[promises] Don't allocate new array before filling up existing array

src/builtins/builtins-promise.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/builtins/builtins-utils.h"
 #include "src/builtins/builtins.h"
 #include "src/code-factory.h"
 #include "src/code-stub-assembler.h"
 #include "src/promise-utils.h"
 
@@ skipping 461 matching lines @@
     }
 
     a->Bind(&if_existingcallbacks);
     {
       Label if_singlecallback(a), if_multiplecallbacks(a);
       a->BranchIfJSObject(existing_deferred, &if_singlecallback,
                           &if_multiplecallbacks);
 
       a->Bind(&if_singlecallback);
       {
-        // Create new FixedArrays to store callbacks.
+        // Create new FixedArrays to store callbacks, and migrate
+        // existing callbacks.
         Node* const deferreds =
             a->AllocateFixedArray(FAST_ELEMENTS, a->Int32Constant(2));
+        a->StoreFixedArrayElement(deferreds, 0, existing_deferred);
+        a->StoreFixedArrayElement(deferreds, 1, deferred);
+
         Node* const fulfill_reactions =
             a->AllocateFixedArray(FAST_ELEMENTS, a->Int32Constant(2));
-        Node* const reject_reactions =
-            a->AllocateFixedArray(FAST_ELEMENTS, a->Int32Constant(2));
-
-        // Store existing callbacks in FixedArrays.
-        a->StoreFixedArrayElement(deferreds, 0, existing_deferred);
         a->StoreFixedArrayElement(
             fulfill_reactions, 0,
             a->LoadObjectField(promise, JSPromise::kFulfillReactionsOffset));
+        a->StoreFixedArrayElement(fulfill_reactions, 1, var_on_resolve.value());
+
+        Node* const reject_reactions =
+            a->AllocateFixedArray(FAST_ELEMENTS, a->Int32Constant(2));
         a->StoreFixedArrayElement(
             reject_reactions, 0,
             a->LoadObjectField(promise, JSPromise::kRejectReactionsOffset));
-
-        // Store new callbacks in FixedArrays.
-        a->StoreFixedArrayElement(deferreds, 1, deferred);
-        a->StoreFixedArrayElement(fulfill_reactions, 1, var_on_resolve.value());
         a->StoreFixedArrayElement(reject_reactions, 1, var_on_reject.value());
 
         // Store new FixedArrays in promise.
         a->StoreObjectField(promise, JSPromise::kDeferredOffset, deferreds);
         a->StoreObjectField(promise, JSPromise::kFulfillReactionsOffset,
                             fulfill_reactions);
         a->StoreObjectField(promise, JSPromise::kRejectReactionsOffset,
                             reject_reactions);
         a->Goto(&out);
       }
@@ skipping 151 matching lines @@
   // 5. Return PerformPromiseThen(promise, onFulfilled, onRejected,
   //    resultCapability).
   a.Bind(&perform_promise_then);
   Node* const result = InternalPerformPromiseThen(
       &a, context, promise, on_resolve, on_reject, var_deferred.value());
   a.Return(result);
 }
 
 }  // namespace internal
 }  // namespace v8