Bug 1203766 - Part 6: Cache resolved style contexts on nsComputedDOMStyle to avoid re-resolving if styles haven't changed. r=bzbarsky

layout/style/nsComputedDOMStyle.cpp

@@ -222,7 +222,9 @@ nsComputedDOMStyle::nsComputedDOMStyle(dom::Element* aElement,
   : mDocumentWeak(nullptr), mOuterFrame(nullptr),
     mInnerFrame(nullptr), mPresShell(nullptr),
     mStyleType(aStyleType),
-    mExposeVisitedStyle(false)
+    mStyleContextGeneration(0),
+    mExposeVisitedStyle(false),
+    mResolvedStyleContext(false)
 {
   MOZ_ASSERT(aElement && aPresShell);
 
@@ -572,10 +574,10 @@ nsComputedDOMStyle::GetCSSParsingEnvironment(CSSParsingEnvironment& aCSSParseEnv
 void
 nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush)
 {
-  MOZ_ASSERT(!mStyleContext);
-
   nsCOMPtr<nsIDocument> document = do_QueryReferent(mDocumentWeak);
   if (!document) {
+    mResolvedStyleContext = false;
+    mStyleContext = nullptr;
     return;
   }
 
@@ -593,9 +595,24 @@ nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush)
 
   mPresShell = document->GetShell();
   if (!mPresShell || !mPresShell->GetPresContext()) {
+    mResolvedStyleContext = false;
+    mStyleContext = nullptr;
     return;
   }
 
+  uint64_t currentGeneration =
+    mPresShell->GetPresContext()->GetRestyleGeneration();
+
+  if (mStyleContext) {
+    if (mStyleContextGeneration == currentGeneration) {
+      // Our cached style context is still valid.
+      return;
+    }
+    // We've processed some restyles, so the cached style context might
+    // be out of date.
+    mStyleContext = nullptr;
+  }
+
   // XXX the !mContent->IsHTMLElement(nsGkAtoms::area)
   // check is needed due to bug 135040 (to avoid using
   // mPrimaryFrame). Remove it once that's fixed.
@@ -616,6 +633,7 @@ nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush)
       }
 
       mStyleContext = mInnerFrame->StyleContext();
+      mResolvedStyleContext = false;
       NS_ASSERTION(mStyleContext, "Frame without style context?");
     }
   }
@@ -649,9 +667,19 @@ nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush)
                                                     mPresShell,
                                                     mStyleType);
     if (!mStyleContext) {
+      mResolvedStyleContext = false;
       return;
     }
 
+    // No need to re-get the generation, even though GetStyleContextForElement
+    // will flush, since we flushed style at the top of this function.
+    NS_ASSERTION(mPresShell &&
+                 currentGeneration ==
+                   mPresShell->GetPresContext()->GetRestyleGeneration(),
+                 "why should we have flushed style again?");
+
+    mResolvedStyleContext = true;
+    mStyleContextGeneration = currentGeneration;
     NS_ASSERTION(mPseudo || !mStyleContext->HasPseudoElementData(),
                  "should not have pseudo-element data");
   }
@@ -675,9 +703,12 @@ nsComputedDOMStyle::ClearCurrentStyleSources()
   mInnerFrame = nullptr;
   mPresShell = nullptr;
 
-  // Release the current style context for it should be re-resolved
-  // whenever a frame is not available.
-  mStyleContext = nullptr;
+  // Release the current style context if we got it off the frame.
+  // For a style context we resolved, keep it around so that we
+  // can re-use it next time this object is queried.
+  if (!mResolvedStyleContext) {
+    mStyleContext = nullptr;
+  }
 }
 
 already_AddRefed<CSSValue>

layout/style/nsComputedDOMStyle.h

@@ -9,6 +9,8 @@
 #define nsComputedDOMStyle_h__
 
 #include "nsAutoPtr.h"
+#include "mozilla/ArenaRefPtr.h"
+#include "mozilla/ArenaRefPtrInlines.h"
 #include "mozilla/Attributes.h"
 #include "nsCOMPtr.h"
 #include "nscore.h"
@@ -595,12 +597,23 @@ private:
   nsWeakPtr mDocumentWeak;
   nsCOMPtr<nsIContent> mContent;
 
-  /*
-   * Strong reference to the style context while we're accessing the data from
-   * it.  This can be either a style context we resolved ourselves or a style
-   * context we got from our frame.
+  /**
+   * Strong reference to the style context we access data from.  This can be
+   * either a style context we resolved ourselves or a style context we got
+   * from our frame.
+   *
+   * If we got the style context from the frame, we clear out mStyleContext
+   * in ClearCurrentStyleSources.  If we resolved one ourselves, then
+   * ClearCurrentStyleSources leaves it in mStyleContext for use the next
+   * time this nsComputedDOMStyle object is queried.  UpdateCurrentStyleSources
+   * in this case will check that the style context is still valid to be used,
+   * by checking whether flush styles results in any restyles having been
+   * processed.
+   *
+   * Since an ArenaRefPtr is used to hold the style context, it will be cleared
+   * if the pres arena from which it was allocated goes away.
    */
-  nsRefPtr<nsStyleContext> mStyleContext;
+  mozilla::ArenaRefPtr<nsStyleContext> mStyleContext;
   nsCOMPtr<nsIAtom> mPseudo;
 
   /*
@@ -626,8 +639,20 @@ private:
    */
   StyleType mStyleType;
 
+  /**
+   * The nsComputedDOMStyle generation at the time we last resolved a style
+   * context and stored it in mStyleContext.
+   */
+  uint64_t mStyleContextGeneration;
+
   bool mExposeVisitedStyle;
 
+  /**
+   * Whether we resolved a style context last time we called
+   * UpdateCurrentStyleSources.  Initially false.
+   */
+  bool mResolvedStyleContext;
+
 #ifdef DEBUG
   bool mFlushedPendingReflows;
 #endif