Refactor how LayoutContext structure works (reduce TLS lookups + simplify fns used by seq/parallel code paths).

- LayoutContext is renamed to SharedLayoutContext.
- SharedLayoutContext is immutable.
- LayoutContext is a wrapper around SharedLayoutContext + access to local caches (font, style etc).
- Creating a LayoutContext does a single local_data lookup to fetch the cache information.
- Android shares same implementation of context.rs as other platforms.
- LayoutContext can be used from both green thread (parallel layout) and native thread (sequential layout).
- Removes the need for other types (such as FontContext, StyleSharingCandidateCache etc) to be passed around.

16 files changed, 212 insertions, 355 deletions

diff --git a/src/components/layout/block.rs b/src/components/layout/block.rs
index 23a402f24c9..e72515088c2 100644
--- a/src/components/layout/block.rs
+++ b/src/components/layout/block.rs
@@ -402,7 +402,7 @@ fn translate_including_floats(cur_b: &mut Au, delta: Au, floats: &mut Floats) {
 ///
 /// Note that flows with position 'fixed' just form a flat list as they all
 /// have the Root flow as their CB.
-struct AbsoluteAssignBSizesTraversal<'a>(&'a mut LayoutContext);
+struct AbsoluteAssignBSizesTraversal<'a>(&'a LayoutContext<'a>);
 
 impl<'a> PreorderFlowTraversal for AbsoluteAssignBSizesTraversal<'a> {
     #[inline]
@@ -429,7 +429,7 @@ impl<'a> PreorderFlowTraversal for AbsoluteAssignBSizesTraversal<'a> {
 /// After that, it is up to the normal store-overflow traversal to propagate
 /// it further up.
 struct AbsoluteStoreOverflowTraversal<'a>{
-    layout_context: &'a mut LayoutContext,
+    layout_context: &'a LayoutContext<'a>,
 }
 
 impl<'a> PostorderFlowTraversal for AbsoluteStoreOverflowTraversal<'a> {
@@ -579,7 +579,7 @@ impl BlockFlow {
     }
 
     /// Compute the used value of inline-size for this Block.
-    fn compute_used_inline_size(&mut self, ctx: &mut LayoutContext, containing_block_inline_size: Au) {
+    fn compute_used_inline_size(&mut self, ctx: &LayoutContext, containing_block_inline_size: Au) {
         let block_type = self.block_type();
         match block_type {
             AbsoluteReplacedType => {
@@ -805,8 +805,8 @@ impl BlockFlow {
     /// `inline(always)` because this is only ever called by in-order or non-in-order top-level
     /// methods
     #[inline(always)]
-    pub fn assign_block_size_block_base(&mut self,
-                                    layout_context: &mut LayoutContext,
+    pub fn assign_block_size_block_base<'a>(&mut self,
+                                    layout_context: &'a LayoutContext<'a>,
                                     margins_may_collapse: MarginsMayCollapseFlag) {
         // Our current border-box position.
         let mut cur_b = Au(0);
@@ -949,7 +949,7 @@ impl BlockFlow {
         let mut block_size = cur_b - block_start_offset;
         if self.is_root() {
             let screen_size = LogicalSize::from_physical(
-                self.fragment.style.writing_mode, layout_context.screen_size);
+                self.fragment.style.writing_mode, layout_context.shared.screen_size);
             block_size = Au::max(screen_size.block, block_size)
         }
 
@@ -1053,7 +1053,7 @@ impl BlockFlow {
     /// should be calculated using CSS Section 10.6.7
     ///
     /// It does not calculate the block-size of the flow itself.
-    pub fn assign_block_size_float(&mut self, ctx: &mut LayoutContext) {
+    pub fn assign_block_size_float<'a>(&mut self, ctx: &'a LayoutContext<'a>) {
         let mut floats = Floats::new(self.fragment.style.writing_mode);
         for kid in self.base.child_iter() {
             flow::mut_base(kid).floats = floats.clone();
@@ -1179,7 +1179,7 @@ impl BlockFlow {
     /// + y-coordinate of the flow wrt its Containing Block.
     /// + block-size, vertical margins, and y-coordinate for the flow's box.
     fn calculate_abs_block_size_and_margins(&mut self, ctx: &LayoutContext) {
-        let containing_block_block_size = self.containing_block_size(ctx.screen_size).block;
+        let containing_block_block_size = self.containing_block_size(ctx.shared.screen_size).block;
         let static_b_offset = self.static_b_offset;
 
         // This is the stored content block-size value from assign-block-size
@@ -1445,7 +1445,7 @@ impl Flow for BlockFlow {
     /// any fragments it is responsible for flowing.
     ///
     /// TODO(pcwalton): Inline blocks.
-    fn bubble_inline_sizes(&mut self, _: &mut LayoutContext) {
+    fn bubble_inline_sizes(&mut self, _: &LayoutContext) {
         let mut flags = self.base.flags;
         flags.set_has_left_floated_descendants(false);
         flags.set_has_right_floated_descendants(false);
@@ -1489,7 +1489,7 @@ impl Flow for BlockFlow {
     ///
     /// Dual fragments consume some inline-size first, and the remainder is assigned to all child (block)
     /// contexts.
-    fn assign_inline_sizes(&mut self, layout_context: &mut LayoutContext) {
+    fn assign_inline_sizes(&mut self, layout_context: &LayoutContext) {
         debug!("assign_inline_sizes({}): assigning inline_size for flow",
                if self.is_float() {
                    "float"
@@ -1501,7 +1501,7 @@ impl Flow for BlockFlow {
             debug!("Setting root position");
             self.base.position.start = LogicalPoint::zero(self.base.writing_mode);
             self.base.position.size.inline = LogicalSize::from_physical(
-                self.base.writing_mode, layout_context.screen_size).inline;
+                self.base.writing_mode, layout_context.shared.screen_size).inline;
             self.base.floats = Floats::new(self.base.writing_mode);
 
             // The root element is never impacted by floats.
@@ -1559,7 +1559,7 @@ impl Flow for BlockFlow {
     ///
     /// This is called on child flows by the parent. Hence, we can assume that `assign_block-size` has
     /// already been called on the child (because of the bottom-up traversal).
-    fn assign_block_size_for_inorder_child_if_necessary(&mut self, layout_context: &mut LayoutContext)
+    fn assign_block_size_for_inorder_child_if_necessary<'a>(&mut self, layout_context: &'a LayoutContext<'a>)
                                                     -> bool {
         if self.is_float() {
             self.place_float();
@@ -1573,7 +1573,7 @@ impl Flow for BlockFlow {
         impacted
     }
 
-    fn assign_block_size(&mut self, ctx: &mut LayoutContext) {
+    fn assign_block_size<'a>(&mut self, ctx: &'a LayoutContext<'a>) {
         // Assign block-size for fragment if it is an image fragment.
         self.fragment.assign_replaced_block_size_if_necessary();
 
@@ -1794,7 +1794,7 @@ pub trait ISizeAndMarginsComputer {
     fn compute_inline_size_constraint_inputs(&self,
                                        block: &mut BlockFlow,
                                        parent_flow_inline_size: Au,
-                                       ctx: &mut LayoutContext)
+                                       ctx: &LayoutContext)
                                        -> ISizeConstraintInput {
         let containing_block_inline_size = self.containing_block_inline_size(block, parent_flow_inline_size, ctx);
         let computed_inline_size = self.initial_computed_inline_size(block, parent_flow_inline_size, ctx);
@@ -1864,7 +1864,7 @@ pub trait ISizeAndMarginsComputer {
     fn initial_computed_inline_size(&self,
                               block: &mut BlockFlow,
                               parent_flow_inline_size: Au,
-                              ctx: &mut LayoutContext)
+                              ctx: &LayoutContext)
                               -> MaybeAuto {
         MaybeAuto::from_style(block.fragment().style().content_inline_size(),
                               self.containing_block_inline_size(block, parent_flow_inline_size, ctx))
@@ -1873,7 +1873,7 @@ pub trait ISizeAndMarginsComputer {
     fn containing_block_inline_size(&self,
                               _: &mut BlockFlow,
                               parent_flow_inline_size: Au,
-                              _: &mut LayoutContext)
+                              _: &LayoutContext)
                               -> Au {
         parent_flow_inline_size
     }
@@ -1883,7 +1883,7 @@ pub trait ISizeAndMarginsComputer {
     /// CSS Section 10.4: Minimum and Maximum inline-sizes
     fn compute_used_inline_size(&self,
                           block: &mut BlockFlow,
-                          ctx: &mut LayoutContext,
+                          ctx: &LayoutContext,
                           parent_flow_inline_size: Au) {
         let mut input = self.compute_inline_size_constraint_inputs(block, parent_flow_inline_size, ctx);
 
@@ -2127,8 +2127,8 @@ impl ISizeAndMarginsComputer for AbsoluteNonReplaced {
         ISizeConstraintSolution::for_absolute_flow(inline_start, inline_end, inline_size, margin_inline_start, margin_inline_end)
     }
 
