Remove parallel display list construction

Parallel display list construction hasn't been shown to give any
performance gains. It is also incompatible with the current flat display
list implementation. Once flat display lists have landed, we can explore
possible benefits of parallel construction once again.

components/layout/layout_thread.rs

@@ -855,19 +855,7 @@ impl LayoutThread {
             flow::mut_base(flow_ref::deref_mut(layout_root)).clip =
                 ClippingRegion::from_rect(&data.page_clip_rect);
 
-            match (&mut self.parallel_traversal, opts::get().parallel_display_list_building) {
+            sequential::build_display_list_for_subtree(layout_root, shared_layout_context);
-                (&mut Some(ref mut traversal), true) => {
-                    parallel::build_display_list_for_subtree(layout_root,
-                                                             metadata,
-                                                             sender,
-                                                             shared_layout_context,
-                                                             traversal);
-                }
-                _ => {
-                    sequential::build_display_list_for_subtree(layout_root,
-                                                               shared_layout_context);
-                }
-            }
 
             if data.goal == ReflowGoal::ForDisplay {
                 debug!("Done building display list.");

components/layout/parallel.rs

@@ -17,8 +17,8 @@ use std::sync::atomic::{AtomicIsize, Ordering};
 use style::dom::{TNode, UnsafeNode};
 use style::parallel::{CHUNK_SIZE, WorkQueueData};
 use style::parallel::{run_queue_with_custom_work_data_type};
-use traversal::{AssignBSizesAndStoreOverflow, AssignISizes, BubbleISizes};
+use traversal::AssignBSizesAndStoreOverflow;
-use traversal::{BuildDisplayList, ComputeAbsolutePositions, PostorderNodeMutTraversal};
+use traversal::{AssignISizes, BubbleISizes, PostorderNodeMutTraversal};
 use util::opts;
 use util::workqueue::{WorkQueue, WorkUnit, WorkerProxy};
 
@@ -200,23 +200,6 @@ impl<'a> ParallelPreorderFlowTraversal for AssignISizes<'a> {
 
 impl<'a> ParallelPostorderFlowTraversal for AssignBSizesAndStoreOverflow<'a> {}
 
-impl<'a> ParallelPreorderFlowTraversal for ComputeAbsolutePositions<'a> {
-    fn run_parallel(&self,
-                    unsafe_flows: UnsafeFlowList,
-                    proxy: &mut WorkerProxy<SharedLayoutContext, UnsafeFlowList>) {
-        self.run_parallel_helper(unsafe_flows,
-                                 proxy,
-                                 compute_absolute_positions,
-                                 build_display_list)
-    }
-
-    fn should_record_thread_ids(&self) -> bool {
-        false
-    }
-}
-
-impl<'a> ParallelPostorderFlowTraversal for BuildDisplayList<'a> {}
-
 fn assign_inline_sizes(unsafe_flows: UnsafeFlowList,
                        proxy: &mut WorkerProxy<SharedLayoutContext, UnsafeFlowList>) {
     let shared_layout_context = proxy.user_data();
@@ -237,28 +220,6 @@ fn assign_block_sizes_and_store_overflow(
     assign_block_sizes_traversal.run_parallel(unsafe_flow)
 }
 
-fn compute_absolute_positions(
-        unsafe_flows: UnsafeFlowList,
-        proxy: &mut WorkerProxy<SharedLayoutContext, UnsafeFlowList>) {
-    let shared_layout_context = proxy.user_data();
-    let layout_context = LayoutContext::new(shared_layout_context);
-    let compute_absolute_positions_traversal = ComputeAbsolutePositions {
-        layout_context: &layout_context,
-    };
-    compute_absolute_positions_traversal.run_parallel(unsafe_flows, proxy);
-}
-
-fn build_display_list(unsafe_flow: UnsafeFlow,
-                      shared_layout_context: &SharedLayoutContext) {
-    let layout_context = LayoutContext::new(shared_layout_context);
-
-    let build_display_list_traversal = BuildDisplayList {
-        layout_context: &layout_context,
-    };
-
-    build_display_list_traversal.run_parallel(unsafe_flow);
-}
-
 pub fn traverse_flow_tree_preorder(
         root: &mut FlowRef,
         profiler_metadata: Option<TimerMetadata>,
@@ -281,20 +242,3 @@ pub fn traverse_flow_tree_preorder(
         });
     }, shared_layout_context);
 }
-
-pub fn build_display_list_for_subtree(
-        root: &mut FlowRef,
-        profiler_metadata: Option<TimerMetadata>,
-        time_profiler_chan: time::ProfilerChan,
-        shared_layout_context: &SharedLayoutContext,
-        queue: &mut WorkQueue<SharedLayoutContext, WorkQueueData>) {
-    run_queue_with_custom_work_data_type(queue, |queue| {
-        profile(time::ProfilerCategory::LayoutParallelWarmup, profiler_metadata,
-                time_profiler_chan, || {
-            queue.push(WorkUnit {
-                fun: compute_absolute_positions,
-                data: (box vec![mut_owned_flow_to_unsafe_flow(root)], 0),
-            })
-        });
-    }, shared_layout_context);
-}

components/util/opts.rs

@@ -169,9 +169,6 @@ pub struct Opts {
     /// Whether Style Sharing Cache is used
     pub disable_share_style_cache: bool,
 
-    /// Whether to run absolute position calculation and display list construction in parallel.
-    pub parallel_display_list_building: bool,
-
     /// Translate mouse input into touch events.
     pub convert_mouse_to_touch: bool,
 
@@ -263,9 +260,6 @@ pub struct DebugOptions {
     /// Disable the style sharing cache.
     pub disable_share_style_cache: bool,
 
-    /// Build display lists in parallel.
-    pub parallel_display_list_building: bool,
-
     /// Translate mouse input into touch events.
     pub convert_mouse_to_touch: bool,
 
@@ -316,7 +310,6 @@ impl DebugOptions {
                 "trace-layout" => debug_options.trace_layout = true,
                 "validate-display-list-geometry" => debug_options.validate_display_list_geometry = true,
                 "disable-share-style-cache" => debug_options.disable_share_style_cache = true,
-                "parallel-display-list-building" => debug_options.parallel_display_list_building = true,
                 "convert-mouse-to-touch" => debug_options.convert_mouse_to_touch = true,
                 "replace-surrogates" => debug_options.replace_surrogates = true,
                 "gc-profile" => debug_options.gc_profile = true,
@@ -497,7 +490,6 @@ pub fn default_opts() -> Opts {
         profile_script_events: false,
         profile_heartbeats: false,
         disable_share_style_cache: false,
-        parallel_display_list_building: false,
         convert_mouse_to_touch: false,
         exit_after_load: false,
         no_native_titlebar: false,
@@ -737,7 +729,6 @@ pub fn from_cmdline_args(args: &[String]) -> ArgumentParsingResult {
         relayout_event: debug_options.relayout_event,
         validate_display_list_geometry: debug_options.validate_display_list_geometry,
         disable_share_style_cache: debug_options.disable_share_style_cache,
-        parallel_display_list_building: debug_options.parallel_display_list_building,
         convert_mouse_to_touch: debug_options.convert_mouse_to_touch,
         exit_after_load: opt_match.opt_present("x"),
         no_native_titlebar: opt_match.opt_present("b"),