Bug 1371095 - Avoid doing unnecessary hashtable lookups in ImageTracker::Add/Remove. r=froydnj

MozReview-Commit-ID: AUpTHGH3JQV

dom/base/ImageTracker.cpp

@@ -28,26 +28,27 @@ ImageTracker::Add(imgIRequest* aImage)
 {
   MOZ_ASSERT(aImage);
 
-  // See if the image is already in the hashtable. If it is, get the old count.
-  uint32_t oldCount = 0;
-  mImages.Get(aImage, &oldCount);
-
-  // Put the image in the hashtable, with the proper count.
-  mImages.Put(aImage, oldCount + 1);
-
   nsresult rv = NS_OK;
+  auto entry = mImages.LookupForAdd(aImage);
+  if (entry) {
+    // The image is already in the hashtable.  Increment its count.
+    uint32_t oldCount = entry.Data();
+    MOZ_ASSERT(oldCount > 0, "Entry in the image tracker with count 0!");
+    entry.Data() = oldCount + 1;
+  } else {
+    // A new entry was inserted - set the count to 1.
+    entry.OrInsert([]() { return 1; });
 
-  // If this is the first insertion and we're locking images, lock this image
-  // too.
-  if (oldCount == 0 && mLocking) {
-    rv = aImage->LockImage();
-  }
+    // If we're locking images, lock this image too.
+    if (mLocking) {
+      rv = aImage->LockImage();
+    }
 
-  // If this is the first insertion and we're animating images, request
-  // that this image be animated too.
-  if (oldCount == 0 && mAnimating) {
-    nsresult rv2 = aImage->IncrementAnimationConsumers();
-    rv = NS_SUCCEEDED(rv) ? rv2 : rv;
+    // If we're animating images, request that this image be animated too.
+    if (mAnimating) {
+      nsresult rv2 = aImage->IncrementAnimationConsumers();
+      rv = NS_SUCCEEDED(rv) ? rv2 : rv;
+    }
   }
 
   return rv;
@@ -59,23 +60,23 @@ ImageTracker::Remove(imgIRequest* aImage, uint32_t aFlags)
   NS_ENSURE_ARG_POINTER(aImage);
 
   // Get the old count. It should exist and be > 0.
-  uint32_t count = 0;
-  DebugOnly<bool> found = mImages.Get(aImage, &count);
+  DebugOnly<bool> found = false;
+  bool remove = false;
+  mImages.LookupRemoveIf(aImage,
+    [&found, &remove] (uint32_t& aCount) {
+      found = true;
+      MOZ_ASSERT(aCount > 0, "Entry in the image tracker with count 0!");
+      --aCount;
+      // If the count is now zero, remove it from the tracker.
+      remove = aCount == 0;
+      return remove;
+    });
+
   MOZ_ASSERT(found, "Removing image that wasn't in the tracker!");
-  MOZ_ASSERT(count > 0, "Entry in the cache tracker with count 0!");
-
-  // We're removing, so decrement the count.
-  count--;
-
-  // If the count is now zero, remove from the tracker.
-  // Otherwise, set the new value.
-  if (count != 0) {
-    mImages.Put(aImage, count);
+  if (!remove) {
     return NS_OK;
   }
 
-  mImages.Remove(aImage);
-
   nsresult rv = NS_OK;
 
   // Now that we're no longer tracking this image, unlock it if we'd