-    fn containing_block_inline_size(&self, block: &mut BlockFlow, _: Au, ctx: &mut LayoutContext) -> Au {
-        block.containing_block_size(ctx.screen_size).inline
+    fn containing_block_inline_size(&self, block: &mut BlockFlow, _: Au, ctx: &LayoutContext) -> Au {
+        block.containing_block_size(ctx.shared.screen_size).inline
     }
 
     fn set_flow_x_coord_if_necessary(&self,
@@ -2241,9 +2241,9 @@ impl ISizeAndMarginsComputer for AbsoluteReplaced {
     fn initial_computed_inline_size(&self,
                               block: &mut BlockFlow,
                               _: Au,
-                              ctx: &mut LayoutContext)
+                              ctx: &LayoutContext)
                               -> MaybeAuto {
-        let containing_block_inline_size = block.containing_block_size(ctx.screen_size).inline;
+        let containing_block_inline_size = block.containing_block_size(ctx.shared.screen_size).inline;
         let fragment = block.fragment();
         fragment.assign_replaced_inline_size_if_necessary(containing_block_inline_size, None);
         // For replaced absolute flow, the rest of the constraint solving will
@@ -2251,8 +2251,8 @@ impl ISizeAndMarginsComputer for AbsoluteReplaced {
         Specified(fragment.content_inline_size())
     }
 
-    fn containing_block_inline_size(&self, block: &mut BlockFlow, _: Au, ctx: &mut LayoutContext) -> Au {
-        block.containing_block_size(ctx.screen_size).inline
+    fn containing_block_inline_size(&self, block: &mut BlockFlow, _: Au, ctx: &LayoutContext) -> Au {
+        block.containing_block_size(ctx.shared.screen_size).inline
     }
 
     fn set_flow_x_coord_if_necessary(&self, block: &mut BlockFlow, solution: ISizeConstraintSolution) {
@@ -2291,7 +2291,7 @@ impl ISizeAndMarginsComputer for BlockReplaced {
     fn initial_computed_inline_size(&self,
                               block: &mut BlockFlow,
                               parent_flow_inline_size: Au,
-                              _: &mut LayoutContext)
+                              _: &LayoutContext)
                               -> MaybeAuto {
         let fragment = block.fragment();
         fragment.assign_replaced_inline_size_if_necessary(parent_flow_inline_size, None);
@@ -2347,7 +2347,7 @@ impl ISizeAndMarginsComputer for FloatReplaced {
     fn initial_computed_inline_size(&self,
                               block: &mut BlockFlow,
                               parent_flow_inline_size: Au,
-                              _: &mut LayoutContext)
+                              _: &LayoutContext)
                               -> MaybeAuto {
         let fragment = block.fragment();
         fragment.assign_replaced_inline_size_if_necessary(parent_flow_inline_size, None);
diff --git a/src/components/layout/construct.rs b/src/components/layout/construct.rs
index a780c073f17..db6084a1c02 100644
--- a/src/components/layout/construct.rs
+++ b/src/components/layout/construct.rs
@@ -47,7 +47,6 @@ use wrapper::{PostorderNodeMutTraversal, TLayoutNode, ThreadSafeLayoutNode};
 use wrapper::{Before, BeforeBlock, After, AfterBlock, Normal};
 
 use gfx::display_list::OpaqueNode;
-use gfx::font_context::FontContext;
 use script::dom::element::{HTMLIFrameElementTypeId, HTMLImageElementTypeId};
 use script::dom::element::{HTMLObjectElementTypeId};
 use script::dom::element::{HTMLTableColElementTypeId, HTMLTableDataCellElementTypeId};
@@ -184,47 +183,20 @@ enum WhitespaceStrippingMode {
 }
 
 /// An object that knows how to create flows.
-pub struct FlowConstructor<'a> {
+pub struct FlowConstructor<'a, 'b> {
     /// The layout context.
-    pub layout_context: &'a mut LayoutContext,
-
-    /// An optional font context. If this is `None`, then we fetch the font context from the
-    /// layout context.
-    ///
-    /// FIXME(pcwalton): This is pretty bogus and is basically just a workaround for libgreen
-    /// having slow TLS.
-    pub font_context: Option<Box<FontContext>>,
+    pub layout_context: &'b LayoutContext<'b>,
 }
 
-impl<'a> FlowConstructor<'a> {
+impl<'a, 'b> FlowConstructor<'a, 'b> {
     /// Creates a new flow constructor.
-    pub fn new(layout_context: &'a mut LayoutContext, font_context: Option<Box<FontContext>>)
-               -> FlowConstructor<'a> {
+    pub fn new<'b>(layout_context: &'b LayoutContext)
+               -> FlowConstructor<'a, 'b> {
         FlowConstructor {
             layout_context: layout_context,
-            font_context: font_context,
         }
     }
 
-    fn font_context<'a>(&'a mut self) -> &'a mut FontContext {
-        match self.font_context {
-            Some(ref mut font_context) => {
-                let font_context: &mut FontContext = &mut **font_context;
-                font_context
-            }
-            None => self.layout_context.font_context(),
-        }
-    }
-
-    /// Destroys this flow constructor and retrieves the font context.
-    pub fn unwrap_font_context(self) -> Option<Box<FontContext>> {
-        let FlowConstructor {
-            font_context,
-            ..
-        } = self;
-        font_context
-    }
-
     /// Builds the `ImageFragmentInfo` for the given image. This is out of line to guide inlining.
     fn build_fragment_info_for_image(&mut self, node: &ThreadSafeLayoutNode, url: Option<Url>)
                                 -> SpecificFragmentInfo {
@@ -233,7 +205,7 @@ impl<'a> FlowConstructor<'a> {
             Some(url) => {
                 // FIXME(pcwalton): The fact that image fragments store the cache within them makes
                 // little sense to me.
-                ImageFragment(ImageFragmentInfo::new(node, url, self.layout_context.image_cache.clone()))
+                ImageFragment(ImageFragmentInfo::new(node, url, self.layout_context.shared.image_cache.clone()))
             }
         }
     }
@@ -293,11 +265,11 @@ impl<'a> FlowConstructor<'a> {
         }
 
         let mut inline_flow = box InlineFlow::from_fragments((*node).clone(), fragments);
-        let (ascent, descent) = inline_flow.compute_minimum_ascent_and_descent(self.font_context(), &**node.style());
+        let (ascent, descent) = inline_flow.compute_minimum_ascent_and_descent(self.layout_context.font_context(), &**node.style());
         inline_flow.minimum_block_size_above_baseline = ascent;
         inline_flow.minimum_depth_below_baseline = descent;
         let mut inline_flow = inline_flow as Box<Flow>;
-        TextRunScanner::new().scan_for_runs(self.font_context(), inline_flow);
+        TextRunScanner::new().scan_for_runs(self.layout_context.font_context(), inline_flow);
         let mut inline_flow = FlowRef::new(inline_flow);
         inline_flow.finish(self.layout_context);
 
@@ -797,7 +769,7 @@ impl<'a> FlowConstructor<'a> {
     }
 }
 
-impl<'a> PostorderNodeMutTraversal for FlowConstructor<'a> {
+impl<'a, 'b> PostorderNodeMutTraversal for FlowConstructor<'a, 'b> {
     // Construct Flow based on 'display', 'position', and 'float' values.
     //
     // CSS 2.1 Section 9.7
@@ -1039,7 +1011,7 @@ pub trait FlowConstructionUtils {
     ///
     /// All flows must be finished at some point, or they will not have their intrinsic inline-sizes
     /// properly computed. (This is not, however, a memory safety problem.)
-    fn finish(&mut self, context: &mut LayoutContext);
+    fn finish(&mut self, context: &LayoutContext);
 }
 
 impl FlowConstructionUtils for FlowRef {
@@ -1066,8 +1038,8 @@ impl FlowConstructionUtils for FlowRef {
     /// properly computed. (This is not, however, a memory safety problem.)
     ///
     /// This must not be public because only the layout constructor can do this.
-    fn finish(&mut self, context: &mut LayoutContext) {
-        if !context.opts.bubble_inline_sizes_separately {
+    fn finish(&mut self, context: &LayoutContext) {
+        if !context.shared.opts.bubble_inline_sizes_separately {
             self.get_mut().bubble_inline_sizes(context)
         }
     }
diff --git a/src/components/layout/context.rs b/src/components/layout/context.rs
index 0a58e3a761a..936314dcb63 100644
--- a/src/components/layout/context.rs
+++ b/src/components/layout/context.rs
@@ -10,8 +10,6 @@ use geom::{Rect, Size2D};
 use gfx::display_list::OpaqueNode;
 use gfx::font_context::FontContext;
 use gfx::font_cache_task::FontCacheTask;
-#[cfg(not(target_os="android"))]
-use green::task::GreenTask;
 use script::layout_interface::LayoutChan;
 use servo_msg::constellation_msg::ConstellationChan;
 use servo_net::local_image_cache::LocalImageCache;
@@ -19,42 +17,37 @@ use servo_util::geometry::Au;
 use servo_util::opts::Opts;
 use sync::{Arc, Mutex};
 use std::mem;
-#[cfg(not(target_os="android"))]
-use std::ptr;
-#[cfg(not(target_os="android"))]
-use std::rt::local::Local;
-#[cfg(not(target_os="android"))]
-use std::rt::task::Task;
 use style::Stylist;
 use url::Url;
 
