,7 @@ fn traverse_subtree(element: GeckoElement, raw_data: RawServoStyleSetBorrowed,
     debug!("{:?}", ShowSubtreeData(element.as_node()));
 
     let shared_style_context = create_shared_context(&per_doc_data);
-    let traversal_driver = if per_doc_data.num_threads == 1 || per_doc_data.work_queue.is_none() {
+    let traversal_driver = if STYLE_THREAD_POOL.is_none() {
         TraversalDriver::Sequential
     } else {
         TraversalDriver::Parallel
@@ -169,7 +184,7 @@ fn traverse_subtree(element: GeckoElement, raw_data: RawServoStyleSetBorrowed,
     let known_depth = None;
     if traversal_driver.is_parallel() {
         parallel::traverse_dom(&traversal, element, known_depth, token,
-                               per_doc_data.work_queue.as_mut().unwrap());
+                               STYLE_THREAD_POOL.as_ref().unwrap());
     } else {
         sequential::traverse_dom(&traversal, element, token);
     }

ports/geckolib/lib.rs

@@ -9,9 +9,11 @@ extern crate atomic_refcell;
 extern crate cssparser;
 extern crate env_logger;
 extern crate euclid;
+#[macro_use] extern crate lazy_static;
 extern crate libc;
 #[macro_use] extern crate log;
 extern crate parking_lot;
+extern crate rayon;
 extern crate selectors;
 extern crate servo_url;
 extern crate style;