Optimize layout flattening

CHANGELOG.md

@@ -10,6 +10,7 @@
 - Clean up unused/unneeded header macros. [Adlai Holler](https://github.com/Adlai-Holler)
 - Clean up C-function `extern` decorators. [Adlai Holler](https://github.com/Adlai-Holler)
 - Add an experiment to reduce work involved in collection teardown. [Adlai Holler](https://github.com/Adlai-Holler)
+- Optimize layout flattening, particularly reducing retain/release operations. [Adlai Holler](https://github.com/Adlai-Holler)
 
 ## 2.7
 - Fix pager node for interface coalescing. [Max Wang](https://github.com/wsdwsd0829) [#877](https://github.com/TextureGroup/Texture/pull/877)

Source/Base/ASBaseDefines.h

@@ -18,6 +18,7 @@
 #import <Foundation/NSObjCRuntime.h>
 
 #define AS_EXTERN FOUNDATION_EXTERN
+#define unowned __unsafe_unretained
 
 #ifdef __GNUC__
 # define ASDISPLAYNODE_GNUC(major, minor) \

Source/Layout/ASLayout.mm

@@ -55,22 +55,6 @@ ASDISPLAYNODE_INLINE AS_WARN_UNUSED_RESULT BOOL ASLayoutIsDisplayNodeType(ASLayo
   return layout.type == ASLayoutElementTypeDisplayNode;
 }
 
-ASDISPLAYNODE_INLINE AS_WARN_UNUSED_RESULT BOOL ASLayoutIsFlattened(ASLayout *layout)
-{
-  // A layout is flattened if its position is null, and all of its sublayouts are of type displaynode with no sublayouts.
-  if (! ASPointIsNull(layout.position)) {
-    return NO;
-  }
-
-  for (ASLayout *sublayout in layout.sublayouts) {
-    if (ASLayoutIsDisplayNodeType(sublayout) == NO || sublayout.sublayouts.count > 0) {
-      return NO;
-    }
-  }
-
-  return YES;
-}
-
 @interface ASLayout () <ASDescriptionProvider>
 {
   ASLayoutElementType _layoutElementType;
@@ -216,52 +200,70 @@ - (void)setRetainSublayoutLayoutElements:(BOOL)retainSublayoutLayoutElements
 
 #pragma mark - Layout Flattening
 
+- (BOOL)isFlattened
+{
+  // A layout is flattened if its position is null, and all of its sublayouts are of type displaynode with no sublayouts.
+  if (! ASPointIsNull(_position)) {
+    return NO;
+  }
+
+  for (ASLayout *sublayout in _sublayouts) {
+    if (ASLayoutIsDisplayNodeType(sublayout) == NO || sublayout->_sublayouts.count > 0) {
+      return NO;
+    }
+  }
+
+  return YES;
+}
+
 - (ASLayout *)filteredNodeLayoutTree NS_RETURNS_RETAINED
 {
-  if (ASLayoutIsFlattened(self)) {
+  if ([self isFlattened]) {
     // All flattened layouts must have this flag enabled
     // to ensure sublayout elements are retained until the layouts are applied.
     self.retainSublayoutLayoutElements = YES;
     return self;
   }
 
   struct Context {
-    ASLayout *layout;
+    unowned ASLayout *layout;
     CGPoint absolutePosition;
   };
 
   // Queue used to keep track of sublayouts while traversing this layout in a DFS fashion.
   std::deque<Context> queue;
-  for (ASLayout *sublayout in self.sublayouts) {
+  for (ASLayout *sublayout in _sublayouts) {
     queue.push_back({sublayout, sublayout.position});
   }
 
-  NSMutableArray *flattenedSublayouts = [[NSMutableArray alloc] init];
+  auto flattenedSublayouts = [[NSMutableArray<ASLayout *> alloc] init];
 
   while (!queue.empty()) {
     const Context context = std::move(queue.front());
     queue.pop_front();
 
-    ASLayout *layout = context.layout;
-    const NSArray<ASLayout *> *sublayouts = layout.sublayouts;
-    const NSUInteger sublayoutsCount = sublayouts.count;
+    unowned ASLayout *layout = context.layout;
+    const NSUInteger sublayoutsCount = layout->_sublayouts.count;
     const CGPoint absolutePosition = context.absolutePosition;
 
     if (ASLayoutIsDisplayNodeType(layout)) {
       if (sublayoutsCount > 0 || CGPointEqualToPoint(ASCeilPointValues(absolutePosition), layout.position) == NO) {
         // Only create a new layout if the existing one can't be reused, which means it has either some sublayouts or an invalid absolute position.
-        layout = [ASLayout layoutWithLayoutElement:layout.layoutElement
-                                              size:layout.size
-                                          position:absolutePosition
-                                        sublayouts:@[]];
+        auto newLayout = [ASLayout layoutWithLayoutElement:layout->_layoutElement
+                                                      size:layout.size
+                                                  position:absolutePosition
+                                                sublayouts:@[]];
+        [flattenedSublayouts addObject:newLayout];
+      } else {
+        [flattenedSublayouts addObject:layout];
       }
-      [flattenedSublayouts addObject:layout];
-    } else if (sublayoutsCount > 0){
-      std::vector<Context> sublayoutContexts;
-      for (ASLayout *sublayout in sublayouts) {
-        sublayoutContexts.push_back({sublayout, absolutePosition + sublayout.position});
+    } else if (sublayoutsCount > 0) {
+      // Fast-reverse-enumerate the sublayouts array by copying it into a C-array and push_front'ing each into the queue.
+      unowned ASLayout *rawSublayouts[sublayoutsCount];
+      [layout->_sublayouts getObjects:rawSublayouts range:NSMakeRange(0, sublayoutsCount)];
+      for (NSInteger i = sublayoutsCount - 1; i >= 0; i--) {
+        queue.push_front({rawSublayouts[i], absolutePosition + rawSublayouts[i].position});
       }
-      queue.insert(queue.cbegin(), sublayoutContexts.begin(), sublayoutContexts.end());
     }
   }