-#[cfg(not(target_os="android"))]
-#[thread_local]
-static mut FONT_CONTEXT: *mut FontContext = 0 as *mut FontContext;
-
-#[cfg(target_os="android")]
-local_data_key!(font_context: *mut FontContext)
-
-#[cfg(not(target_os="android"))]
-#[thread_local]
-static mut APPLICABLE_DECLARATIONS_CACHE: *mut ApplicableDeclarationsCache =
-    0 as *mut ApplicableDeclarationsCache;
-
-#[cfg(target_os="android")]
-local_data_key!(applicable_declarations_cache: *mut ApplicableDeclarationsCache)
-
-#[cfg(not(target_os="android"))]
-#[thread_local]
-static mut STYLE_SHARING_CANDIDATE_CACHE: *mut StyleSharingCandidateCache =
-    0 as *mut StyleSharingCandidateCache;
+struct LocalLayoutContext {
+    font_context: FontContext,
+    applicable_declarations_cache: ApplicableDeclarationsCache,
+    style_sharing_candidate_cache: StyleSharingCandidateCache,
+}
 
-#[cfg(target_os="android")]
-local_data_key!(style_sharing_candidate_cache: *mut StyleSharingCandidateCache)
+local_data_key!(local_context_key: *mut LocalLayoutContext)
+
+fn create_or_get_local_context(shared_layout_context: &SharedLayoutContext) -> *mut LocalLayoutContext {
+    let maybe_context = local_context_key.get();
+
+    let context = match maybe_context {
+        None => {
+            let context = box LocalLayoutContext {
+                font_context: FontContext::new(shared_layout_context.font_cache_task.clone()),
+                applicable_declarations_cache: ApplicableDeclarationsCache::new(),
+                style_sharing_candidate_cache: StyleSharingCandidateCache::new(),
+            };
+            local_context_key.replace(Some(unsafe { mem::transmute(context) }));
+            local_context_key.get().unwrap()
+        },
+        Some(context) => context
+    };
+
+    *context
+}
 
-/// Data shared by all layout workers.
-#[allow(raw_pointer_deriving)]
-#[deriving(Clone)]
-pub struct LayoutContext {
+pub struct SharedLayoutContext {
     /// The local image cache.
     pub image_cache: Arc<Mutex<LocalImageCache>>,
 
@@ -88,126 +81,43 @@ pub struct LayoutContext {
     pub dirty: Rect<Au>,
 }
 
-#[cfg(not(target_os="android"))]
-impl LayoutContext {
-    pub fn font_context<'a>(&'a mut self) -> &'a mut FontContext {
-        // Sanity check.
-        {
-            let mut task = Local::borrow(None::<Task>);
-            match task.maybe_take_runtime::<GreenTask>() {
-                Some(green) => {
-                    task.put_runtime(green);
-                    fail!("can't call this on a green task!")
-                }
-                None => {}
-            }
-        }
-
-        unsafe {
-            if FONT_CONTEXT == ptr::mut_null() {
-                let context = box FontContext::new(self.font_cache_task.clone());
-                FONT_CONTEXT = mem::transmute(context)
-            }
-            mem::transmute(FONT_CONTEXT)
-        }
-    }
-
-    pub fn applicable_declarations_cache<'a>(&'a self) -> &'a mut ApplicableDeclarationsCache {
-        // Sanity check.
-        {
-            let mut task = Local::borrow(None::<Task>);
-            match task.maybe_take_runtime::<GreenTask>() {
-                Some(green) => {
-                    task.put_runtime(green);
-                    fail!("can't call this on a green task!")
-                }
-                None => {}
-            }
-        }
+pub struct LayoutContext<'a> {
+    pub shared: &'a SharedLayoutContext,
+    cached_local_layout_context: *mut LocalLayoutContext,
+}
 
-        unsafe {
-            if APPLICABLE_DECLARATIONS_CACHE == ptr::mut_null() {
-                let cache = box ApplicableDeclarationsCache::new();
-                APPLICABLE_DECLARATIONS_CACHE = mem::transmute(cache)
-            }
-            mem::transmute(APPLICABLE_DECLARATIONS_CACHE)
-        }
-    }
+impl<'a> LayoutContext<'a> {
+    pub fn new(shared_layout_context: &'a SharedLayoutContext) -> LayoutContext<'a> {
 
-    pub fn style_sharing_candidate_cache<'a>(&'a self) -> &'a mut StyleSharingCandidateCache {
-        // Sanity check.
-        {
-            let mut task = Local::borrow(None::<Task>);
-            match task.maybe_take_runtime::<GreenTask>() {
-                Some(green) => {
-                    task.put_runtime(green);
-                    fail!("can't call this on a green task!")
-                }
-                None => {}
-            }
-        }
+        let local_context = create_or_get_local_context(shared_layout_context);
 
-        unsafe {
-            if STYLE_SHARING_CANDIDATE_CACHE == ptr::mut_null() {
-                let cache = box StyleSharingCandidateCache::new();
-                STYLE_SHARING_CANDIDATE_CACHE = mem::transmute(cache)
-            }
-            mem::transmute(STYLE_SHARING_CANDIDATE_CACHE)
+        LayoutContext {
+            shared: shared_layout_context,
+            cached_local_layout_context: local_context,
         }
     }
-}
-
 
-// On Android, we don't have the __tls_* functions emitted by rustc, so we
-// need to use the slower local_data functions.
-// Making matters worse, the local_data functions are very particular about
-// enforcing the lifetimes associated with objects that they hold onto,
-// which causes us some trouble we work around as below.
-#[cfg(target_os="android")]
-impl LayoutContext {
-    pub fn font_context<'a>(&'a mut self) -> &'a mut FontContext {
+    #[inline(always)]
+    pub fn font_context<'a>(&'a self) -> &'a mut FontContext {
         unsafe {
-            let opt = font_context.replace(None);
-            let mut context;
-            match opt {
-                Some(c) => context = mem::transmute(c),
-                None => {
-                    context = mem::transmute(box FontContext::new(self.font_cache_task.clone()))
-                }
-            }
-            font_context.replace(Some(context));
-            mem::transmute(context)
+            let cached_context = &*self.cached_local_layout_context;
+            mem::transmute(&cached_context.font_context)
         }
     }
 
+    #[inline(always)]
     pub fn applicable_declarations_cache<'a>(&'a self) -> &'a mut ApplicableDeclarationsCache {
         unsafe {
-            let opt = applicable_declarations_cache.replace(None);
-            let mut cache;
-            match opt {
-                Some(c) => cache = mem::transmute(c),
-                None => {
-                    cache = mem::transmute(box ApplicableDeclarationsCache::new());
-                }
-            }
-            applicable_declarations_cache.replace(Some(cache));
-            mem::transmute(cache)
+            let cached_context = &*self.cached_local_layout_context;
+            mem::transmute(&cached_context.applicable_declarations_cache)
         }
     }
 
+    #[inline(always)]
     pub fn style_sharing_candidate_cache<'a>(&'a self) -> &'a mut StyleSharingCandidateCache {
         unsafe {
-            let opt = style_sharing_candidate_cache.replace(None);
-            let mut cache;
-            match opt {
-                Some(c) => cache = mem::transmute(c),
-                None => {
-                    cache = mem::transmute(box StyleSharingCandidateCache::new());
-                }
-            }
-            style_sharing_candidate_cache.replace(Some(cache));
-            mem::transmute(cache)
+            let cached_context = &*self.cached_local_layout_context;
+            mem::transmute(&cached_context.style_sharing_candidate_cache)
         }
     }
 }
-
diff --git a/src/components/layout/css/matching.rs b/src/components/layout/css/matching.rs
index f6104195dab..3c7ee0d9a4c 100644
--- a/src/components/layout/css/matching.rs
+++ b/src/components/layout/css/matching.rs
@@ -11,7 +11,6 @@ use extra::LayoutAuxMethods;
 use util::{LayoutDataAccess, LayoutDataWrapper};
 use wrapper::{LayoutElement, LayoutNode, PostorderNodeMutTraversal, ThreadSafeLayoutNode};
 
-use gfx::font_context::FontContext;
 use servo_util::atom::Atom;
 use servo_util::cache::{Cache, LRUCache, SimpleHashCache};
 use servo_util::namespace::Null;
@@ -283,13 +282,9 @@ pub trait MatchMethods {
     /// sequentially.
     fn recalc_style_for_subtree(&self,
                                 stylist: &Stylist,
-                                layout_context: &mut LayoutContext,
-                                mut font_context: Box<FontContext>,
+                                layout_context: &LayoutContext,
                                 applicable_declarations: &mut ApplicableDeclarations,
-                                applicable_declarations_cache: &mut ApplicableDeclarationsCache,
-                                style_sharing_candidate_cache: &mut StyleSharingCandidateCache,
-                                parent: Option<LayoutNode>)
-                                -> Box<FontContext>;
+                                parent: Option<LayoutNode>);
 
     fn match_node(&self,
                   stylist: &Stylist,
@@ -462,18 +457,14 @@ impl<'ln> MatchMethods for LayoutNode<'ln> {
 
     fn recalc_style_for_subtree(&self,
                                 stylist: &Stylist,
-                                layout_context: &mut LayoutContext,
-                                mut font_context: Box<FontContext>,
+                                layout_context: &LayoutContext,
                                 applicable_declarations: &mut ApplicableDeclarations,
-                                applicable_declarations_cache: &mut ApplicableDeclarationsCache,
-                                style_sharing_candidate_cache: &mut StyleSharingCandidateCache,
-                                parent: Option<LayoutNode>)
-                                -> Box<FontContext> {
-        self.initialize_layout_data(layout_context.layout_chan.clone());
+                                parent: Option<LayoutNode>) {
+        self.initialize_layout_data(layout_context.shared.layout_chan.clone());
 
