Auto merge of #11394 - mbrubeck:stacking-context-id, r=pcwalton

Reduce the size of StackingContextId

r? @pcwalton

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2 files changed, 8 insertions, 7 deletions

diff --git a/components/gfx/display_list/mod.rs b/components/gfx/display_list/mod.rs
index 73071d91ec0..091e9a08c5a 100644
--- a/components/gfx/display_list/mod.rs
+++ b/components/gfx/display_list/mod.rs
@@ -1409,20 +1409,21 @@ pub enum FragmentType {
 /// A unique ID for every stacking context.
 #[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, HeapSizeOf, PartialEq, RustcEncodable, Serialize)]
 pub struct StackingContextId(
-    /// The type of the fragment for this StackingContext. This serves to differentiate
-    /// StackingContexts that share fragments.
-    FragmentType,
-    /// The identifier for this StackingContexts, derived from the Flow's memory address.
+    /// The identifier for this StackingContext, derived from the Flow's memory address
+    /// and fragment type.  As a space optimization, these are combined into a single word.
     usize
 );
 
 impl StackingContextId {
+    #[inline(always)]
     pub fn new(id: usize) -> StackingContextId {
-        StackingContextId(FragmentType::FragmentBody, id)
+        StackingContextId::new_of_type(id, FragmentType::FragmentBody)
     }
 
+    #[inline(always)]
     pub fn new_of_type(id: usize, fragment_type: FragmentType) -> StackingContextId {
-        StackingContextId(fragment_type, id)
+        debug_assert_eq!(id & fragment_type as usize, 0);
+        StackingContextId(id | fragment_type as usize)
     }
 }
 
diff --git a/tests/unit/layout/size_of.rs b/tests/unit/layout/size_of.rs
index 87f1dcb3d44..033916647ee 100644
--- a/tests/unit/layout/size_of.rs
+++ b/tests/unit/layout/size_of.rs
@@ -8,7 +8,7 @@ use std::mem::size_of;
 
 #[test]
 fn test_size_of_fragment() {
-    let expected = 168;
+    let expected = 160;
     let actual = size_of::<Fragment>();
 
     if actual < expected {