Bug 256311. Improve block reflow performance in the presence of many empty lines, by being more aggressive about caching block and line emptiness state, and by stashing a reference to a line that we know has nothing but empty lines above it. Relanding because I accidentally backed it out. r+sr=dbaron

layout/generic/nsBlockFrame.cpp

@@ -3124,6 +3124,24 @@ nsBlockFrame::IsSelfEmpty()
   return PR_TRUE;
 }
 
+PRBool
+nsBlockFrame::CachedIsEmpty()
+{
+  if (!IsSelfEmpty()) {
+    return PR_FALSE;
+  }
+
+  for (line_iterator line = begin_lines(), line_end = end_lines();
+       line != line_end;
+       ++line)
+  {
+    if (!line->CachedIsEmpty())
+      return PR_FALSE;
+  }
+
+  return PR_TRUE;
+}
+
 PRBool
 nsBlockFrame::IsEmpty()
 {
@@ -3160,7 +3178,11 @@ nsBlockFrame::ShouldApplyTopMargin(nsBlockReflowState& aState,
   }
 
   // Determine if this line is "essentially" the first line
-  for (line_iterator line = begin_lines(); line != aLine; ++line) {
+  line_iterator line = begin_lines();
+  if (aState.GetFlag(BRS_HAVELINEADJACENTTOTOP)) {
+    line = aState.mLineAdjacentToTop;
+  }
+  while (line != aLine) {
     if (!line->CachedIsEmpty() || line->HasClearance()) {
       // A line which precedes aLine is non-empty, or has clearance,
       // so therefore the top margin applies.
@@ -3169,6 +3191,9 @@ nsBlockFrame::ShouldApplyTopMargin(nsBlockReflowState& aState,
     }
     // No need to apply the top margin if the line has floats.  We
     // should collapse anyway (bug 44419)
+    ++line;
+    aState.SetFlag(BRS_HAVELINEADJACENTTOTOP, PR_TRUE);
+    aState.mLineAdjacentToTop = line;
   }
 
   // The line being reflowed is "essentially" the first line in the

layout/generic/nsBlockFrame.h

@@ -202,6 +202,7 @@ public:
   virtual PRBool IsVisibleInSelection(nsISelection* aSelection);
 
   virtual PRBool IsEmpty();
+  virtual PRBool CachedIsEmpty();
   virtual PRBool IsSelfEmpty();
 
   // nsIHTMLReflow

layout/generic/nsBlockReflowContext.cpp

@@ -90,7 +90,8 @@ static nsIFrame* DescendIntoBlockLevelFrame(nsIFrame* aFrame)
 
 PRBool
 nsBlockReflowContext::ComputeCollapsedTopMargin(const nsHTMLReflowState& aRS,
-  nsCollapsingMargin* aMargin, nsIFrame* aClearanceFrame, PRBool* aMayNeedRetry)
+  nsCollapsingMargin* aMargin, nsIFrame* aClearanceFrame,
+  PRBool* aMayNeedRetry, PRBool* aBlockIsEmpty)
 {
   // Include frame's top margin
   aMargin->Include(aRS.mComputedMargin.top);
@@ -105,6 +106,7 @@ nsBlockReflowContext::ComputeCollapsedTopMargin(const nsHTMLReflowState& aRS,
 #endif
 
   PRBool dirtiedLine = PR_FALSE;
+  PRBool setBlockIsEmpty = PR_FALSE;
 
   // Calculate the frame's generational top-margin from its child
   // blocks. Note that if the frame has a non-zero top-border or
@@ -151,8 +153,10 @@ nsBlockReflowContext::ComputeCollapsedTopMargin(const nsHTMLReflowState& aRS,
             dirtiedLine = PR_TRUE;
           }
           
