Bug 931668 - Part 18: Add a function to swap style structs between style contexts. r=dbaron

--HG--
extra : rebase_source : 4d32ed1f140db975b2fffe76ac6bd793287c916c

layout/style/nsStyleContext.cpp

@@ -1039,3 +1039,52 @@ nsStyleContext::HasSameCachedStyleData(nsStyleContext* aOther,
 {
   return GetCachedStyleData(aSID) == aOther->GetCachedStyleData(aSID);
 }
+
+void
+nsStyleContext::SwapStyleData(nsStyleContext* aNewContext, uint32_t aStructs)
+{
+  static_assert(nsStyleStructID_Length <= 32, "aStructs is not big enough");
+
+  for (nsStyleStructID i = nsStyleStructID_Inherited_Start;
+       i < nsStyleStructID_Inherited_Start + nsStyleStructID_Inherited_Count;
+       i = nsStyleStructID(i + 1)) {
+    uint32_t bit = nsCachedStyleData::GetBitForSID(i);
+    if (!(aStructs & bit)) {
+      continue;
+    }
+    void*& thisData = mCachedInheritedData.mStyleStructs[i];
+    void*& otherData = aNewContext->mCachedInheritedData.mStyleStructs[i];
+    if (mBits & bit) {
+      if (thisData == otherData) {
+        thisData = nullptr;
+      }
+    } else if (!(aNewContext->mBits & bit) && thisData && otherData) {
+      std::swap(thisData, otherData);
+    }
+  }
+
+  for (nsStyleStructID i = nsStyleStructID_Reset_Start;
+       i < nsStyleStructID_Reset_Start + nsStyleStructID_Reset_Count;
+       i = nsStyleStructID(i + 1)) {
+    uint32_t bit = nsCachedStyleData::GetBitForSID(i);
+    if (!(aStructs & bit)) {
+      continue;
+    }
+    if (!mCachedResetData) {
+      mCachedResetData = new (mRuleNode->PresContext()) nsResetStyleData;
+    }
+    if (!aNewContext->mCachedResetData) {
+      aNewContext->mCachedResetData =
+        new (mRuleNode->PresContext()) nsResetStyleData;
+    }
+    void*& thisData = mCachedResetData->mStyleStructs[i];
+    void*& otherData = aNewContext->mCachedResetData->mStyleStructs[i];
+    if (mBits & bit) {
+      if (thisData == otherData) {
+        thisData = nullptr;
+      }
+    } else if (!(aNewContext->mBits & bit) && thisData && otherData) {
+      std::swap(thisData, otherData);
+    }
+  }
+}

layout/style/nsStyleContext.h

@@ -360,6 +360,26 @@ public:
    */
   void MoveTo(nsStyleContext* aNewParent);
 
+  /**
+   * Swaps owned style struct pointers between this and aNewContext, on
+   * the assumption that aNewContext is the new style context for a frame
+   * and this is the old one.  aStructs indicates which structs to consider
+   * swapping; only those which are owned in both this and aNewContext
+   * will be swapped.
+   *
+   * Additionally, if there are identical struct pointers for one of the
+   * structs indicated by aStructs, and it is not an owned struct on this,
+   * then the cached struct slot on this will be set to null.  If the struct
+   * has been swapped on an ancestor, this style context (being the old one)
+   * will be left caching the struct pointer on the new ancestor, despite
+   * inheriting from the old ancestor.  This is not normally a problem, as
+   * this style context will usually be destroyed by being released at the
+   * end of ElementRestyler::Restyle; but for style contexts held on to outside
+   * of the frame, we need to clear out the cached pointer so that if we need
+   * it again we'll re-fetch it from the new ancestor.
+   */
+  void SwapStyleData(nsStyleContext* aNewContext, uint32_t aStructs);
+
 #ifdef DEBUG
   void List(FILE* out, int32_t aIndent);
   static void AssertStyleStructMaxDifferenceValid();