[es6] implement default parameters via desugaring

src/parser.cc

 // Copyright 2012 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/v8.h"
 
 #include "src/api.h"
 #include "src/ast.h"
 #include "src/bailout-reason.h"
 #include "src/base/platform/platform.h"
@@ skipping 855 matching lines @@
   set_allow_harmony_arrow_functions(FLAG_harmony_arrow_functions);
   set_allow_harmony_classes(FLAG_harmony_classes);
   set_allow_harmony_object_literals(FLAG_harmony_object_literals);
   set_allow_harmony_sloppy(FLAG_harmony_sloppy);
   set_allow_harmony_unicode(FLAG_harmony_unicode);
   set_allow_harmony_computed_property_names(
       FLAG_harmony_computed_property_names);
   set_allow_harmony_rest_params(FLAG_harmony_rest_parameters);
   set_allow_harmony_spreadcalls(FLAG_harmony_spreadcalls);
   set_allow_harmony_destructuring(FLAG_harmony_destructuring);
+  set_allow_harmony_optional_params(FLAG_harmony_optional_params);
   set_allow_strong_mode(FLAG_strong_mode);
   for (int feature = 0; feature < v8::Isolate::kUseCounterFeatureCount;
        ++feature) {
     use_counts_[feature] = 0;
   }
   if (info->ast_value_factory() == NULL) {
     // info takes ownership of AstValueFactory.
     info->set_ast_value_factory(new AstValueFactory(zone(), info->hash_seed()));
     info->set_ast_value_factory_owned();
     ast_value_factory_ = info->ast_value_factory();
@@ skipping 241 matching lines @@
               ? FunctionLiteral::ANONYMOUS_EXPRESSION
               : FunctionLiteral::NAMED_EXPRESSION)
         : FunctionLiteral::DECLARATION;
     bool ok = true;
 
     if (shared_info->is_arrow()) {
       Scope* scope = NewScope(scope_, ARROW_SCOPE);
       scope->set_start_position(shared_info->start_position());
       FormalParameterErrorLocations error_locs;
       bool has_rest = false;
+      bool has_initializers = false;
+      ZoneList<Expression*>* initializers =
+          new (zone()) ZoneList<Expression*>(0, zone());
       if (Check(Token::LPAREN)) {
         // '(' StrictFormalParameters ')'
-        ParseFormalParameterList(scope, &error_locs, &has_rest, &ok);
+        ParseFormalParameterList(scope, &error_locs, initializers,
+                                 &has_initializers, &has_rest, &ok);
         if (ok) ok = Check(Token::RPAREN);
       } else {
         // BindingIdentifier
-        ParseFormalParameter(scope, &error_locs, has_rest, &ok);
+        Expression** initializer_ptr = nullptr;
+        bool** has_initializer_ptr = nullptr;
+        ParseFormalParameter(scope, &error_locs, initializer_ptr,
+                             has_initializer_ptr, has_rest, &ok);
       }
 
       if (ok) {
         ExpressionClassifier classifier;
         Expression* expression = ParseArrowFunctionLiteral(
             scope, error_locs, has_rest, &classifier, &ok);
         ValidateExpression(&classifier, &ok);
         if (ok) {
           // Scanning must end at the same position that was recorded
           // previously. If not, parsing has been interrupted due to a stack
@@ skipping 903 matching lines @@
   // initialization code. Thus, inside the 'with' statement, we need
   // both access to the static and the dynamic context chain; the
   // runtime needs to provide both.
   if (resolve && var != NULL) {
     proxy->BindTo(var);
   }
   return var;
 }
 
