Bug 269905. Remember emptiness state on line boxes, but be selective about to to minimize the invalidation burden. r+sr=dbaron

layout/generic/nsBlockFrame.cpp

@@ -2211,7 +2211,7 @@ nsBlockFrame::ReflowDirtyLines(nsBlockReflowState& aState)
       // and also because our final height may depend on it. Only
       // update mY if the line is not empty, because that's what
       // PlaceLine does.
-      if (!line->IsEmpty()) {
+      if (!line->CachedIsEmpty()) {
         aState.mY = line->mBounds.YMost();
         // This will include any pending float clearing height, so
         // don't bother clearing previous lines' floats
@@ -2483,6 +2483,7 @@ nsBlockFrame::ReflowLine(nsBlockReflowState& aState,
   // Setup the line-layout for the new line
   aState.mCurrentLine = aLine;
   aLine->ClearDirty();
+  aLine->InvalidateCachedIsEmpty();
   
   // Now that we know what kind of line we have, reflow it
   if (aLine->IsBlock()) {
@@ -3027,7 +3028,7 @@ nsBlockFrame::ShouldApplyTopMargin(nsBlockReflowState& aState,
 
   // Determine if this line is "essentially" the first line
   for (line_iterator line = begin_lines(); line != aLine; ++line) {
-    if (!line->IsEmpty()) {
+    if (!line->CachedIsEmpty()) {
       // A line which preceeds aLine is non-empty, so therefore the
       // top margin applies.
       aState.SetFlag(BRS_APPLYTOPMARGIN, PR_TRUE);
@@ -3053,7 +3054,7 @@ nsBlockFrame::GetTopBlockChild(nsPresContext* aPresContext)
   if (firstLine->IsBlock())
     return firstLine->mFirstChild;
 
-  if (!firstLine->IsEmpty())
+  if (!firstLine->CachedIsEmpty())
     return nsnull;
 
   line_iterator secondLine = begin_lines();
@@ -4195,7 +4196,7 @@ nsBlockFrame::PlaceLine(nsBlockReflowState& aState,
   // collapsed with a block that follows.
   nscoord newY;
 
-  if (!aLine->IsEmpty()) {
+  if (!aLine->CachedIsEmpty()) {
     // This line has some height. Therefore the application of the
     // previous-bottom-margin should stick.
     aState.mPrevBottomMargin.Zero();

layout/generic/nsLineBox.cpp

@@ -288,6 +288,23 @@ nsLineBox::IsEmpty() const
   return PR_TRUE;
 }
 
+PRBool
+nsLineBox::CachedIsEmpty()
+{
+  if (mFlags.mDirty) {
+    return IsEmpty();
+  }
+  
+  if (mFlags.mEmptyCacheValid) {
+    return mFlags.mEmptyCacheState;
+  }
+
+  PRBool result = IsEmpty();
+  mFlags.mEmptyCacheValid = PR_TRUE;
+  mFlags.mEmptyCacheState = result;
+  return result;
+}
+
 void
 nsLineBox::DeleteLineList(nsPresContext* aPresContext, nsLineList& aLines)
 {

layout/generic/nsLineBox.h

@@ -400,6 +400,20 @@ public:
   // whether the line box is "logically" empty (just like nsIFrame::IsEmpty)
   PRBool IsEmpty() const;
 
+  // Call this only while in Reflow() for the block the line belongs
+  // to, only between reflowing the line (or sliding it, if we skip
+  // reflowing it) and the end of reflowing the the block.
+  PRBool CachedIsEmpty();
+
+  void InvalidateCachedIsEmpty() {
+    mFlags.mEmptyCacheValid = PR_FALSE;
+  }
+
+  // For debugging purposes
+  PRBool IsValidCachedIsEmpty() {
+    return mFlags.mEmptyCacheValid;
+  }
+
 #ifdef DEBUG
   static PRInt32 GetCtorCount();
 #endif
@@ -418,9 +432,11 @@ public:
     PRUint32 mHasPercentageChild : 1;
     PRUint32 mLineWrapped: 1;
     PRUint32 mResizeReflowOptimizationDisabled: 1;  // default 0 = means that the opt potentially applies to this line. 1 = never skip reflowing this line for a resize reflow
+    PRUint32 mEmptyCacheValid: 1;
+    PRUint32 mEmptyCacheState: 1;
     PRUint32 mBreakType : 4;
 
