Bug 1597679 - part 1: Cache multiple `TextControlState` instances for reuse r=Ehsan

Previously, we cache only one `TextControlState` instance to reuse.  However,
`HTMLTextAreaElement` also started to allocate it in the heap rather than
a part of its instance.  Therefore, we should have more room to cache the
instances for saving the allocation cost.

Differential Revision: https://phabricator.services.mozilla.com/D54326

--HG--
extra : moz-landing-system : lando

dom/html/TextControlState.cpp

@@ -1334,7 +1334,8 @@ class MOZ_STACK_CLASS AutoTextControlHandlingState {
  * mozilla::TextControlState
  *****************************************************************************/
 
-TextControlState* TextControlState::sReleasedInstance = nullptr;
+AutoTArray<TextControlState*, TextControlState::kMaxCountOfCacheToReuse>*
+    TextControlState::sReleasedInstances = nullptr;
 bool TextControlState::sHasShutDown = false;
 
 TextControlState::TextControlState(nsITextControlElement* aOwningElement)
@@ -1357,10 +1358,9 @@ TextControlState::TextControlState(nsITextControlElement* aOwningElement)
 
 TextControlState* TextControlState::Construct(
     nsITextControlElement* aOwningElement) {
-  if (sReleasedInstance) {
-    MOZ_ASSERT(!sReleasedInstance->IsBusy());
-    TextControlState* state = sReleasedInstance;
-    sReleasedInstance = nullptr;
+  if (sReleasedInstances && !sReleasedInstances->IsEmpty()) {
+    TextControlState* state = sReleasedInstances->LastElement();
+    sReleasedInstances->RemoveLastElement();
     state->mTextCtrlElement = aOwningElement;
     state->mBoundFrame = nullptr;
     state->mSelectionProperties = SelectionProperties();
@@ -1387,9 +1387,11 @@ TextControlState::~TextControlState() {
 
 void TextControlState::Shutdown() {
   sHasShutDown = true;
-  if (sReleasedInstance) {
-    sReleasedInstance->DeleteOrCacheForReuse();
-    sReleasedInstance = nullptr;
+  if (sReleasedInstances) {
+    for (TextControlState* textControlState : *sReleasedInstances) {
+      textControlState->DeleteOrCacheForReuse();
+    }
+    delete sReleasedInstances;
   }
 }
 
@@ -1406,9 +1408,14 @@ void TextControlState::DeleteOrCacheForReuse() {
   MOZ_ASSERT(!IsBusy());
 
   // If we can cache this instance, we should do it instead of deleting it.
-  if (!sHasShutDown && !sReleasedInstance) {
-    sReleasedInstance = this;
-    sReleasedInstance->PrepareForReuse();
+  if (!sHasShutDown && (!sReleasedInstances || sReleasedInstances->Length() <
+                                                   kMaxCountOfCacheToReuse)) {
+    PrepareForReuse();
+    if (!sReleasedInstances) {
+      sReleasedInstances =
+          new AutoTArray<TextControlState*, kMaxCountOfCacheToReuse>;
+    }
+    sReleasedInstances->AppendElement(this);
     return;
   }
   delete this;

dom/html/TextControlState.h

@@ -465,11 +465,14 @@ class TextControlState final : public SupportsWeakPtr<TextControlState> {
   /**
    * For avoiding allocation cost of the instance, we should reuse instances
    * as far as possible.
-   * TODO: Maybe, we should cache more instances with array.  Then, it must
-   *       be faster to load pages which have a lot of `<input type="text">`
-   *       elements.
+   *
+   * FYI: `25` is just a magic number considered without enough investigation,
+   *      but at least, this value must not make damage for footprint.
+   *      Feel free to change it if you find better number.
    */
-  static TextControlState* sReleasedInstance;
+  static const size_t kMaxCountOfCacheToReuse = 25;
+  static AutoTArray<TextControlState*, kMaxCountOfCacheToReuse>*
+      sReleasedInstances;
   static bool sHasShutDown;
 
   friend class AutoTextControlHandlingState;