         // First, check to see whether we can share a style with someone.
         let sharing_result = unsafe {
-            self.share_style_if_possible(style_sharing_candidate_cache, parent.clone())
+            self.share_style_if_possible(layout_context.style_sharing_candidate_cache(), parent.clone())
         };
 
         // Otherwise, match and cascade selectors.
@@ -486,34 +477,30 @@ impl<'ln> MatchMethods for LayoutNode<'ln> {
                 unsafe {
                     self.cascade_node(parent,
                                       applicable_declarations,
-                                      applicable_declarations_cache)
+                                      layout_context.applicable_declarations_cache())
                 }
 
                 applicable_declarations.clear();
 
                 // Add ourselves to the LRU cache.
                 if shareable {
-                    style_sharing_candidate_cache.insert_if_possible(self)
+                    layout_context.style_sharing_candidate_cache().insert_if_possible(self)
                 }
             }
-            StyleWasShared(index) => style_sharing_candidate_cache.touch(index),
+            StyleWasShared(index) => layout_context.style_sharing_candidate_cache().touch(index),
         }
 
         for kid in self.children() {
-            font_context = kid.recalc_style_for_subtree(stylist,
-                                                        layout_context,
-                                                        font_context,
-                                                        applicable_declarations,
-                                                        applicable_declarations_cache,
-                                                        style_sharing_candidate_cache,
-                                                        Some(self.clone()))
+            kid.recalc_style_for_subtree(stylist,
+                                        layout_context,
+                                        applicable_declarations,
+                                        Some(self.clone()))
         }
 
         // Construct flows.
         let layout_node = ThreadSafeLayoutNode::new(self);
-        let mut flow_constructor = FlowConstructor::new(layout_context, Some(font_context));
+        let mut flow_constructor = FlowConstructor::new(layout_context);
         flow_constructor.process(&layout_node);
-        flow_constructor.unwrap_font_context().unwrap()
     }
 
     unsafe fn cascade_node(&self,
diff --git a/src/components/layout/flow.rs b/src/components/layout/flow.rs
index 6f124ca81f1..e401a1f75c6 100644
--- a/src/components/layout/flow.rs
+++ b/src/components/layout/flow.rs
@@ -149,24 +149,24 @@ pub trait Flow: fmt::Show + ToString + Share {
     /// this flow, all child flows have had their minimum and preferred inline-sizes set. This function
     /// must decide minimum/preferred inline-sizes based on its children's inline-sizes and the dimensions of
     /// any boxes it is responsible for flowing.
-    fn bubble_inline_sizes(&mut self, _ctx: &mut LayoutContext) {
+    fn bubble_inline_sizes(&mut self, _ctx: &LayoutContext) {
         fail!("bubble_inline_sizes not yet implemented")
     }
 
     /// Pass 2 of reflow: computes inline-size.
-    fn assign_inline_sizes(&mut self, _ctx: &mut LayoutContext) {
+    fn assign_inline_sizes(&mut self, _ctx: &LayoutContext) {
         fail!("assign_inline_sizes not yet implemented")
     }
 
     /// Pass 3a of reflow: computes block-size.
-    fn assign_block_size(&mut self, _ctx: &mut LayoutContext) {
+    fn assign_block_size<'a>(&mut self, _ctx: &'a LayoutContext<'a>) {
         fail!("assign_block_size not yet implemented")
     }
 
     /// Assigns block-sizes in-order; or, if this is a float, places the float. The default
     /// implementation simply assigns block-sizes if this flow is impacted by floats. Returns true if
     /// this child was impacted by floats or false otherwise.
-    fn assign_block_size_for_inorder_child_if_necessary(&mut self, layout_context: &mut LayoutContext)
+    fn assign_block_size_for_inorder_child_if_necessary<'a>(&mut self, layout_context: &'a LayoutContext<'a>)
                                                     -> bool {
         let impacted = base(&*self).flags.impacted_by_floats();
         if impacted {
@@ -365,7 +365,7 @@ pub trait MutableFlowUtils {
     // Mutators
 
     /// Computes the overflow region for this flow.
-    fn store_overflow(self, _: &mut LayoutContext);
+    fn store_overflow(self, _: &LayoutContext);
 
     /// Builds the display lists for this flow.
     fn build_display_list(self, layout_context: &LayoutContext);
@@ -963,7 +963,7 @@ impl<'a> MutableFlowUtils for &'a mut Flow {
     /// Assumption: This is called in a bottom-up traversal, so kids' overflows have
     /// already been set.
     /// Assumption: Absolute descendants have had their overflow calculated.
-    fn store_overflow(self, _: &mut LayoutContext) {
+    fn store_overflow(self, _: &LayoutContext) {
         let my_position = mut_base(self).position;
         let mut overflow = my_position;
 
diff --git a/src/components/layout/fragment.rs b/src/components/layout/fragment.rs
index ab004a3a17d..66ccb6b2d86 100644
--- a/src/components/layout/fragment.rs
+++ b/src/components/layout/fragment.rs
@@ -660,7 +660,7 @@ impl Fragment {
             Some(ref image_url) => image_url,
         };
 
-        let mut holder = ImageHolder::new(image_url.clone(), layout_context.image_cache.clone());
+        let mut holder = ImageHolder::new(image_url.clone(), layout_context.shared.image_cache.clone());
         let image = match holder.get_image() {
             None => {
                 // No image data at all? Do nothing.
@@ -860,7 +860,7 @@ impl Fragment {
                absolute_fragment_bounds,
                self);
         debug!("Fragment::build_display_list: dirty={}, flow_origin={}",
-               layout_context.dirty,
+               layout_context.shared.dirty,
                flow_origin);
 
         let mut accumulator = ChildDisplayListAccumulator::new(self.style(),
@@ -871,7 +871,7 @@ impl Fragment {
             return accumulator
         }
 
-        if !absolute_fragment_bounds.intersects(&layout_context.dirty) {
+        if !absolute_fragment_bounds.intersects(&layout_context.shared.dirty) {
             debug!("Fragment::build_display_list: Did not intersect...");
             return accumulator
         }
@@ -1431,7 +1431,7 @@ impl Fragment {
                iframe_fragment.pipeline_id,
                iframe_fragment.subpage_id);
         let msg = FrameRectMsg(iframe_fragment.pipeline_id, iframe_fragment.subpage_id, rect);
-        let ConstellationChan(ref chan) = layout_context.constellation_chan;
+        let ConstellationChan(ref chan) = layout_context.shared.constellation_chan;
         chan.send(msg)
     }
 }
diff --git a/src/components/layout/inline.rs b/src/components/layout/inline.rs
index e3aabadff35..89449ca5571 100644
--- a/src/components/layout/inline.rs
+++ b/src/components/layout/inline.rs
@@ -932,7 +932,7 @@ impl InlineFlow {
         // FIXME(#2795): Get the real container size
         let container_size = Size2D::zero();
         if !abs_rect.to_physical(self.base.writing_mode, container_size)
-                    .intersects(&layout_context.dirty) {
+                    .intersects(&layout_context.shared.dirty) {
             return
         }
 
@@ -1090,7 +1090,7 @@ impl Flow for InlineFlow {
         self
     }
 
-    fn bubble_inline_sizes(&mut self, _: &mut LayoutContext) {
+    fn bubble_inline_sizes(&mut self, _: &LayoutContext) {
         let writing_mode = self.base.writing_mode;
         for kid in self.base.child_iter() {
             flow::mut_base(kid).floats = Floats::new(writing_mode);
@@ -1115,7 +1115,7 @@ impl Flow for InlineFlow {
 
     /// Recursively (top-down) determines the actual inline-size of child contexts and fragments. When called
     /// on this context, the context has had its inline-size set by the parent context.
-    fn assign_inline_sizes(&mut self, _: &mut LayoutContext) {
+    fn assign_inline_sizes(&mut self, _: &LayoutContext) {
         // Initialize content fragment inline-sizes if they haven't been initialized already.
         //
         // TODO: Combine this with `LineBreaker`'s walk in the fragment list, or put this into `Fragment`.
@@ -1144,7 +1144,7 @@ impl Flow for InlineFlow {
     }
 
     /// Calculate and set the block-size of this flow. See CSS 2.1 § 10.6.1.
-    fn assign_block_size(&mut self, _: &mut LayoutContext) {
+    fn assign_block_size(&mut self, _: &LayoutContext) {
         debug!("assign_block_size_inline: assigning block_size for flow");
 
         // Divide the fragments into lines.
diff --git a/src/components/layout/layout_task.rs b/src/components/layout/layout_task.rs
index 83809aa137e..0edb9436771 100644
--- a/src/components/layout/layout_task.rs
+++ b/src/components/layout/layout_task.rs
@@ -5,12 +5,11 @@
 //! The layout task. Performs layout on the DOM, builds display lists and sends them to be
 //! rendered.
 
