Bug 728460, part 2: remove childless nodes in slice before CC. r=smaug

dom/base/nsJSEnvironment.cpp

@@ -144,7 +144,11 @@ static PRLogModuleInfo* gJSDiagnostics;
 
 #define NS_CC_SKIPPABLE_DELAY       400 // ms
 
-#define NS_CC_FORCED                (5 * 60 * PR_USEC_PER_SEC) // 5 min
+// Force a CC after this long if there's anything in the purple buffer.
+#define NS_CC_FORCED                (2 * 60 * PR_USEC_PER_SEC) // 2 min
+
+// Trigger a CC if the purple buffer exceeds this size when we check it.
+#define NS_CC_PURPLE_LIMIT          250
 
 #define JAVASCRIPT nsIProgrammingLanguage::JAVASCRIPT
 
@@ -3286,46 +3290,70 @@ ShrinkGCBuffersTimerFired(nsITimer *aTimer, void *aClosure)
   nsJSContext::ShrinkGCBuffersNow();
 }
 
-// static
-void
+static bool
+ShouldTriggerCC(PRUint32 aSuspected)
+{
+  return sNeedsFullCC ||
+         aSuspected > NS_CC_PURPLE_LIMIT ||
+         sLastCCEndTime + NS_CC_FORCED < PR_Now();
+}
+
+static void
+TimerFireForgetSkippable(PRUint32 aSuspected, bool aRemoveChildless)
+{
+  PRTime startTime = PR_Now();
+  nsCycleCollector_forgetSkippable(aRemoveChildless);
+  sPreviousSuspectedCount = nsCycleCollector_suspectedCount();
+  sCleanupSinceLastGC = true;
+  PRTime delta = PR_Now() - startTime;
+  if (sMinForgetSkippableTime > delta) {
+    sMinForgetSkippableTime = delta;
+  }
+  if (sMaxForgetSkippableTime < delta) {
+    sMaxForgetSkippableTime = delta;
+  }
+  sTotalForgetSkippableTime += delta;
+  sRemovedPurples += (aSuspected - sPreviousSuspectedCount);
+  ++sForgetSkippableBeforeCC;
+}
+
+static void
 CCTimerFired(nsITimer *aTimer, void *aClosure)
 {
-  if (sDidShutdown) {
-    return;
-  }
-  if (sCCLockedOut) {
+  if (sDidShutdown || sCCLockedOut) {
     return;
   }
   ++sCCTimerFireCount;
-  if (sCCTimerFireCount < (NS_CC_DELAY / NS_CC_SKIPPABLE_DELAY)) {
-    PRUint32 suspected = nsCycleCollector_suspectedCount();
-    if ((sPreviousSuspectedCount + 100) > suspected) {
-      // Just few new suspected objects, return early.
-      return;
-    }
-    
-    PRTime startTime = PR_Now();
-    nsCycleCollector_forgetSkippable();
-    sPreviousSuspectedCount = nsCycleCollector_suspectedCount();
-    sCleanupSinceLastGC = true;
-    PRTime delta = PR_Now() - startTime;
-    if (sMinForgetSkippableTime > delta) {
-      sMinForgetSkippableTime = delta;
-    }
-    if (sMaxForgetSkippableTime < delta) {
-      sMaxForgetSkippableTime = delta;
-    }
-    sTotalForgetSkippableTime += delta;
-    sRemovedPurples += (suspected - sPreviousSuspectedCount);
-    ++sForgetSkippableBeforeCC;
-  } else {
-    sPreviousSuspectedCount = 0;
-    nsJSContext::KillCCTimer();
-    if (sNeedsFullCC ||
-        nsCycleCollector_suspectedCount() > 500 ||
-        sLastCCEndTime + NS_CC_FORCED < PR_Now()) {
+
+  // During early timer fires, we only run forgetSkippable. During the first
+  // late timer fire, we decide if we are going to have a second and final
+  // late timer fire, where we may run the CC.
+  const PRUint32 numEarlyTimerFires = NS_CC_DELAY / NS_CC_SKIPPABLE_DELAY - 2;
+  bool isLateTimerFire = sCCTimerFireCount > numEarlyTimerFires;
+  PRUint32 suspected = nsCycleCollector_suspectedCount();
+  if (isLateTimerFire && ShouldTriggerCC(suspected)) {
+    if (sCCTimerFireCount == numEarlyTimerFires + 1) {
+      TimerFireForgetSkippable(suspected, true);
+      if (ShouldTriggerCC(nsCycleCollector_suspectedCount())) {
+        // Our efforts to avoid a CC have failed, so we return to let the
+        // timer fire once more to trigger a CC.
+        return;
+      }
+    } else {
+      // We are in the final timer fire and still meet the conditions for
+      // triggering a CC.
       nsJSContext::CycleCollectNow();
     }
+  } else if ((sPreviousSuspectedCount + 100) <= suspected) {
+    // Only do a forget skippable if there are more than a few new objects.
+    TimerFireForgetSkippable(suspected, false);
+  }
+
+  if (isLateTimerFire) {
+    // We have either just run the CC or decided we don't want to run the CC
+    // next time, so kill the timer.
+    sPreviousSuspectedCount = 0;
+    nsJSContext::KillCCTimer();
   }
 }