Bug 1372022 - Replace a series of Contains+Put+Get+Put hashtable lookups with a single LookupForAdd call. r=froydnj

MozReview-Commit-ID: GOSe2Ztahlr

layout/painting/nsDisplayList.cpp

@@ -1719,12 +1719,15 @@ nsDisplayListBuilder::AddToWillChangeBudget(nsIFrame* aFrame,
   }
 
   nsPresContext* key = aFrame->PresContext();
-  if (!mWillChangeBudget.Contains(key)) {
-    mWillChangeBudget.Put(key, DocumentWillChangeBudget());
-  }
-
   DocumentWillChangeBudget budget;
-  mWillChangeBudget.Get(key, &budget);
+  auto willChangeBudgetEntry = mWillChangeBudget.LookupForAdd(key);
+  if (willChangeBudgetEntry) {
+    // We have an existing entry.
+    budget = willChangeBudgetEntry.Data();
+  } else {
+    budget = DocumentWillChangeBudget();
+    willChangeBudgetEntry.OrInsert([&budget] () { return budget; });
+  }
 
   nsRect area = aFrame->PresContext()->GetVisibleArea();
   uint32_t budgetLimit = nsPresContext::AppUnitsToIntCSSPixels(area.width) *
@@ -1736,7 +1739,7 @@ nsDisplayListBuilder::AddToWillChangeBudget(nsIFrame* aFrame,
 
   if (onBudget) {
     budget.mBudget += cost;
-    mWillChangeBudget.Put(key, budget);
+    willChangeBudgetEntry.Data() = budget;
     mWillChangeBudgetSet.PutEntry(aFrame);
   }