Make GraphWalker use templates instead of virtual functions. (Bug 537547) r=smaug

xpcom/base/nsCycleCollector.cpp

@@ -1042,9 +1042,18 @@ struct DoWalkDebugInfo
     EdgePool::Iterator mCurrentChild;
 };
 
+/**
+ * GraphWalker is templatized over a Visitor class that must provide
+ * the following two methods:
+ *
+ * PRBool ShouldVisitNode(PtrInfo const *pi);
+ * void VisitNode(PtrInfo *pi);
+ */
+template <class Visitor>
 class GraphWalker
 {
 private:
+    Visitor mVisitor;
     DoWalkDebugInfo *mDebugInfo;
 
     void DoWalk(nsDeque &aQueue);
@@ -1052,10 +1061,9 @@ private:
 public:
     void Walk(PtrInfo *s0);
     void WalkFromRoots(GCGraph &aGraph);
-
-    // Provided by concrete walker subtypes.
-    virtual PRBool ShouldVisitNode(PtrInfo const *pi) = 0;
-    virtual void VisitNode(PtrInfo *pi) = 0;
+    // copy-constructing the visitor should be cheap, and less
+    // indirection than using a reference
+    GraphWalker(const Visitor aVisitor) : mVisitor(aVisitor) {}
 };
 
 
@@ -1225,16 +1233,18 @@ nsCycleCollectionXPCOMRuntime::ToParticipant(void *p)
 }
 
 
+template <class Visitor>
 void
-GraphWalker::Walk(PtrInfo *s0)
+GraphWalker<Visitor>::Walk(PtrInfo *s0)
 {
     nsDeque queue;
     queue.Push(s0);
     DoWalk(queue);
 }
 
+template <class Visitor>
 void
-GraphWalker::WalkFromRoots(GCGraph& aGraph)
+GraphWalker<Visitor>::WalkFromRoots(GCGraph& aGraph)
 {
     nsDeque queue;
     NodePool::Enumerator etor(aGraph.mNodes);
@@ -1286,8 +1296,9 @@ NodePool::CheckPtrInfo(PtrInfo *aPtrInfo)
     CC_RUNTIME_ABORT_IF_FALSE(block, "Pointer is outside blocks.");
 }
 
+template <class Visitor>
 void
-GraphWalker::DoWalk(nsDeque &aQueue)
+GraphWalker<Visitor>::DoWalk(nsDeque &aQueue)
 {
     // Use a aQueue to match the breadth-first traversal used when we
     // built the graph, for hopefully-better locality.
@@ -1300,8 +1311,8 @@ GraphWalker::DoWalk(nsDeque &aQueue)
         sCollector->mGraph.mNodes.CheckPtrInfo(pi);
 
         debugInfo.mCurrentPI = pi;
-        if (this->ShouldVisitNode(pi)) {
-            this->VisitNode(pi);
+        if (mVisitor.ShouldVisitNode(pi)) {
+            mVisitor.VisitNode(pi);
             debugInfo.mFirstChild = pi->mFirstChild;
             debugInfo.mLastChild = pi->mLastChild;
             debugInfo.mCurrentChild = pi->mFirstChild;
@@ -1693,9 +1704,10 @@ nsCycleCollector::MarkRoots(GCGraphBuilder &builder)
 ////////////////////////////////////////////////////////////////////////
 
 
-struct ScanBlackWalker : public GraphWalker
+struct ScanBlackVisitor
 {
-    ScanBlackWalker(PRUint32 &aWhiteNodeCount) : mWhiteNodeCount(aWhiteNodeCount)
+    ScanBlackVisitor(PRUint32 &aWhiteNodeCount)
+        : mWhiteNodeCount(aWhiteNodeCount)
     {
     }
 
@@ -1718,9 +1730,9 @@ struct ScanBlackWalker : public GraphWalker
 };
 
 
-struct scanWalker : public GraphWalker
+struct scanVisitor
 {
-    scanWalker(PRUint32 &aWhiteNodeCount) : mWhiteNodeCount(aWhiteNodeCount)
+    scanVisitor(PRUint32 &aWhiteNodeCount) : mWhiteNodeCount(aWhiteNodeCount)
     {
     }
 
@@ -1741,9 +1753,9 @@ struct scanWalker : public GraphWalker
             sCollector->mStats.mSetColorWhite++;
 #endif
         } else {
-            ScanBlackWalker(mWhiteNodeCount).Walk(pi);
+            GraphWalker<ScanBlackVisitor>(ScanBlackVisitor(mWhiteNodeCount)).Walk(pi);
             NS_ASSERTION(pi->mColor == black,
-                         "Why didn't ScanBlackWalker make pi black?");
+                         "Why didn't ScanBlackVisitor make pi black?");
         }
     }
 
@@ -1758,7 +1770,7 @@ nsCycleCollector::ScanRoots()
     // On the assumption that most nodes will be black, it's
     // probably faster to use a GraphWalker than a
     // NodePool::Enumerator.
-    scanWalker(mWhiteNodeCount).WalkFromRoots(mGraph); 
+    GraphWalker<scanVisitor>(scanVisitor(mWhiteNodeCount)).WalkFromRoots(mGraph); 
 
 #ifdef DEBUG_CC
     // Sanity check: scan should have colored all grey nodes black or
@@ -2760,16 +2772,16 @@ AddExpectedGarbage(nsVoidPtrHashKey *p, void *arg)
     return PL_DHASH_NEXT;
 }
 
-struct SetSCCWalker : public GraphWalker
+struct SetSCCVisitor
 {
-    SetSCCWalker(PRUint32 aIndex) : mIndex(aIndex) {}
+    SetSCCVisitor(PRUint32 aIndex) : mIndex(aIndex) {}
     PRBool ShouldVisitNode(PtrInfo const *pi) { return pi->mSCCIndex == 0; }
     void VisitNode(PtrInfo *pi) { pi->mSCCIndex = mIndex; }
 private:
     PRUint32 mIndex;
 };
 
-struct SetNonRootGreyWalker : public GraphWalker
+struct SetNonRootGreyVisitor
 {
     PRBool ShouldVisitNode(PtrInfo const *pi) { return pi->mColor == white; }
     void VisitNode(PtrInfo *pi) { pi->mColor = grey; }
@@ -3005,7 +3017,7 @@ nsCycleCollector::ExplainLiveExpectedGarbage()
                     PRUint32 currentSCC = 1;
 
                     while (DFSPostOrder.GetSize() > 0) {
-                        SetSCCWalker(currentSCC).Walk((PtrInfo*)DFSPostOrder.PopFront());
+                        GraphWalker<SetSCCVisitor>(SetSCCVisitor(currentSCC)).Walk((PtrInfo*)DFSPostOrder.PopFront());
                         ++currentSCC;
                     }
                 }
@@ -3022,7 +3034,7 @@ nsCycleCollector::ExplainLiveExpectedGarbage()
                                             child_end = pi->mLastChild;
                              child != child_end; ++child) {
                             if ((*child)->mSCCIndex != pi->mSCCIndex) {
-                                SetNonRootGreyWalker().Walk(*child);
+                                GraphWalker<SetNonRootGreyVisitor>(SetNonRootGreyVisitor()).Walk(*child);
                             }
                         }
                     }