-use css::matching::{ApplicableDeclarations, ApplicableDeclarationsCache, MatchMethods};
-use css::matching::{StyleSharingCandidateCache};
+use css::matching::{ApplicableDeclarations, MatchMethods};
 use css::select::new_stylist;
 use css::node_style::StyledNode;
 use construct::{FlowConstructionResult, NoConstructionResult};
-use context::LayoutContext;
+use context::{LayoutContext, SharedLayoutContext};
 use flow::{Flow, ImmutableFlowUtils, MutableFlowUtils, MutableOwnedFlowUtils};
 use flow::{PreorderFlowTraversal, PostorderFlowTraversal};
 use flow;
@@ -27,7 +26,6 @@ use geom::rect::Rect;
 use geom::size::Size2D;
 use gfx::display_list::{ClipDisplayItemClass, ContentStackingLevel, DisplayItem};
 use gfx::display_list::{DisplayItemIterator, DisplayList, OpaqueNode};
-use gfx::font_context::FontContext;
 use gfx::render_task::{RenderInitMsg, RenderChan, RenderLayer};
 use gfx::{render_task, color};
 use layout_traits;
@@ -104,7 +102,7 @@ pub struct LayoutTask {
     pub stylist: Box<Stylist>,
 
     /// The workers that we use for parallel operation.
-    pub parallel_traversal: Option<WorkQueue<*mut LayoutContext,UnsafeFlow>>,
+    pub parallel_traversal: Option<WorkQueue<*const SharedLayoutContext,UnsafeFlow>>,
 
     /// The channel on which messages can be sent to the time profiler.
     pub time_profiler_chan: TimeProfilerChan,
@@ -178,7 +176,7 @@ impl PreorderFlowTraversal for FlowTreeVerificationTraversal {
 /// The bubble-inline-sizes traversal, the first part of layout computation. This computes preferred
 /// and intrinsic inline-sizes and bubbles them up the tree.
 pub struct BubbleISizesTraversal<'a> {
-    pub layout_context: &'a mut LayoutContext,
+    pub layout_context: &'a LayoutContext<'a>,
 }
 
 impl<'a> PostorderFlowTraversal for BubbleISizesTraversal<'a> {
@@ -199,7 +197,7 @@ impl<'a> PostorderFlowTraversal for BubbleISizesTraversal<'a> {
 
 /// The assign-inline-sizes traversal. In Gecko this corresponds to `Reflow`.
 pub struct AssignISizesTraversal<'a> {
-    pub layout_context: &'a mut LayoutContext,
+    pub layout_context: &'a LayoutContext<'a>,
 }
 
 impl<'a> PreorderFlowTraversal for AssignISizesTraversal<'a> {
@@ -214,7 +212,7 @@ impl<'a> PreorderFlowTraversal for AssignISizesTraversal<'a> {
 /// computation. Determines the final block-sizes for all layout objects, computes positions, and
 /// computes overflow regions. In Gecko this corresponds to `FinishAndStoreOverflow`.
 pub struct AssignBSizesAndStoreOverflowTraversal<'a> {
-    pub layout_context: &'a mut LayoutContext,
+    pub layout_context: &'a LayoutContext<'a>,
 }
 
 impl<'a> PostorderFlowTraversal for AssignBSizesAndStoreOverflowTraversal<'a> {
@@ -237,7 +235,7 @@ impl<'a> PostorderFlowTraversal for AssignBSizesAndStoreOverflowTraversal<'a> {
 
 /// The display list construction traversal.
 pub struct BuildDisplayListTraversal<'a> {
-    layout_context: &'a LayoutContext,
+    layout_context: &'a LayoutContext<'a>,
 }
 
 impl<'a> BuildDisplayListTraversal<'a> {
@@ -330,7 +328,7 @@ impl LayoutTask {
         let local_image_cache = Arc::new(Mutex::new(LocalImageCache::new(image_cache_task.clone())));
         let screen_size = Size2D(Au(0), Au(0));
         let parallel_traversal = if opts.layout_threads != 1 {
-            Some(WorkQueue::new("LayoutWorker", opts.layout_threads, ptr::mut_null()))
+            Some(WorkQueue::new("LayoutWorker", opts.layout_threads, ptr::null()))
         } else {
             None
         };
@@ -365,8 +363,8 @@ impl LayoutTask {
     }
 
     // Create a layout context for use in building display lists, hit testing, &c.
-    fn build_layout_context(&self, reflow_root: &LayoutNode, url: &Url) -> LayoutContext {
-        LayoutContext {
+    fn build_shared_layout_context(&self, reflow_root: &LayoutNode, url: &Url) -> SharedLayoutContext {
+        SharedLayoutContext {
             image_cache: self.local_image_cache.clone(),
             screen_size: self.screen_size.clone(),
             constellation_chan: self.constellation_chan.clone(),
@@ -542,10 +540,10 @@ impl LayoutTask {
     /// This corresponds to `Reflow()` in Gecko and `layout()` in WebKit/Blink and should be
     /// benchmarked against those two. It is marked `#[inline(never)]` to aid profiling.
     #[inline(never)]
-    fn solve_constraints(&mut self,
+    fn solve_constraints<'a>(&mut self,
                          layout_root: &mut Flow,
-                         layout_context: &mut LayoutContext) {
-        if layout_context.opts.bubble_inline_sizes_separately {
+                         layout_context: &'a LayoutContext<'a>) {
+        if layout_context.shared.opts.bubble_inline_sizes_separately {
             let mut traversal = BubbleISizesTraversal {
                 layout_context: layout_context,
             };
@@ -580,10 +578,10 @@ impl LayoutTask {
     #[inline(never)]
     fn solve_constraints_parallel(&mut self,
                                   layout_root: &mut FlowRef,
-                                  layout_context: &mut LayoutContext) {
-        if layout_context.opts.bubble_inline_sizes_separately {
+                                  shared_layout_context: &SharedLayoutContext) {
+        if shared_layout_context.opts.bubble_inline_sizes_separately {
             let mut traversal = BubbleISizesTraversal {
-                layout_context: layout_context,
+                layout_context: &LayoutContext::new(shared_layout_context),
             };
             layout_root.get_mut().traverse_postorder(&mut traversal);
         }
@@ -595,7 +593,7 @@ impl LayoutTask {
                 // operation out.
                 parallel::traverse_flow_tree_preorder(layout_root,
                                                       self.time_profiler_chan.clone(),
-                                                      layout_context,
+                                                      shared_layout_context,
                                                       traversal);
             }
         }
@@ -654,17 +652,7 @@ impl LayoutTask {
         self.screen_size = current_screen_size;
 
         // Create a layout context for use throughout the following passes.
-        let mut layout_ctx = self.build_layout_context(node, &data.url);
-
-        // Create a font context, if this is sequential.
-        //
-        // FIXME(pcwalton): This is a pretty bogus thing to do. Essentially this is a workaround
-        // for libgreen having slow TLS.
-        let mut font_context_opt = if self.parallel_traversal.is_none() {
-            Some(box FontContext::new(layout_ctx.font_cache_task.clone()))
-        } else {
-            None
-        };
+        let mut shared_layout_ctx = self.build_shared_layout_context(node, &data.url);
 
         let mut layout_root = profile(time::LayoutStyleRecalcCategory,
                                       self.time_profiler_chan.clone(),
@@ -672,19 +660,15 @@ impl LayoutTask {
             // Perform CSS selector matching and flow construction.
             match self.parallel_traversal {
                 None => {
+                    let layout_ctx = LayoutContext::new(&shared_layout_ctx);
                     let mut applicable_declarations = ApplicableDeclarations::new();
-                    let mut applicable_declarations_cache = ApplicableDeclarationsCache::new();
-                    let mut style_sharing_candidate_cache = StyleSharingCandidateCache::new();
-                    drop(node.recalc_style_for_subtree(&*self.stylist,
-                                                       &mut layout_ctx,
-                                                       font_context_opt.take_unwrap(),
-                                                       &mut applicable_declarations,
-                                                       &mut applicable_declarations_cache,
-                                                       &mut style_sharing_candidate_cache,
-                                                       None))
+                    node.recalc_style_for_subtree(&*self.stylist,
+                                                   &layout_ctx,
+                                                   &mut applicable_declarations,
+                                                   None)
                 }
                 Some(ref mut traversal) => {
-                    parallel::recalc_style_for_subtree(node, &mut layout_ctx, traversal)
+                    parallel::recalc_style_for_subtree(node, &mut shared_layout_ctx, traversal)
                 }
             }
 
