[ic] Restore Function.prototype fast path for LoadIC_Uninitialized

src/ic/accessor-assembler.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/ic/accessor-assembler.h"
 
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
 #include "src/counters.h"
 #include "src/ic/handler-configuration.h"
@@ skipping 1859 matching lines @@
   Node* receiver = p->receiver;
   GotoIf(TaggedIsSmi(receiver), &miss);
   Node* receiver_map = LoadMap(receiver);
   Node* instance_type = LoadMapInstanceType(receiver_map);
 
   // Optimistically write the state transition to the vector.
   StoreFixedArrayElement(p->vector, p->slot,
                          LoadRoot(Heap::kpremonomorphic_symbolRootIndex),
                          SKIP_WRITE_BARRIER, 0, SMI_PARAMETERS);
 
+  {
+    // Special case for Function.prototype load, because it's very common
+    // for ICs that are only executed once (MyFunc.prototype.foo = ...).
+    Label not_function_prototype(this);
+    GotoIf(Word32NotEqual(instance_type, Int32Constant(JS_FUNCTION_TYPE)),
+           &not_function_prototype);
+    GotoIfNot(IsPrototypeString(p->name), &not_function_prototype);
+    Node* bit_field = LoadMapBitField(receiver_map);
+    GotoIf(IsSetWord32(bit_field, 1 << Map::kHasNonInstancePrototype),
+           &not_function_prototype);
+    Return(LoadJSFunctionPrototype(receiver, &miss));
+    BIND(&not_function_prototype);
+  }
+
   GenericPropertyLoad(receiver, receiver_map, instance_type, p->name, p, &miss,
                       kDontUseStubCache);
 
   BIND(&miss);
   {
     // Undo the optimistic state transition.
     StoreFixedArrayElement(p->vector, p->slot,
                            LoadRoot(Heap::kuninitialized_symbolRootIndex),
                            SKIP_WRITE_BARRIER, 0, SMI_PARAMETERS);
 
@@ skipping 583 matching lines @@
   Node* context = Parameter(Descriptor::kContext);
   Node* vector = LoadFeedbackVectorForStub();
 
   Callable callable =
       CodeFactory::KeyedStoreICInOptimizedCode(isolate(), language_mode);
   TailCallStub(callable, context, receiver, name, value, slot, vector);
 }
 
 }  // namespace internal
 }  // namespace v8