Issue #2678 - Don't use QI to canonicalize nsISupports pointers in the purple buffer.

The nsISupports objects added to the purple buffer are already canonical,
so we can avoid some overhead by not QIing them to
`nsCycleCollectionISupports`.

xpcom/base/nsCycleCollector.cpp

@@ -961,27 +961,6 @@ CanonicalizeXPCOMParticipant(nsISupports* aIn)
   return out;
 }
 
-static inline void
-ToParticipant(nsISupports* aPtr, nsXPCOMCycleCollectionParticipant** aCp);
-
-static void
-CanonicalizeParticipant(void** aParti, nsCycleCollectionParticipant** aCp)
-{
-  // If the participant is null, this is an nsISupports participant,
-  // so we must QI to get the real participant.
-
-  if (!*aCp) {
-    nsISupports* nsparti = static_cast<nsISupports*>(*aParti);
-    nsparti = CanonicalizeXPCOMParticipant(nsparti);
-    NS_ASSERTION(nsparti,
-                 "Don't add objects that don't participate in collection!");
-    nsXPCOMCycleCollectionParticipant* xcp;
-    ToParticipant(nsparti, &xcp);
-    *aParti = nsparti;
-    *aCp = xcp;
-  }
-}
-
 struct nsPurpleBufferEntry
 {
   nsPurpleBufferEntry(void* aObject, nsCycleCollectingAutoRefCnt* aRefCnt,
@@ -1414,6 +1393,21 @@ ToParticipant(nsISupports* aPtr, nsXPCOMCycleCollectionParticipant** aCp)
   CallQueryInterface(aPtr, aCp);
 }
 
+static void
+ToParticipant(void* aParti, nsCycleCollectionParticipant** aCp)
+{
+  // If the participant is null, this is an nsISupports participant,
+  // so we must QI to get the real participant.
+
+  if (!*aCp) {
+    nsISupports* nsparti = static_cast<nsISupports*>(aParti);
+    MOZ_ASSERT(CanonicalizeXPCOMParticipant(nsparti) == nsparti);
+    nsXPCOMCycleCollectionParticipant* xcp;
+    ToParticipant(nsparti, &xcp);
+    *aCp = xcp;
+  }
+}
+
 template<class Visitor>
 MOZ_NEVER_INLINE void
 GraphWalker<Visitor>::Walk(PtrInfo* aPi)
@@ -2220,7 +2214,7 @@ CCGraphBuilder::AddNode(void* aPtr, nsCycleCollectionParticipant* aParticipant)
 bool
 CCGraphBuilder::AddPurpleRoot(void* aRoot, nsCycleCollectionParticipant* aParti)
 {
-  CanonicalizeParticipant(&aRoot, &aParti);
+  ToParticipant(aRoot, &aParti);
 
   if (WantAllTraces() || !aParti->CanSkipInCC(aRoot)) {
     PtrInfo* pinfo = AddNode(aRoot, aParti);
@@ -2658,7 +2652,7 @@ public:
     if (!aEntry->mRefCnt->get()) {
       void* o = aEntry->mObject;
       nsCycleCollectionParticipant* cp = aEntry->mParticipant;
-      CanonicalizeParticipant(&o, &cp);
+      ToParticipant(o, &cp);
       SnowWhiteObject swo = { o, cp, aEntry->mRefCnt };
       mObjects.InfallibleAppend(swo);
       aBuffer.Remove(aEntry);
@@ -2774,7 +2768,7 @@ public:
     }
     void* o = aEntry->mObject;
     nsCycleCollectionParticipant* cp = aEntry->mParticipant;
-    CanonicalizeParticipant(&o, &cp);
+    ToParticipant(o, &cp);
     if (aEntry->mRefCnt->IsPurple() && !cp->CanSkip(o, false) &&
         (!mRemoveChildlessNodes || MayHaveChild(o, cp))) {
       return;
@@ -2981,10 +2975,8 @@ public:
                "Snow-white objects shouldn't be in the purple buffer.");
 
     void* obj = aEntry->mObject;
-    if (!aEntry->mParticipant) {
+    MOZ_ASSERT(aEntry->mParticipant || CanonicalizeXPCOMParticipant(static_cast<nsISupports*>(obj)) == obj,
-      obj = CanonicalizeXPCOMParticipant(static_cast<nsISupports*>(obj));
+               "Suspect nsISupports pointer must be canonical");
-      MOZ_ASSERT(obj, "Don't add objects that don't participate in collection!");
-    }
 
     PtrInfo* pi = mGraph.FindNode(obj);
     if (!pi) {
@@ -3455,6 +3447,9 @@ nsCycleCollector::Suspect(void* aPtr, nsCycleCollectionParticipant* aParti,
   MOZ_ASSERT(HasParticipant(aPtr, aParti),
              "Suspected nsISupports pointer must QI to nsXPCOMCycleCollectionParticipant");
 
+  MOZ_ASSERT(aParti || CanonicalizeXPCOMParticipant(static_cast<nsISupports*>(aPtr)) == aPtr,
+             "Suspect nsISupports pointer must be canonical");
+
   mPurpleBuf.Put(aPtr, aParti, aRefCnt);
 }
 
@@ -3953,7 +3948,7 @@ SuspectAfterShutdown(void* aPtr, nsCycleCollectionParticipant* aCp,
   if (aRefCnt->get() == 0) {
     if (!aShouldDelete) {
       // The CC is shut down, so we can't be in the middle of an ICC.
-      CanonicalizeParticipant(&aPtr, &aCp);
+      ToParticipant(aPtr, &aCp);
       aRefCnt->stabilizeForDeletion();
       aCp->DeleteCycleCollectable(aPtr);
     } else {