@@ -710,11 +694,12 @@ impl LayoutTask {
             match self.parallel_traversal {
                 None => {
                     // Sequential mode.
-                    self.solve_constraints(layout_root.get_mut(), &mut layout_ctx)
+                    let layout_ctx = LayoutContext::new(&shared_layout_ctx);
+                    self.solve_constraints(layout_root.get_mut(), &layout_ctx)
                 }
                 Some(_) => {
                     // Parallel mode.
-                    self.solve_constraints_parallel(&mut layout_root, &mut layout_ctx)
+                    self.solve_constraints_parallel(&mut layout_root, &mut shared_layout_ctx)
                 }
             }
         });
@@ -725,11 +710,12 @@ impl LayoutTask {
             profile(time::LayoutDispListBuildCategory, self.time_profiler_chan.clone(), || {
                 // FIXME(#2795): Get the real container size
                 let container_size = Size2D::zero();
-                layout_ctx.dirty = flow::base(layout_root.get()).position.to_physical(
+                shared_layout_ctx.dirty = flow::base(layout_root.get()).position.to_physical(
                     writing_mode, container_size);
 
                 match self.parallel_traversal {
                     None => {
+                        let layout_ctx = LayoutContext::new(&shared_layout_ctx);
                         let mut traversal = BuildDisplayListTraversal {
                             layout_context: &layout_ctx,
                         };
@@ -738,7 +724,7 @@ impl LayoutTask {
                     Some(ref mut traversal) => {
                         parallel::build_display_list_for_subtree(&mut layout_root,
                                                                  self.time_profiler_chan.clone(),
-                                                                 &mut layout_ctx,
+                                                                 &mut shared_layout_ctx,
                                                                  traversal);
                     }
                 }
diff --git a/src/components/layout/parallel.rs b/src/components/layout/parallel.rs
index aa184cafda6..a2786e8ba91 100644
--- a/src/components/layout/parallel.rs
+++ b/src/components/layout/parallel.rs
@@ -8,7 +8,7 @@
 
 use css::matching::{ApplicableDeclarations, CannotShare, MatchMethods, StyleWasShared};
 use construct::FlowConstructor;
-use context::LayoutContext;
+use context::{LayoutContext, SharedLayoutContext};
 use extra::LayoutAuxMethods;
 use flow::{Flow, MutableFlowUtils, PreorderFlowTraversal, PostorderFlowTraversal};
 use flow;
@@ -116,7 +116,7 @@ trait ParallelPostorderFlowTraversal : PostorderFlowTraversal {
     /// fetch-and-subtract the parent's children count.
     fn run_parallel(&mut self,
                     mut unsafe_flow: UnsafeFlow,
-                    _: &mut WorkerProxy<*mut LayoutContext,UnsafeFlow>) {
+                    _: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>) {
         loop {
             unsafe {
                 // Get a real flow.
@@ -160,17 +160,17 @@ trait ParallelPostorderFlowTraversal : PostorderFlowTraversal {
 trait ParallelPreorderFlowTraversal : PreorderFlowTraversal {
     fn run_parallel(&mut self,
                     unsafe_flow: UnsafeFlow,
-                    proxy: &mut WorkerProxy<*mut LayoutContext,UnsafeFlow>);
+                    proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>);
 
     #[inline(always)]
     fn run_parallel_helper(&mut self,
                            unsafe_flow: UnsafeFlow,
-                           proxy: &mut WorkerProxy<*mut LayoutContext,UnsafeFlow>,
+                           proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>,
                            top_down_func: extern "Rust" fn(UnsafeFlow,
-                                                           &mut WorkerProxy<*mut LayoutContext,
+                                                           &mut WorkerProxy<*const SharedLayoutContext,
                                                                             UnsafeFlow>),
                            bottom_up_func: extern "Rust" fn(UnsafeFlow,
-                                                            &mut WorkerProxy<*mut LayoutContext,
+                                                            &mut WorkerProxy<*const SharedLayoutContext,
                                                                              UnsafeFlow>)) {
         let mut had_children = false;
         unsafe {
@@ -203,7 +203,7 @@ impl<'a> ParallelPostorderFlowTraversal for BubbleISizesTraversal<'a> {}
 impl<'a> ParallelPreorderFlowTraversal for AssignISizesTraversal<'a> {
     fn run_parallel(&mut self,
                     unsafe_flow: UnsafeFlow,
-                    proxy: &mut WorkerProxy<*mut LayoutContext,UnsafeFlow>) {
+                    proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>) {
         self.run_parallel_helper(unsafe_flow,
                                  proxy,
                                  assign_inline_sizes,
@@ -214,8 +214,9 @@ impl<'a> ParallelPreorderFlowTraversal for AssignISizesTraversal<'a> {
 impl<'a> ParallelPostorderFlowTraversal for AssignBSizesAndStoreOverflowTraversal<'a> {}
 
 fn recalc_style_for_node(unsafe_layout_node: UnsafeLayoutNode,
-                         proxy: &mut WorkerProxy<*mut LayoutContext,UnsafeLayoutNode>) {
-    let layout_context = unsafe { &mut **proxy.user_data() };
+                         proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeLayoutNode>) {
+    let shared_layout_context = unsafe { &**proxy.user_data() };
+    let layout_context = LayoutContext::new(shared_layout_context);
 
     // Get a real layout node.
     let node: LayoutNode = unsafe {
@@ -226,11 +227,11 @@ fn recalc_style_for_node(unsafe_layout_node: UnsafeLayoutNode,
     //
     // FIXME(pcwalton): Stop allocating here. Ideally this should just be done by the HTML
     // parser.
-    node.initialize_layout_data(layout_context.layout_chan.clone());
+    node.initialize_layout_data(layout_context.shared.layout_chan.clone());
 
     // Get the parent node.
     let opaque_node: OpaqueNode = OpaqueNodeMethods::from_layout_node(&node);
-    let parent_opt = if opaque_node == layout_context.reflow_root {
+    let parent_opt = if opaque_node == layout_context.shared.reflow_root {
         None
     } else {
         node.parent_node()
@@ -250,7 +251,7 @@ fn recalc_style_for_node(unsafe_layout_node: UnsafeLayoutNode,
 
             if node.is_element() {
                 // Perform the CSS selector matching.
-                let stylist = unsafe { &*layout_context.stylist };
+                let stylist = unsafe { &*layout_context.shared.stylist };
                 node.match_node(stylist, &mut applicable_declarations, &mut shareable);
             }
 
@@ -302,14 +303,12 @@ fn recalc_style_for_node(unsafe_layout_node: UnsafeLayoutNode,
     }
 
     // If we got here, we're a leaf. Start construction of flows for this node.
-    construct_flows(unsafe_layout_node, proxy)
+    construct_flows(unsafe_layout_node, &layout_context)
 }
 
 fn construct_flows(mut unsafe_layout_node: UnsafeLayoutNode,
-                   proxy: &mut WorkerProxy<*mut LayoutContext,UnsafeLayoutNode>) {
+                   layout_context: &LayoutContext) {
     loop {
-        let layout_context = unsafe { &mut **proxy.user_data() };
-
         // Get a real layout node.
         let node: LayoutNode = unsafe {
             layout_node_from_unsafe_layout_node(&unsafe_layout_node)
@@ -317,7 +316,7 @@ fn construct_flows(mut unsafe_layout_node: UnsafeLayoutNode,
 
         // Construct flows for this node.
         {
-            let mut flow_constructor = FlowConstructor::new(layout_context, None);
+            let mut flow_constructor = FlowConstructor::new(layout_context);
             flow_constructor.process(&ThreadSafeLayoutNode::new(&node));
         }
 
@@ -340,7 +339,7 @@ fn construct_flows(mut unsafe_layout_node: UnsafeLayoutNode,
 
         // If this is the reflow root, we're done.
         let opaque_node: OpaqueNode = OpaqueNodeMethods::from_layout_node(&node);
-        if layout_context.reflow_root == opaque_node {
+        if layout_context.shared.reflow_root == opaque_node {
             break
         }
 
@@ -376,25 +375,27 @@ fn construct_flows(mut unsafe_layout_node: UnsafeLayoutNode,
 }
 
 fn assign_inline_sizes(unsafe_flow: UnsafeFlow,
-                 proxy: &mut WorkerProxy<*mut LayoutContext,UnsafeFlow>) {
-    let layout_context = unsafe { &mut **proxy.user_data() };
+                 proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>) {
+    let shared_layout_context = unsafe { &**proxy.user_data() };
+    let layout_context = LayoutContext::new(shared_layout_context);
     let mut assign_inline_sizes_traversal = AssignISizesTraversal {
-        layout_context: layout_context,
+        layout_context: &layout_context,
     };
     assign_inline_sizes_traversal.run_parallel(unsafe_flow, proxy)
 }
 
 fn assign_block_sizes_and_store_overflow(unsafe_flow: UnsafeFlow,
-                                     proxy: &mut WorkerProxy<*mut LayoutContext,UnsafeFlow>) {
-    let layout_context = unsafe { &mut **proxy.user_data() };
+                                     proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>) {
+    let shared_layout_context = unsafe { &**proxy.user_data() };
+    let layout_context = LayoutContext::new(shared_layout_context);
     let mut assign_block_sizes_traversal = AssignBSizesAndStoreOverflowTraversal {
-        layout_context: layout_context,
+        layout_context: &layout_context,
     };
     assign_block_sizes_traversal.run_parallel(unsafe_flow, proxy)
 }
 
 fn compute_absolute_position(unsafe_flow: UnsafeFlow,
-                             proxy: &mut WorkerProxy<*mut LayoutContext,UnsafeFlow>) {
+                             proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>) {
     let mut had_descendants = false;
     unsafe {
         // Get a real flow.
@@ -448,8 +449,9 @@ fn compute_absolute_position(unsafe_flow: UnsafeFlow,
 }
 
 fn build_display_list(mut unsafe_flow: UnsafeFlow,
-                      proxy: &mut WorkerProxy<*mut LayoutContext,UnsafeFlow>) {
-    let layout_context = unsafe { &mut **proxy.user_data() };
+                      proxy: &mut WorkerProxy<*const SharedLayoutContext,UnsafeFlow>) {
+    let shared_layout_context = unsafe { &**proxy.user_data() };
+    let layout_context = LayoutContext::new(shared_layout_context);
 
     loop {
         unsafe {
@@ -457,7 +459,7 @@ fn build_display_list(mut unsafe_flow: UnsafeFlow,
             let flow: &mut FlowRef = mem::transmute(&unsafe_flow);
 