 
+void Parser::ShadowParametersForExpressions(Scope* scope) {
+  scope->ShadowParametersForExpressions();
+}
+
+
 // Language extension which is only enabled for source files loaded
 // through the API's extension mechanism.  A native function
 // declaration is resolved by looking up the function through a
 // callback provided by the extension.
 Statement* Parser::ParseNativeDeclaration(bool* ok) {
   int pos = peek_position();
   Expect(Token::FUNCTION, CHECK_OK);
   // Allow "eval" or "arguments" for backward compatibility.
   const AstRawString* name =
       ParseIdentifier(kAllowRestrictedIdentifiers, CHECK_OK);
@@ skipping 1438 matching lines @@
 
   inner_scope->set_end_position(scanner()->location().end_pos);
   inner_block->set_scope(inner_scope);
   scope_ = for_scope;
 
   outer_loop->Initialize(NULL, NULL, NULL, inner_block);
   return outer_block;
 }
 
 
+ZoneList<Statement*>* Parser::DesugarInitializeParameters(
+    Scope* scope, bool has_initializers, ZoneList<Expression*>* initializers) {
+  DCHECK(scope->is_function_scope());
+
+  if (has_initializers) {
+    // If hasParameterExpressions for the function is true, each parameter is
+    // desugared as follows:
+    //
+    // SingleNameBinding :
+    //   let <name> = %_Arguments(<index>);
+    // SingleNameBinding Initializer
+    //   let <name> = IS_UNDEFINED(%_Arguments(<index>)) ? <initializer>
+    //                                                   : %_Arguments(<index>);
+    //
+    // TODO(caitp, dslomov): support BindingPatterns & rest parameters
+    //
+
+    ShadowParametersForExpressions(scope);
+    ZoneList<Statement*>* body = new (zone()) ZoneList<Statement*>(0, zone());
+    for (int i = 0; i < initializers->length(); ++i) {
+      Expression* initializer = initializers->at(i);
+
+      // Position of parameter VariableProxy, for hole-checking
+      int pos = scope->parameter_position(i);
+
+      // Lexically declare the initialized variable
+      static const VariableMode mode = LET;
+      VariableProxy* proxy =
+          NewUnresolved(scope->parameter(i)->raw_name(), mode);
+      VariableDeclaration* declaration = factory()->NewVariableDeclaration(
+          proxy, mode, scope, RelocInfo::kNoPosition);
+      bool ok = true;
+
+      // All formal parameters have been removed from the scope VariableMap,
+      // and so Declare() should not be able to fail.
+      proxy = factory()->NewVariableProxy(Declare(declaration, true, &ok), pos);
+      DCHECK(ok);
+      proxy->var()->set_maybe_assigned();

[rossberg, 2015/05/12 14:04:53]

Why is this set?

[rossberg, 2015/05/20 07:41:47]

Ping

[caitp (gmail), 2015/05/20 11:29:38]

Maybe it's not needed, but it reads like it does the right thing, since the
variable gets assigned immediately

[rossberg, 2015/05/20 12:33:55]

Ah, note that these are initialisations, not assignments. This flag tries to
conservatively indicate whether a variable is ever assigned, i.e., can change
its value observably, or is quasi const (which is relevant for optimisations
later). For lexical declarations, which have a proper TDZ, initialisation does
not observably change their value (though it does for 'var').

+
+      const AstRawString* fn_name = ast_value_factory()->empty_string();
+      const Runtime::Function* arguments =
+          Runtime::FunctionForId(Runtime::kInlineArguments);
+      ZoneList<Expression*>* arguments_i0 =
+          new (zone()) ZoneList<Expression*>(0, zone());
+      arguments_i0->Add(factory()->NewSmiLiteral(i, RelocInfo::kNoPosition),
+                        zone());
+
+      // TODO(caitp): ensure proper TDZ behaviour --- need hole-check for

[rossberg, 2015/05/12 14:04:53]