-    PRUint32 mChildCount : 21;
+    PRUint32 mChildCount : 19;
   };
 
   struct ExtraData {

layout/html/base/src/nsBlockFrame.cpp

@@ -2211,7 +2211,7 @@ nsBlockFrame::ReflowDirtyLines(nsBlockReflowState& aState)
       // and also because our final height may depend on it. Only
       // update mY if the line is not empty, because that's what
       // PlaceLine does.
-      if (!line->IsEmpty()) {
+      if (!line->CachedIsEmpty()) {
         aState.mY = line->mBounds.YMost();
         // This will include any pending float clearing height, so
         // don't bother clearing previous lines' floats
@@ -2483,6 +2483,7 @@ nsBlockFrame::ReflowLine(nsBlockReflowState& aState,
   // Setup the line-layout for the new line
   aState.mCurrentLine = aLine;
   aLine->ClearDirty();
+  aLine->InvalidateCachedIsEmpty();
   
   // Now that we know what kind of line we have, reflow it
   if (aLine->IsBlock()) {
@@ -3027,7 +3028,7 @@ nsBlockFrame::ShouldApplyTopMargin(nsBlockReflowState& aState,
 
   // Determine if this line is "essentially" the first line
   for (line_iterator line = begin_lines(); line != aLine; ++line) {
-    if (!line->IsEmpty()) {
+    if (!line->CachedIsEmpty()) {
       // A line which preceeds aLine is non-empty, so therefore the
       // top margin applies.
       aState.SetFlag(BRS_APPLYTOPMARGIN, PR_TRUE);
@@ -3053,7 +3054,7 @@ nsBlockFrame::GetTopBlockChild(nsPresContext* aPresContext)
   if (firstLine->IsBlock())
     return firstLine->mFirstChild;
 
-  if (!firstLine->IsEmpty())
+  if (!firstLine->CachedIsEmpty())
     return nsnull;
 
   line_iterator secondLine = begin_lines();
@@ -4195,7 +4196,7 @@ nsBlockFrame::PlaceLine(nsBlockReflowState& aState,
   // collapsed with a block that follows.
   nscoord newY;
 
-  if (!aLine->IsEmpty()) {
+  if (!aLine->CachedIsEmpty()) {
     // This line has some height. Therefore the application of the
     // previous-bottom-margin should stick.
     aState.mPrevBottomMargin.Zero();

layout/html/base/src/nsLineBox.cpp

@@ -288,6 +288,23 @@ nsLineBox::IsEmpty() const
   return PR_TRUE;
 }
 
+PRBool
+nsLineBox::CachedIsEmpty()
+{
+  if (mFlags.mDirty) {
+    return IsEmpty();
+  }
+  
+  if (mFlags.mEmptyCacheValid) {
+    return mFlags.mEmptyCacheState;
+  }
+
+  PRBool result = IsEmpty();
+  mFlags.mEmptyCacheValid = PR_TRUE;
+  mFlags.mEmptyCacheState = result;
+  return result;
+}
+
 void
 nsLineBox::DeleteLineList(nsPresContext* aPresContext, nsLineList& aLines)
 {

layout/html/base/src/nsLineBox.h

@@ -400,6 +400,20 @@ public:
   // whether the line box is "logically" empty (just like nsIFrame::IsEmpty)
   PRBool IsEmpty() const;
 
+  // Call this only while in Reflow() for the block the line belongs
+  // to, only between reflowing the line (or sliding it, if we skip
+  // reflowing it) and the end of reflowing the the block.
+  PRBool CachedIsEmpty();
+
+  void InvalidateCachedIsEmpty() {
+    mFlags.mEmptyCacheValid = PR_FALSE;
+  }
+
+  // For debugging purposes
+  PRBool IsValidCachedIsEmpty() {
+    return mFlags.mEmptyCacheValid;
+  }
+
 #ifdef DEBUG
   static PRInt32 GetCtorCount();
 #endif
@@ -418,9 +432,11 @@ public:
     PRUint32 mHasPercentageChild : 1;
     PRUint32 mLineWrapped: 1;
     PRUint32 mResizeReflowOptimizationDisabled: 1;  // default 0 = means that the opt potentially applies to this line. 1 = never skip reflowing this line for a resize reflow
+    PRUint32 mEmptyCacheValid: 1;
+    PRUint32 mEmptyCacheState: 1;
     PRUint32 mBreakType : 4;
 
-    PRUint32 mChildCount : 21;
+    PRUint32 mChildCount : 19;
   };
 
   struct ExtraData {