             // Build display lists.
-            flow.get_mut().build_display_list(layout_context);
+            flow.get_mut().build_display_list(&layout_context);
 
             {
                 let base = flow::mut_base(flow.get_mut());
@@ -506,9 +508,9 @@ fn build_display_list(mut unsafe_flow: UnsafeFlow,
 }
 
 pub fn recalc_style_for_subtree(root_node: &LayoutNode,
-                                layout_context: &mut LayoutContext,
-                                queue: &mut WorkQueue<*mut LayoutContext,UnsafeLayoutNode>) {
-    queue.data = layout_context as *mut _;
+                                shared_layout_context: &SharedLayoutContext,
+                                queue: &mut WorkQueue<*const SharedLayoutContext,UnsafeLayoutNode>) {
+    queue.data = shared_layout_context as *const _;
 
     // Enqueue the root node.
     queue.push(WorkUnit {
@@ -518,14 +520,14 @@ pub fn recalc_style_for_subtree(root_node: &LayoutNode,
 
     queue.run();
 
-    queue.data = ptr::mut_null()
+    queue.data = ptr::null()
 }
 
 pub fn traverse_flow_tree_preorder(root: &mut FlowRef,
                                    time_profiler_chan: TimeProfilerChan,
-                                   layout_context: &mut LayoutContext,
-                                   queue: &mut WorkQueue<*mut LayoutContext,UnsafeFlow>) {
-    queue.data = layout_context as *mut _;
+                                   shared_layout_context: &SharedLayoutContext,
+                                   queue: &mut WorkQueue<*const SharedLayoutContext,UnsafeFlow>) {
+    queue.data = shared_layout_context as *const _;
 
     profile(time::LayoutParallelWarmupCategory, time_profiler_chan, || {
         queue.push(WorkUnit {
@@ -536,14 +538,14 @@ pub fn traverse_flow_tree_preorder(root: &mut FlowRef,
 
     queue.run();
 
-    queue.data = ptr::mut_null()
+    queue.data = ptr::null()
 }
 
 pub fn build_display_list_for_subtree(root: &mut FlowRef,
                                       time_profiler_chan: TimeProfilerChan,
-                                      layout_context: &mut LayoutContext,
-                                      queue: &mut WorkQueue<*mut LayoutContext,UnsafeFlow>) {
-    queue.data = layout_context as *mut _;
+                                      shared_layout_context: &SharedLayoutContext,
+                                      queue: &mut WorkQueue<*const SharedLayoutContext,UnsafeFlow>) {
+    queue.data = shared_layout_context as *const _;
 
     profile(time::LayoutParallelWarmupCategory, time_profiler_chan, || {
         queue.push(WorkUnit {
@@ -554,6 +556,6 @@ pub fn build_display_list_for_subtree(root: &mut FlowRef,
 
     queue.run();
 
-    queue.data = ptr::mut_null()
+    queue.data = ptr::null()
 }
 
diff --git a/src/components/layout/table.rs b/src/components/layout/table.rs
index db6993c2a5e..98569d68c95 100644
--- a/src/components/layout/table.rs
+++ b/src/components/layout/table.rs
@@ -125,7 +125,7 @@ impl TableFlow {
     /// inline(always) because this is only ever called by in-order or non-in-order top-level
     /// methods
     #[inline(always)]
-    fn assign_block_size_table_base(&mut self, layout_context: &mut LayoutContext) {
+    fn assign_block_size_table_base<'a>(&mut self, layout_context: &'a LayoutContext<'a>) {
         self.block_flow.assign_block_size_block_base(layout_context, MarginsMayNotCollapse);
     }
 
@@ -164,7 +164,7 @@ impl Flow for TableFlow {
     /// table layout calculation.
     /// The maximum min/pref inline-sizes of each column are set from the rows for the automatic
     /// table layout calculation.
-    fn bubble_inline_sizes(&mut self, _: &mut LayoutContext) {
+    fn bubble_inline_sizes(&mut self, _: &LayoutContext) {
         let mut min_inline_size = Au(0);
         let mut pref_inline_size = Au(0);
         let mut did_first_row = false;
@@ -235,7 +235,7 @@ impl Flow for TableFlow {
 
     /// Recursively (top-down) determines the actual inline-size of child contexts and fragments. When
     /// called on this context, the context has had its inline-size set by the parent context.
-    fn assign_inline_sizes(&mut self, ctx: &mut LayoutContext) {
+    fn assign_inline_sizes(&mut self, ctx: &LayoutContext) {
         debug!("assign_inline_sizes({}): assigning inline_size for flow", "table");
 
         // The position was set to the containing block by the flow's parent.
@@ -282,7 +282,7 @@ impl Flow for TableFlow {
         self.block_flow.propagate_assigned_inline_size_to_children(inline_start_content_edge, content_inline_size, Some(self.col_inline_sizes.clone()));
     }
 
-    fn assign_block_size(&mut self, ctx: &mut LayoutContext) {
+    fn assign_block_size<'a>(&mut self, ctx: &'a LayoutContext<'a>) {
         debug!("assign_block_size: assigning block_size for table");
         self.assign_block_size_table_base(ctx);
     }
@@ -309,7 +309,7 @@ impl ISizeAndMarginsComputer for InternalTable {
     /// CSS Section 10.4: Minimum and Maximum inline-sizes
     fn compute_used_inline_size(&self,
                           block: &mut BlockFlow,
-                          ctx: &mut LayoutContext,
+                          ctx: &LayoutContext,
                           parent_flow_inline_size: Au) {
         let input = self.compute_inline_size_constraint_inputs(block, parent_flow_inline_size, ctx);
         let solution = self.solve_inline_size_constraints(block, &input);
diff --git a/src/components/layout/table_caption.rs b/src/components/layout/table_caption.rs
index 9ad0e017813..8c1dba3e7ca 100644
--- a/src/components/layout/table_caption.rs
+++ b/src/components/layout/table_caption.rs
@@ -47,16 +47,16 @@ impl Flow for TableCaptionFlow {
         &mut self.block_flow
     }
 
-    fn bubble_inline_sizes(&mut self, ctx: &mut LayoutContext) {
+    fn bubble_inline_sizes(&mut self, ctx: &LayoutContext) {
         self.block_flow.bubble_inline_sizes(ctx);
     }
 
-    fn assign_inline_sizes(&mut self, ctx: &mut LayoutContext) {
+    fn assign_inline_sizes(&mut self, ctx: &LayoutContext) {
         debug!("assign_inline_sizes({}): assigning inline_size for flow", "table_caption");
         self.block_flow.assign_inline_sizes(ctx);
     }
 
-    fn assign_block_size(&mut self, ctx: &mut LayoutContext) {
+    fn assign_block_size<'a>(&mut self, ctx: &'a LayoutContext<'a>) {
         debug!("assign_block_size: assigning block_size for table_caption");
         self.block_flow.assign_block_size(ctx);
     }
diff --git a/src/components/layout/table_cell.rs b/src/components/layout/table_cell.rs
index 57bc806f7fc..0011cd29fbc 100644
--- a/src/components/layout/table_cell.rs
+++ b/src/components/layout/table_cell.rs
@@ -45,7 +45,7 @@ impl TableCellFlow {
     /// inline(always) because this is only ever called by in-order or non-in-order top-level
     /// methods
     #[inline(always)]
-    fn assign_block_size_table_cell_base(&mut self, layout_context: &mut LayoutContext) {
+    fn assign_block_size_table_cell_base<'a>(&mut self, layout_context: &'a LayoutContext<'a>) {
         self.block_flow.assign_block_size_block_base(layout_context, MarginsMayNotCollapse)
     }
 
@@ -69,7 +69,7 @@ impl Flow for TableCellFlow {
     }
 
     /// Minimum/preferred inline-sizes set by this function are used in automatic table layout calculation.
-    fn bubble_inline_sizes(&mut self, ctx: &mut LayoutContext) {
+    fn bubble_inline_sizes(&mut self, ctx: &LayoutContext) {
         self.block_flow.bubble_inline_sizes(ctx);
         let specified_inline_size = MaybeAuto::from_style(self.block_flow.fragment.style().content_inline_size(),
                                                     Au::new(0)).specified_or_zero();
@@ -85,7 +85,7 @@ impl Flow for TableCellFlow {
 
     /// Recursively (top-down) determines the actual inline-size of child contexts and fragments. When
     /// called on this context, the context has had its inline-size set by the parent table row.
-    fn assign_inline_sizes(&mut self, ctx: &mut LayoutContext) {
+    fn assign_inline_sizes(&mut self, ctx: &LayoutContext) {
         debug!("assign_inline_sizes({}): assigning inline_size for flow", "table_cell");
 
         // The position was set to the column inline-size by the parent flow, table row flow.
@@ -104,7 +104,7 @@ impl Flow for TableCellFlow {
                                                              None);
     }
 
-    fn assign_block_size(&mut self, ctx: &mut LayoutContext) {
+    fn assign_block_size<'a>(&mut self, ctx: &'a LayoutContext<'a>) {
         debug!("assign_block_size: assigning block_size for table_cell");
         self.assign_block_size_table_cell_base(ctx);
     }
diff --git a/src/components/layout/table_colgroup.rs b/src/components/layout/table_colgroup.rs
index a27cff86deb..270f55970b2 100644
--- a/src/components/layout/table_colgroup.rs
+++ b/src/components/layout/table_colgroup.rs
@@ -52,7 +52,7 @@ impl Flow for TableColGroupFlow {
         self
     }
 