Nit: this comment seems obsolete

+      // all parameter bindings, including ones without initializers
+      if (initializer) {
+        // IS_UNDEFINED(%_Arguments(i)) ? <initializer> : %_Arguments(i);
+        ZoneList<Expression*>* arguments_i1 =
+            new (zone()) ZoneList<Expression*>(0, zone());
+        arguments_i1->Add(factory()->NewSmiLiteral(i, RelocInfo::kNoPosition),
+                          zone());
+
+        Expression* arg_or_default = factory()->NewConditional(
+            // condition:
+            factory()->NewCompareOperation(
+                Token::EQ_STRICT,
+                factory()->NewCallRuntime(fn_name, arguments, arguments_i0,
+                                          RelocInfo::kNoPosition),
+                factory()->NewUndefinedLiteral(RelocInfo::kNoPosition),
+                RelocInfo::kNoPosition),
+            // if true:
+            initializer,
+            // if false:
+            factory()->NewCallRuntime(fn_name, arguments, arguments_i1,
+                                      RelocInfo::kNoPosition),
+            RelocInfo::kNoPosition);
+
+        Expression* assign = factory()->NewAssignment(
+            Token::INIT_LET, proxy, arg_or_default, RelocInfo::kNoPosition);
+
+        body->Add(
+            factory()->NewExpressionStatement(assign, RelocInfo::kNoPosition),
+            zone());
+        proxy->var()->set_initializer_position(initializer->position());
+      } else {
+        // let <name> = %_Arguments(i)
+        Expression* assign = factory()->NewAssignment(
+            Token::INIT_LET, proxy,
+            factory()->NewCallRuntime(fn_name, arguments, arguments_i0,
+                                      RelocInfo::kNoPosition),
+            RelocInfo::kNoPosition);
+        body->Add(
+            factory()->NewExpressionStatement(assign, RelocInfo::kNoPosition),
+            zone());
+        proxy->var()->set_initializer_position(pos);
+      }
+    }
+    return body;
+  } else {
+    // If hasParameterExpressions is false, remove the unnecessary parameter
+    // block scopes.
+    ZoneList<Scope*>* scopes = scope->inner_scopes();
+    for (int i = 0; i < scopes->length(); ++i) {
+      Scope* scope = scopes->at(i);
+      DCHECK(scope->is_block_scope());
+      scope->FinalizeBlockScope();
+    }
+    return nullptr;
+  }
+}
+
+
 Statement* Parser::ParseForStatement(ZoneList<const AstRawString*>* labels,
                                      bool* ok) {
   // ForStatement ::
   //   'for' '(' Expression? ';' Expression? ';' Expression? ')' Statement
 
   int stmt_pos = peek_position();
   bool is_const = false;
   Statement* init = NULL;
   ZoneList<const AstRawString*> lexical_bindings(1, zone());
 
@@ skipping 390 matching lines @@
     error_locs->reserved = param_location;
   if (!error_locs->undefined.IsValid() && IsUndefined(raw_name))
     error_locs->undefined = param_location;
 
   // When the formal parameter was originally seen, it was parsed as a
   // VariableProxy and recorded as unresolved in the scope.  Here we undo that
   // parse-time side-effect.
   parser_->scope_->RemoveUnresolved(expr->AsVariableProxy());
 
   bool is_rest = false;
-  bool is_duplicate = DeclareFormalParameter(scope, raw_name, is_rest);
+  int pos = expr->position();
+  bool is_duplicate = DeclareFormalParameter(scope, raw_name, is_rest, pos);
 
   if (is_duplicate) {
     // Arrow function parameter lists are parsed as StrictFormalParameters,
     // which means that they cannot have duplicates.  Note that this is a subset
     // of the restrictions placed on parameters to functions whose body is
     // strict.
     ReportMessageAt(param_location,
                     "duplicate_arrow_function_formal_parameter");
     *ok = false;
     return;
@@ skipping 109 matching lines @@
       // expressions. This also marks the FunctionState as a generator.
       Variable* temp = scope_->DeclarationScope()->NewTemporary(
           ast_value_factory()->dot_generator_object_string());
       function_state.set_generator_object_variable(temp);
     }
 
     bool has_rest = false;
     Expect(Token::LPAREN, CHECK_OK);
     int start_position = scanner()->location().beg_pos;
     scope_->set_start_position(start_position);
+    ZoneList<Expression*>* initializers =
+        new (zone()) ZoneList<Expression*>(0, zone());
+    bool has_initializers = false;
     num_parameters =
-        ParseFormalParameterList(scope, &error_locs, &has_rest, CHECK_OK);
+        ParseFormalParameterList(scope, &error_locs, initializers,
+                                 &has_initializers, &has_rest, CHECK_OK);
     Expect(Token::RPAREN, CHECK_OK);
     int formals_end_position = scanner()->location().end_pos;
 
