Optimize LocalVarDescriptor objects for functions that have no local variables

Allocate a canonical “empty” variable descriptor object. Saves about 10% of
variable descriptor space when running dart2js.

R=asiva@google.com

Review URL: https://codereview.chromium.org//565513002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@40158 260f80e4-7a28-3924-810f-c04153c831b5

runtime/vm/object.cc

@@ -98,6 +98,7 @@ 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;
@@ -435,6 +436,7 @@ void Object::InitOnce() {
   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();
@@ -665,6 +667,20 @@ void Object::InitOnce() {
     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;
+    // Can't use instance mentod StorePointer() here, but this pointer
+    // assignment is safe (from old space to old space).
+    empty_var_descriptors_->raw_ptr()->names_ = empty_array_->raw_ptr();
+  }
 
   cls = Class::New<Instance>(kDynamicCid);
   cls.set_is_abstract();

runtime/vm/object.h

@@ -393,6 +393,11 @@ class Object {
     return *empty_descriptors_;
   }
 
+  static const LocalVarDescriptors& empty_var_descriptors() {
+    ASSERT(empty_var_descriptors_ != NULL);
+    return *empty_var_descriptors_;
+  }
+
   // The sentinel is a value that cannot be produced by Dart code.
   // It can be used to mark special values, for example to distinguish
   // "uninitialized" fields.
@@ -670,6 +675,7 @@ class Object {
   static Array* empty_array_;
   static Array* zero_array_;
   static PcDescriptors* empty_descriptors_;
+  static LocalVarDescriptors* empty_var_descriptors_;
   static Instance* sentinel_;
   static Instance* transition_sentinel_;
   static Instance* unknown_constant_;
@@ -3099,6 +3105,7 @@ class LocalVarDescriptors : public Object {
  private:
   FINAL_HEAP_OBJECT_IMPLEMENTATION(LocalVarDescriptors, Object);
   friend class Class;
+  friend class Object;
 };
 
 

runtime/vm/raw_object.h

@@ -1102,6 +1102,8 @@ class RawLocalVarDescriptors : public RawObject {
 
   // Variable info with [length_] entries.
   VarInfo* data() { OPEN_ARRAY_START(VarInfo, int32_t); }
+
+  friend class Object;
 };
 
 

runtime/vm/scopes.cc

@@ -266,6 +266,9 @@ RawLocalVarDescriptors* LocalScope::GetVarDescriptors(const Function& func) {
   int16_t scope_id = 0;
   CollectLocalVariables(&vars, &scope_id);
 
+  if (vars.length() == 0) {
+    return Object::empty_var_descriptors().raw();
+  }
   const LocalVarDescriptors& var_desc =
       LocalVarDescriptors::Handle(LocalVarDescriptors::New(vars.length()));
   for (int i = 0; i < vars.length(); i++) {
@@ -294,7 +297,7 @@ void LocalScope::CollectLocalVariables(GrowableArray<VarDesc>* vars,
     // This is the outermost scope with a context level or this scope's
     // context level differs from its parent's level.
     VarDesc desc;
-    desc.name = &String::Handle();  // No name.
+    desc.name = &Object::null_string();  // No name.
     desc.info.set_kind(RawLocalVarDescriptors::kContextLevel);
     desc.info.scope_id = *scope_id;
     desc.info.begin_pos = begin_token_pos();