-    fn bubble_inline_sizes(&mut self, _: &mut LayoutContext) {
+    fn bubble_inline_sizes(&mut self, _: &LayoutContext) {
         for fragment in self.cols.iter() {
             // get the specified value from inline-size property
             let inline_size = MaybeAuto::from_style(fragment.style().content_inline_size(),
@@ -70,11 +70,11 @@ impl Flow for TableColGroupFlow {
 
     /// Table column inline-sizes are assigned in table flow and propagated to table row or rowgroup flow.
     /// Therefore, table colgroup flow does not need to assign its inline-size.
-    fn assign_inline_sizes(&mut self, _ctx: &mut LayoutContext) {
+    fn assign_inline_sizes(&mut self, _ctx: &LayoutContext) {
     }
 
     /// Table column do not have block-size.
-    fn assign_block_size(&mut self, _ctx: &mut LayoutContext) {
+    fn assign_block_size(&mut self, _ctx: &LayoutContext) {
     }
 }
 
diff --git a/src/components/layout/table_row.rs b/src/components/layout/table_row.rs
index 6e8f5bfcfd7..101f00eb5cc 100644
--- a/src/components/layout/table_row.rs
+++ b/src/components/layout/table_row.rs
@@ -75,7 +75,7 @@ impl TableRowFlow {
     /// inline(always) because this is only ever called by in-order or non-in-order top-level
     /// methods
     #[inline(always)]
-    fn assign_block_size_table_row_base(&mut self, layout_context: &mut LayoutContext) {
+    fn assign_block_size_table_row_base<'a>(&mut self, layout_context: &'a LayoutContext<'a>) {
         let (block_start_offset, _, _) = self.initialize_offsets();
 
         let /* mut */ cur_y = block_start_offset;
@@ -165,7 +165,7 @@ impl Flow for TableRowFlow {
     /// responsible for flowing.
     /// Min/pref inline-sizes set by this function are used in automatic table layout calculation.
     /// The specified column inline-sizes of children cells are used in fixed table layout calculation.
-    fn bubble_inline_sizes(&mut self, _: &mut LayoutContext) {
+    fn bubble_inline_sizes(&mut self, _: &LayoutContext) {
         let mut min_inline_size = Au(0);
         let mut pref_inline_size = Au(0);
         /* find the specified inline_sizes from child table-cell contexts */
@@ -194,7 +194,7 @@ impl Flow for TableRowFlow {
 
     /// Recursively (top-down) determines the actual inline-size of child contexts and fragments. When called
     /// on this context, the context has had its inline-size set by the parent context.
-    fn assign_inline_sizes(&mut self, ctx: &mut LayoutContext) {
+    fn assign_inline_sizes(&mut self, ctx: &LayoutContext) {
         debug!("assign_inline_sizes({}): assigning inline_size for flow", "table_row");
 
         // The position was set to the containing block by the flow's parent.
@@ -208,7 +208,7 @@ impl Flow for TableRowFlow {
         self.block_flow.propagate_assigned_inline_size_to_children(inline_start_content_edge, Au(0), Some(self.col_inline_sizes.clone()));
     }
 
-    fn assign_block_size(&mut self, ctx: &mut LayoutContext) {
+    fn assign_block_size<'a>(&mut self, ctx: &'a LayoutContext<'a>) {
         debug!("assign_block_size: assigning block_size for table_row");
         self.assign_block_size_table_row_base(ctx);
     }
diff --git a/src/components/layout/table_rowgroup.rs b/src/components/layout/table_rowgroup.rs
index b39bee19596..48f9d376af3 100644
--- a/src/components/layout/table_rowgroup.rs
+++ b/src/components/layout/table_rowgroup.rs
@@ -74,7 +74,7 @@ impl TableRowGroupFlow {
     /// inline(always) because this is only ever called by in-order or non-in-order top-level
     /// methods
     #[inline(always)]
-    fn assign_block_size_table_rowgroup_base(&mut self, layout_context: &mut LayoutContext) {
+    fn assign_block_size_table_rowgroup_base<'a>(&mut self, layout_context: &'a LayoutContext<'a>) {
         let (block_start_offset, _, _) = self.initialize_offsets();
 
         let mut cur_y = block_start_offset;
@@ -133,7 +133,7 @@ impl Flow for TableRowGroupFlow {
     /// Min/pref inline-sizes set by this function are used in automatic table layout calculation.
     /// Also, this function finds the specified column inline-sizes from the first row.
     /// Those are used in fixed table layout calculation
-    fn bubble_inline_sizes(&mut self, _: &mut LayoutContext) {
+    fn bubble_inline_sizes(&mut self, _: &LayoutContext) {
         let mut min_inline_size = Au(0);
         let mut pref_inline_size = Au(0);
 
@@ -175,7 +175,7 @@ impl Flow for TableRowGroupFlow {
 
     /// Recursively (top-down) determines the actual inline-size of child contexts and fragments. When
     /// called on this context, the context has had its inline-size set by the parent context.
-    fn assign_inline_sizes(&mut self, ctx: &mut LayoutContext) {
+    fn assign_inline_sizes(&mut self, ctx: &LayoutContext) {
         debug!("assign_inline_sizes({}): assigning inline_size for flow", "table_rowgroup");
 
         // The position was set to the containing block by the flow's parent.
@@ -191,7 +191,7 @@ impl Flow for TableRowGroupFlow {
         self.block_flow.propagate_assigned_inline_size_to_children(inline_start_content_edge, content_inline_size, Some(self.col_inline_sizes.clone()));
     }
 
-    fn assign_block_size(&mut self, ctx: &mut LayoutContext) {
+    fn assign_block_size<'a>(&mut self, ctx: &'a LayoutContext<'a>) {
         debug!("assign_block_size: assigning block_size for table_rowgroup");
         self.assign_block_size_table_rowgroup_base(ctx);
     }
diff --git a/src/components/layout/table_wrapper.rs b/src/components/layout/table_wrapper.rs
index 5d431e18c4e..2084ef52bdc 100644
--- a/src/components/layout/table_wrapper.rs
+++ b/src/components/layout/table_wrapper.rs
@@ -100,7 +100,7 @@ impl TableWrapperFlow {
     /// inline(always) because this is only ever called by in-order or non-in-order top-level
     /// methods
     #[inline(always)]
-    fn assign_block_size_table_wrapper_base(&mut self, layout_context: &mut LayoutContext) {
+    fn assign_block_size_table_wrapper_base<'a>(&mut self, layout_context: &'a LayoutContext<'a>) {
         self.block_flow.assign_block_size_block_base(layout_context, MarginsMayNotCollapse);
     }
 
@@ -129,7 +129,7 @@ impl Flow for TableWrapperFlow {
     min/pref inline_sizes based on child context inline_sizes and dimensions of
     any fragments it is responsible for flowing.  */
 
-    fn bubble_inline_sizes(&mut self, ctx: &mut LayoutContext) {
+    fn bubble_inline_sizes(&mut self, ctx: &LayoutContext) {
         // get column inline-sizes info from table flow
         for kid in self.block_flow.base.child_iter() {
             assert!(kid.is_table_caption() || kid.is_table());
@@ -147,7 +147,7 @@ impl Flow for TableWrapperFlow {
     ///
     /// Dual fragments consume some inline-size first, and the remainder is assigned to all child (block)
     /// contexts.
-    fn assign_inline_sizes(&mut self, ctx: &mut LayoutContext) {
+    fn assign_inline_sizes(&mut self, ctx: &LayoutContext) {
         debug!("assign_inline_sizes({}): assigning inline_size for flow",
                if self.is_float() {
                    "floated table_wrapper"
@@ -178,7 +178,7 @@ impl Flow for TableWrapperFlow {
         self.block_flow.propagate_assigned_inline_size_to_children(inline_start_content_edge, content_inline_size, assigned_col_inline_sizes);
     }
 
-    fn assign_block_size(&mut self, ctx: &mut LayoutContext) {
+    fn assign_block_size<'a>(&mut self, ctx: &'a LayoutContext<'a>) {
         if self.is_float() {
             debug!("assign_block_size_float: assigning block_size for floated table_wrapper");
             self.block_flow.assign_block_size_float(ctx);
@@ -208,7 +208,7 @@ struct TableWrapper;
 impl TableWrapper {
     fn compute_used_inline_size_table_wrapper(&self,
                                         table_wrapper: &mut TableWrapperFlow,
-                                        ctx: &mut LayoutContext,
+                                        ctx: &LayoutContext,
                                         parent_flow_inline_size: Au) {
         let input = self.compute_inline_size_constraint_inputs_table_wrapper(table_wrapper,
                                                                        parent_flow_inline_size,
@@ -223,7 +223,7 @@ impl TableWrapper {
     fn compute_inline_size_constraint_inputs_table_wrapper(&self,
                                                      table_wrapper: &mut TableWrapperFlow,
                                                      parent_flow_inline_size: Au,
-                                                     ctx: &mut LayoutContext)
+                                                     ctx: &LayoutContext)
                                                      -> ISizeConstraintInput {
         let mut input = self.compute_inline_size_constraint_inputs(&mut table_wrapper.block_flow,
                                                              parent_flow_inline_size,