Improve async code in VM by not unnecessarily capturing parameters of async and

runtime/vm/scopes.cc

Index: runtime/vm/scopes.cc
diff --git a/runtime/vm/scopes.cc b/runtime/vm/scopes.cc
index e11fc166824a221e77870d896328f312cb0eb0ab..637e9b72bfa92fa9e1a9a19be20e4290c0722e7c 100644
--- a/runtime/vm/scopes.cc
+++ b/runtime/vm/scopes.cc
@@ -404,18 +404,10 @@ LocalVariable* LocalScope::LookupVariable(const String& name, bool test_only) {
   LocalScope* current_scope = this;
   while (current_scope != NULL) {
     LocalVariable* var = current_scope->LocalLookupVariable(name);
-    if ((var != NULL) && !var->is_invisible_) {
-      if (!test_only) {
-        if (var->owner()->function_level() != function_level()) {
-          var->set_is_captured();
-        }
-        // Insert aliases of the variable in intermediate scopes.
-        LocalScope* intermediate_scope = this;
-        while (intermediate_scope != current_scope) {
-          intermediate_scope->variables_.Add(var);
-          ASSERT(var->owner() != intermediate_scope);  // Item is an alias.
-          intermediate_scope = intermediate_scope->parent();
-        }
+    // If testing only, return the variable even if invisible.
+    if ((var != NULL) && (!var->is_invisible_ || test_only)) {
+      if (!test_only && (var->owner()->function_level() != function_level())) {
+        CaptureVariable(var);
       }
       return var;
     }
@@ -425,32 +417,29 @@ LocalVariable* LocalScope::LookupVariable(const String& name, bool test_only) {
 }
 
 
-bool LocalScope::CaptureVariable(const String& name) {
-  ASSERT(name.IsSymbol());
-  LocalScope* current_scope = this;
-  while (current_scope != NULL) {
-    LocalVariable* var = current_scope->LocalLookupVariable(name);
-    if (var != NULL) {
-      var->set_is_captured();
-      LocalScope* scope = this;
-      while (var->owner()->function_level() != scope->function_level()) {
-        // Insert an alias of the variable in the top scope of each function
-        // level so that the variable is found in the context.
-        LocalScope* parent_scope = scope->parent();
-        while ((parent_scope != NULL) &&
-               (parent_scope->function_level() == scope->function_level())) {
-          scope = parent_scope;
-          parent_scope = scope->parent();
-        }
-        scope->variables_.Add(var);
-        ASSERT(var->owner() != scope);  // Item is an alias.
-        scope = parent_scope;
-      }
-      return true;
+void LocalScope::CaptureVariable(LocalVariable* variable) {
+  ASSERT(variable != NULL);
+  // The variable must exist in an enclosing scope, not necessarily in this one.
+  variable->set_is_captured();
+  const int variable_function_level = variable->owner()->function_level();
+  LocalScope* scope = this;
+  while (scope->function_level() != variable_function_level) {
+    // Insert an alias of the variable in the top scope of each function
+    // level so that the variable is found in the context.
+    LocalScope* parent_scope = scope->parent();
+    while ((parent_scope != NULL) &&
+           (parent_scope->function_level() == scope->function_level())) {
+      scope = parent_scope;
+      parent_scope = scope->parent();
     }
-    current_scope = current_scope->parent();
+    // An alias may already have been added in this scope, and in that case,
+    // in parent scopes as needed. If so, we are done.
+    if (!scope->AddVariable(variable)) {
+      return;
+    }
+    ASSERT(variable->owner() != scope);  // Item is an alias.
+    scope = parent_scope;
   }
-  return false;
 }
 
 
@@ -525,9 +514,10 @@ SourceLabel* LocalScope::CheckUnresolvedLabels() {
 
 int LocalScope::NumCapturedVariables() const {
   // It is not necessary to traverse parent scopes, since we are only interested
-  // in the captured variables referenced in this scope. If this scope
-  // references a captured variable declared in a parent scope, it will contain
-  // an alias for that variable.
+  // in the captured variables referenced in this scope. If this scope is the
+  // top scope at function level 1 and it (or its children scopes) references a
+  // captured variable declared in a parent scope at function level 0, it will
+  // contain an alias for that variable.
 
   // Since code generation for nested functions is postponed until first
   // invocation, the function level of the closure scope can only be 1.
@@ -628,7 +618,6 @@ LocalScope* LocalScope::RestoreOuterScope(const ContextScope& context_scope) {
 
 
 void LocalScope::RecursivelyCaptureAllVariables() {
-  bool found = false;
   for (intptr_t i = 0; i < num_variables(); i++) {
     if ((VariableAt(i)->name().raw() == Symbols::StackTraceVar().raw()) ||
         (VariableAt(i)->name().raw() == Symbols::ExceptionVar().raw()) ||
@@ -637,8 +626,7 @@ void LocalScope::RecursivelyCaptureAllVariables() {
       // stack.
       continue;
     }
-    found = CaptureVariable(VariableAt(i)->name());
-    ASSERT(found);
+    CaptureVariable(VariableAt(i));
   }
   if (sibling() != NULL) { sibling()->RecursivelyCaptureAllVariables(); }
   if (child() != NULL) { child()->RecursivelyCaptureAllVariables(); }