-          PRBool isEmpty = line->IsEmpty();
-          if (line->IsBlock()) {
+          PRBool isEmpty;
+          if (line->IsInline()) {
+            isEmpty = line->IsEmpty();
+          } else {
             nsIFrame* kid = line->mFirstChild;
             if (kid == aClearanceFrame) {
               line->SetHasClearance();
@@ -194,7 +198,7 @@ nsBlockReflowContext::ComputeCollapsedTopMargin(const nsHTMLReflowState& aRS,
               if (kid->GetStyleDisplay()->mBreakType != NS_STYLE_CLEAR_NONE) {
                 *aMayNeedRetry = PR_TRUE;
               }
-              if (ComputeCollapsedTopMargin(innerReflowState, aMargin, aClearanceFrame, aMayNeedRetry)) {
+              if (ComputeCollapsedTopMargin(innerReflowState, aMargin, aClearanceFrame, aMayNeedRetry, &isEmpty)) {
                 line->MarkDirty();
                 dirtiedLine = PR_TRUE;
               }
@@ -205,14 +209,30 @@ nsBlockReflowContext::ComputeCollapsedTopMargin(const nsHTMLReflowState& aRS,
               delete NS_CONST_CAST(nsHTMLReflowState*, outerReflowState);
             }
           }
-          if (!isEmpty)
+          if (!isEmpty) {
+            if (!setBlockIsEmpty && aBlockIsEmpty) {
+              setBlockIsEmpty = PR_TRUE;
+              *aBlockIsEmpty = PR_FALSE;
+            }
             goto done;
+          }
+        }
+        if (!setBlockIsEmpty && aBlockIsEmpty) {
+          // The first time we reach here is when this is the first block
+          // and we have processed all its normal lines.
+          setBlockIsEmpty = PR_TRUE;
+          // All lines are empty, or we wouldn't be here!
+          *aBlockIsEmpty = aRS.frame->IsSelfEmpty();
         }
       }
     }
   done:
     ;
   }
+
+  if (!setBlockIsEmpty && aBlockIsEmpty) {
+    *aBlockIsEmpty = aRS.frame->IsEmpty();
+  }
   
 #ifdef NOISY_VERTICAL_MARGINS
   nsFrame::ListTag(stdout, aRS.frame);

layout/generic/nsBlockReflowContext.h

@@ -122,7 +122,7 @@ public:
    */
   static PRBool ComputeCollapsedTopMargin(const nsHTMLReflowState& aRS,
                                           nsCollapsingMargin* aMargin, nsIFrame* aClearanceFrame,
-                                          PRBool* aMayNeedRetry);
+                                          PRBool* aMayNeedRetry, PRBool* aIsEmpty = nsnull);
 
 protected:
   nsPresContext* mPresContext;

layout/generic/nsBlockReflowState.h

@@ -58,7 +58,9 @@ class nsBlockFrame;
 #define BRS_COMPUTEMAXELEMENTWIDTH 0x00000100
 #define BRS_COMPUTEMAXWIDTH       0x00000200
 #define BRS_ISFIRSTINFLOW         0x00000400
-#define BRS_LASTFLAG              BRS_ISFIRSTINFLOW
+// Set when mLineAdjacentToTop is valid
+#define BRS_HAVELINEADJACENTTOTOP 0x00000800
+#define BRS_LASTFLAG              BRS_HAVELINEADJACENTTOTOP
 
 class nsBlockReflowState {
 public:
@@ -198,6 +200,12 @@ public:
   // If it is mBlock->end_lines(), then it is invalid.
   nsLineList::iterator mCurrentLine;
 
+  // When BRS_HAVELINEADJACENTTOTOP is set, this refers to a line
+  // which we know is adjacent to the top of the block (in other words,
+  // all lines before it are empty and do not have clearance. This line is
+  // always before the current line.
+  nsLineList::iterator mLineAdjacentToTop;
+
   // The current Y coordinate in the block
   nscoord mY;
 

layout/generic/nsFrame.cpp

@@ -3545,6 +3545,14 @@ nsFrame::IsEmpty()
   return PR_FALSE;
 }
 
+PRBool
+nsIFrame::CachedIsEmpty()
+{
+  NS_PRECONDITION(!(GetStateBits() & NS_FRAME_IS_DIRTY),
+                  "Must only be called on reflowed lines");
+  return IsEmpty();
+}
+
 /* virtual */ PRBool
 nsFrame::IsSelfEmpty()
 {

layout/generic/nsIFrame.h

@@ -1402,7 +1402,11 @@ public:
    * should return true.
    */
   virtual PRBool IsEmpty() = 0;
-
+  /**
+   * Return the same as IsEmpty(). This may only be called after the frame
+   * has been reflowed and before any further style or content changes.
+   */
+  virtual PRBool CachedIsEmpty();
   /**
    * Determine whether the frame is logically empty, assuming that all
    * its children are empty.

layout/generic/nsLineBox.cpp

@@ -307,7 +307,24 @@ nsLineBox::CachedIsEmpty()
     return mFlags.mEmptyCacheState;
   }
 
-  PRBool result = IsEmpty();
+  PRBool result;
+  if (IsBlock()) {
+    result = mFirstChild->CachedIsEmpty();
+  } else {
+    PRInt32 n;
+    nsIFrame *kid;
+    result = PR_TRUE;
+    for (n = GetChildCount(), kid = mFirstChild;
+         n > 0;
+         --n, kid = kid->GetNextSibling())
+      {
+        if (!kid->CachedIsEmpty()) {
+          result = PR_FALSE;
+          break;
+        }
+      }
+  }
+
   mFlags.mEmptyCacheValid = PR_TRUE;
   mFlags.mEmptyCacheState = result;
   return result;