1 files changed, 66 insertions, 0 deletions
diff --git a/layout/style/CachedInheritingStyles.h b/layout/style/CachedInheritingStyles.h
new file mode 100644
index 0000000000..cfe86594ca
--- /dev/null
+++ b/layout/style/CachedInheritingStyles.h
@@ -0,0 +1,66 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_CachedInheritingStyles_h
+#define mozilla_CachedInheritingStyles_h
+
+#include "nsAtom.h"
+#include "nsCOMPtr.h"
+#include "nsTArray.h"
+
+class nsWindowSizes;
+
+namespace mozilla {
+
+enum class PseudoStyleType : uint8_t;
+class ComputedStyle;
+
+// Cache of anonymous box and lazy pseudo styles that inherit from a given
+// style.
+//
+// To minimize memory footprint, the cache is word-sized with a tagged pointer
+// If there is only one entry, it's stored inline. If there are more, they're
+// stored in an out-of-line buffer. See bug 1429126 comment 0 and comment 1 for
+// the measurements and rationale that influenced the design.
+class CachedInheritingStyles {
+ public:
+  void Insert(ComputedStyle* aStyle);
+  ComputedStyle* Lookup(PseudoStyleType) const;
+
+  CachedInheritingStyles() : mBits(0) {}
+  ~CachedInheritingStyles() {
+    if (IsIndirect()) {
+      delete AsIndirect();
+    } else if (!IsEmpty()) {
+      RefPtr<ComputedStyle> ref = dont_AddRef(AsDirect());
+    }
+  }
+
+  void AddSizeOfIncludingThis(nsWindowSizes& aSizes, size_t* aCVsSize) const;
+
+ private:
+  // See bug 1429126 comment 1 for the choice of four here.
+  typedef AutoTArray<RefPtr<ComputedStyle>, 4> IndirectCache;
+
+  bool IsEmpty() const { return !mBits; }
+  bool IsIndirect() const { return (mBits & 1); }
+
+  ComputedStyle* AsDirect() const {
+    MOZ_ASSERT(!IsIndirect());
+    return reinterpret_cast<ComputedStyle*>(mBits);
+  }
+
+  IndirectCache* AsIndirect() const {
+    MOZ_ASSERT(IsIndirect());
+    return reinterpret_cast<IndirectCache*>(mBits & ~1);
+  }
+
+  uintptr_t mBits;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_CachedInheritingStyles_h