From 9f4fe4275c9fd67851e6feaca5221833de18abc0 Mon Sep 17 00:00:00 2001
From: Moonchild <moonchild@palemoon.org>
Date: Thu, 16 Jan 2025 21:56:13 +0100
Subject: [PATCH] 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 | 53 +++++++++++++++------------------
 1 file changed, 24 insertions(+), 29 deletions(-)

diff --git a/xpcom/base/nsCycleCollector.cpp b/xpcom/base/nsCycleCollector.cpp
index 3e78bb0d67..257f36cf5f 100644
--- a/xpcom/base/nsCycleCollector.cpp
+++ b/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) {
-      obj = CanonicalizeXPCOMParticipant(static_cast<nsISupports*>(obj));
-      MOZ_ASSERT(obj, "Don't add objects that don't participate in collection!");
-    }
+    MOZ_ASSERT(aEntry->mParticipant || CanonicalizeXPCOMParticipant(static_cast<nsISupports*>(obj)) == obj,
+               "Suspect nsISupports pointer must be canonical");
 
     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 {