[stubs] Introduce a dedicated FastNewObjectStub.

src/compiler/js-create-lowering.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/compiler/js-create-lowering.h"
 
 #include "src/allocation-site-scopes.h"
 #include "src/code-factory.h"
 #include "src/compilation-dependencies.h"
 #include "src/compiler/access-builder.h"
@@ skipping 225 matching lines @@
   // Extract constructor function.
   if (target_type->IsConstant() &&
       target_type->AsConstant()->Value()->IsJSFunction()) {
     Handle<JSFunction> constructor =
         Handle<JSFunction>::cast(target_type->AsConstant()->Value());
     DCHECK(constructor->IsConstructor());
     // Force completion of inobject slack tracking before
     // generating code to finalize the instance size.
     constructor->CompleteInobjectSlackTrackingIfActive();
 
-    // TODO(bmeurer): We fall back to the runtime in case we cannot inline
-    // the allocation here, which is sort of expensive. We should think about
-    // a soft fallback to some NewObjectCodeStub.
+    // Check if we can inline the allocation.
     if (IsAllocationInlineable(constructor)) {
       // Compute instance size from initial map of {constructor}.
       Handle<Map> initial_map(constructor->initial_map(), isolate());
       int const instance_size = initial_map->instance_size();
 
       // Add a dependency on the {initial_map} to make sure that this code is
       // deoptimized whenever the {initial_map} of the {constructor} changes.
       dependencies()->AssumeInitialMapCantChange(initial_map);
 
       // Emit code to allocate the JSObject instance for the {constructor}.
@@ skipping 829 matching lines @@
   return jsgraph()->simplified();
 }
 
 MachineOperatorBuilder* JSCreateLowering::machine() const {
   return jsgraph()->machine();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8