Add a layout debug module. This outputs a trace of the layout process to a JSON

file which can be viewed in an external tool. It provides a timelapse view of
how the flow tree and fragments changed during the layout process, which makes
it easier to debug layout bugs.

1 files changed, 11 insertions, 0 deletions

diff --git a/src/components/layout/layout_task.rs b/src/components/layout/layout_task.rs
index e104ea443fe..93b49652678 100644
--- a/src/components/layout/layout_task.rs
+++ b/src/components/layout/layout_task.rs
@@ -14,6 +14,7 @@ use flow::{PreorderFlowTraversal, PostorderFlowTraversal};
 use flow;
 use flow_ref::FlowRef;
 use incremental::RestyleDamage;
+use layout_debug;
 use parallel::UnsafeFlow;
 use parallel;
 use util::{LayoutDataAccess, LayoutDataWrapper, OpaqueNodeMethods, ToGfxColor};
@@ -530,6 +531,8 @@ impl LayoutTask {
     fn solve_constraints<'a>(&mut self,
                          layout_root: &mut Flow,
                          layout_context: &'a LayoutContext<'a>) {
+        let _scope = layout_debug::Scope::new("solve_constraints".to_string());
+
         if layout_context.shared.opts.bubble_inline_sizes_separately {
             let mut traversal = BubbleISizesTraversal {
                 layout_context: layout_context,
@@ -667,6 +670,10 @@ impl LayoutTask {
         // memory safety but is a useful debugging tool.)
         self.verify_flow_tree(&mut layout_root);
 
+        if self.opts.trace_layout {
+            layout_debug::begin_trace(layout_root.clone());
+        }
+
         // Propagate damage.
         profile(time::LayoutDamagePropagateCategory, self.time_profiler_chan.clone(), || {
             layout_root.get_mut().traverse_preorder(&mut PropagateDamageTraversal {
@@ -778,6 +785,10 @@ impl LayoutTask {
             });
         }
 
+        if self.opts.trace_layout {
+            layout_debug::end_trace();
+        }
+
         // Tell script that we're done.
         //
         // FIXME(pcwalton): This should probably be *one* channel, but we can't fix this without