     CheckArityRestrictions(num_parameters, arity_restriction, start_position,
                            formals_end_position, CHECK_OK);
 
     Expect(Token::LBRACE, CHECK_OK);
 
     // If we have a named function expression, we add a local variable
     // declaration to the body of the function with the name of the
@@ skipping 80 matching lines @@
         is_lazily_parsed = false;
 
         // This is probably an initialization function. Inform the compiler it
         // should also eager-compile this function, and that we expect it to be
         // used once.
         eager_compile_hint = FunctionLiteral::kShouldEagerCompile;
         should_be_used_once_hint = true;
       }
     }
     if (!is_lazily_parsed) {
-      body = ParseEagerFunctionBody(function_name, pos, fvar, fvar_init_op,
-                                    kind, CHECK_OK);
+      body = DesugarInitializeParameters(scope, has_initializers, initializers);
+      if (has_initializers) {
+        // TODO(caitp): Function body scope must be a declaration scope
+        Scope* function_body_scope = NewScope(scope, BLOCK_SCOPE);
+        function_body_scope->set_start_position(scope->start_position());
+        function_body_scope->SetScopeName(function_name);
+        BlockState function_body_state(&scope_, function_body_scope);
+        ZoneList<Statement*>* inner_body = ParseEagerFunctionBody(
+            function_name, pos, fvar, fvar_init_op, kind, CHECK_OK);
+        scope->set_end_position(function_body_scope->end_position());
+        body->AddAll(*inner_body, zone());

[rossberg, 2015/05/12 14:04:53]

Shouldn't this create a block statement instead?

[caitp (gmail), 2015/05/12 14:34:17]

Is this for the extra bailout id? I have mostly been thinking about scoping, so
I'm not sure what effect wrapping it in a block statement has

[rossberg, 2015/05/20 07:41:47]

The scope chain needs to be in sync with the context chain we create at runtime
(up to some optimisations). I'm not sure how it can work without, at least not
if something like eval or with appears in the body, referring to respective
variables (which reminds me that we should probably have tests for those cases
:) ).

+      } else {
+        body = ParseEagerFunctionBody(function_name, pos, fvar, fvar_init_op,
+                                      kind, CHECK_OK);
+      }
       materialized_literal_count = function_state.materialized_literal_count();
       expected_property_count = function_state.expected_property_count();
       handler_count = function_state.handler_count();
 
       if (is_strong(language_mode()) && IsSubclassConstructor(kind)) {
         if (!function_state.super_location().IsValid()) {
           ReportMessageAt(function_name_location,
                           "strong_super_call_missing", kReferenceError);
           *ok = false;
           return nullptr;
@@ skipping 230 matching lines @@
     reusable_preparser_->set_allow_harmony_sloppy(allow_harmony_sloppy());
     reusable_preparser_->set_allow_harmony_unicode(allow_harmony_unicode());
     reusable_preparser_->set_allow_harmony_computed_property_names(
         allow_harmony_computed_property_names());
     reusable_preparser_->set_allow_harmony_rest_params(
         allow_harmony_rest_params());
     reusable_preparser_->set_allow_harmony_spreadcalls(
         allow_harmony_spreadcalls());
     reusable_preparser_->set_allow_harmony_destructuring(
         allow_harmony_destructuring());
+    reusable_preparser_->set_allow_harmony_optional_params(
+        allow_harmony_optional_params());
     reusable_preparser_->set_allow_strong_mode(allow_strong_mode());
   }
   PreParser::PreParseResult result = reusable_preparser_->PreParseLazyFunction(
       language_mode(), function_state_->kind(), logger, bookmark);
   if (pre_parse_timer_ != NULL) {
     pre_parse_timer_->Stop();
   }
   return result;
 }
 
@@ skipping 1491 matching lines @@
 
 Expression* Parser::SpreadCallNew(Expression* function,
                                   ZoneList<v8::internal::Expression*>* args,
                                   int pos) {
   args->InsertAt(0, function, zone());
 
   return factory()->NewCallRuntime(
       ast_value_factory()->reflect_construct_string(), NULL, args, pos);
 }
 } }  // namespace v8::internal