Optimize LocalVarDescriptor objects for functions that have no local variables

runtime/vm/object.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 80 matching lines @@
 #endif
 #define RAW_NULL kHeapObjectTag
 Object* Object::null_object_ = NULL;
 Array* Object::null_array_ = NULL;
 String* Object::null_string_ = NULL;
 Instance* Object::null_instance_ = NULL;
 TypeArguments* Object::null_type_arguments_ = NULL;
 Array* Object::empty_array_ = NULL;
 Array* Object::zero_array_ = NULL;
 PcDescriptors* Object::empty_descriptors_ = NULL;
+LocalVarDescriptors* Object::empty_var_descriptors_ = NULL;
 Instance* Object::sentinel_ = NULL;
 Instance* Object::transition_sentinel_ = NULL;
 Instance* Object::unknown_constant_ = NULL;
 Instance* Object::non_constant_ = NULL;
 Bool* Object::bool_true_ = NULL;
 Bool* Object::bool_false_ = NULL;
 Smi* Object::smi_illegal_cid_ = NULL;
 LanguageError* Object::snapshot_writer_error_ = NULL;
 LanguageError* Object::branch_offset_error_ = NULL;
 
@@ skipping 317 matching lines @@
 
   // Allocate the read only object handles here.
   null_object_ = Object::ReadOnlyHandle();
   null_array_ = Array::ReadOnlyHandle();
   null_string_ = String::ReadOnlyHandle();
   null_instance_ = Instance::ReadOnlyHandle();
   null_type_arguments_ = TypeArguments::ReadOnlyHandle();
   empty_array_ = Array::ReadOnlyHandle();
   zero_array_ = Array::ReadOnlyHandle();
   empty_descriptors_ = PcDescriptors::ReadOnlyHandle();
+  empty_var_descriptors_ = LocalVarDescriptors::ReadOnlyHandle();
   sentinel_ = Instance::ReadOnlyHandle();
   transition_sentinel_ = Instance::ReadOnlyHandle();
   unknown_constant_ =  Instance::ReadOnlyHandle();
   non_constant_ =  Instance::ReadOnlyHandle();
   bool_true_ = Bool::ReadOnlyHandle();
   bool_false_ = Bool::ReadOnlyHandle();
   smi_illegal_cid_ = Smi::ReadOnlyHandle();
   snapshot_writer_error_ = LanguageError::ReadOnlyHandle();
   branch_offset_error_ = LanguageError::ReadOnlyHandle();
 
@@ skipping 210 matching lines @@
         PcDescriptors::InstanceSize(0, RawPcDescriptors::kCompressedRecSize),
         Heap::kOld);
     InitializeObject(address, kPcDescriptorsCid,
         PcDescriptors::InstanceSize(0, RawPcDescriptors::kCompressedRecSize));
     PcDescriptors::initializeHandle(
         empty_descriptors_,
         reinterpret_cast<RawPcDescriptors*>(address + kHeapObjectTag));
     empty_descriptors_->raw_ptr()->length_ = 0;
   }
 
+  // Allocate and initialize the canonical empty variable descriptor object.
+  {
+    uword address = heap->Allocate(
+        LocalVarDescriptors::InstanceSize(0), Heap::kOld);
+    InitializeObject(address, kLocalVarDescriptorsCid,
+        LocalVarDescriptors::InstanceSize(0));
+    LocalVarDescriptors::initializeHandle(
+        empty_var_descriptors_,
+        reinterpret_cast<RawLocalVarDescriptors*>(address + kHeapObjectTag));
+    empty_var_descriptors_->raw_ptr()->length_ = 0;
+    empty_var_descriptors_->raw_ptr()->names_ = empty_array_->raw_ptr();

[siva, 2014/09/11 00:04:29]

to be safe:

StorePointer(&empty_var_descriptors_->raw_ptr()->names_,
empty_array_->raw_ptr());

[hausner, 2014/09/11 16:08:12]

StorePointer() is an instance method, can't use it here. I added a comment
remarking that the pointer assignment is safe here.

On 2014/09/11 00:04:29, siva wrote:

+  }
 
   cls = Class::New<Instance>(kDynamicCid);
   cls.set_is_abstract();
   cls.set_num_type_arguments(0);
   cls.set_num_own_type_arguments(0);
   cls.set_is_type_finalized();
   cls.set_is_finalized();
   dynamic_class_ = cls.raw();
 
   cls = Class::New<Instance>(kVoidCid);
@@ skipping 19684 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart