text: // 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/parsing/rewriter.h"
  
  #include "src/ast/ast.h"
  #include "src/ast/scopes.h"
  #include "src/logging/runtime-call-stats-scope.h"
  #include "src/objects/objects-inl.h"
  #include "src/parsing/parse-info.h"
  #include "src/parsing/parser.h"
  #include "src/zone/zone-list-inl.h"
  
  namespace v8 {
  namespace internal {
  
  class Processor final : public AstVisitor<Processor> {
   public:
    Processor(uintptr_t stack_limit, DeclarationScope* closure_scope,
              Variable* result, AstValueFactory* ast_value_factory, Zone* zone)
        : result_(result),
          replacement_(nullptr),
          zone_(zone),
          closure_scope_(closure_scope),
          factory_(ast_value_factory, zone),
          result_assigned_(false),
          is_set_(false),
          breakable_(false) {
      DCHECK_EQ(closure_scope, closure_scope->GetClosureScope());
      InitializeAstVisitor(stack_limit);
    }
  
    Processor(Parser* parser, DeclarationScope* closure_scope, Variable* result,
              AstValueFactory* ast_value_factory, Zone* zone)
        : result_(result),
          replacement_(nullptr),
          zone_(zone),
          closure_scope_(closure_scope),
          factory_(ast_value_factory, zone_),
          result_assigned_(false),
          is_set_(false),
          breakable_(false) {
      DCHECK_EQ(closure_scope, closure_scope->GetClosureScope());
      InitializeAstVisitor(parser->stack_limit());
    }
  
    void Process(ZonePtrList<Statement>* statements);
    bool result_assigned() const { return result_assigned_; }
  
    Zone* zone() { return zone_; }
    DeclarationScope* closure_scope() { return closure_scope_; }
    AstNodeFactory* factory() { return &factory_; }
  
    // Returns ".result = value"
    Expression* SetResult(Expression* value) {
      result_assigned_ = true;
      VariableProxy* result_proxy = factory()->NewVariableProxy(result_);
      return factory()->NewAssignment(Token::ASSIGN, result_proxy, value,
                                      kNoSourcePosition);
    }
  
    // Inserts '.result = undefined' in front of the given statement.
    Statement* AssignUndefinedBefore(Statement* s);
  
   private:
    Variable* result_;
  
    // When visiting a node, we "return" a replacement for that node in
    // [replacement_].  In many cases this will just be the original node.
    Statement* replacement_;
  
    class V8_NODISCARD BreakableScope final {
     public:
      explicit BreakableScope(Processor* processor, bool breakable = true)
          : processor_(processor), previous_(processor->breakable_) {
        processor->breakable_ = processor->breakable_ || breakable;
      }
  
      ~BreakableScope() { processor_->breakable_ = previous_; }
  
     private:
      Processor* processor_;
      bool previous_;
    };
  
    Zone* zone_;
    DeclarationScope* closure_scope_;
    AstNodeFactory factory_;
  
    // Node visitors.
  #define DEF_VISIT(type) void Visit##type(type* node);
    AST_NODE_LIST(DEF_VISIT)
  #undef DEF_VISIT
  
    void VisitIterationStatement(IterationStatement* stmt);
  
    DEFINE_AST_VISITOR_SUBCLASS_MEMBERS();
  
    // We are not tracking result usage via the result_'s use
    // counts (we leave the accurate computation to the
    // usage analyzer). Instead we simple remember if
    // there was ever an assignment to result_.
    bool result_assigned_;
  
    // To avoid storing to .result all the time, we eliminate some of
    // the stores by keeping track of whether or not we're sure .result
    // will be overwritten anyway. This is a bit more tricky than what I
    // was hoping for.
    bool is_set_;
  
    bool breakable_;
  };
  
  
  Statement* Processor::AssignUndefinedBefore(Statement* s) {
    Expression* undef = factory()->NewUndefinedLiteral(kNoSourcePosition);
    Expression* assignment = SetResult(undef);
    Block* b = factory()->NewBlock(2, false);
    b->statements()->Add(
        factory()->NewExpressionStatement(assignment, kNoSourcePosition), zone());
    b->statements()->Add(s, zone());
    return b;
  }
  
  void Processor::Process(ZonePtrList<Statement>* statements) {
    // If we're in a breakable scope (named block, iteration, or switch), we walk
    // all statements. The last value producing statement before the break needs
    // to assign to .result. If we're not in a breakable scope, only the last
    // value producing statement in the block assigns to .result, so we can stop
    // early.
    for (int i = statements->length() - 1; i >= 0 && (breakable_ || !is_set_);
         --i) {
      Visit(statements->at(i));
      statements->Set(i, replacement_);
    }
  }
  
  
  void Processor::VisitBlock(Block* node) {
    // An initializer block is the rewritten form of a variable declaration
    // with initialization expressions. The initializer block contains the
    // list of assignments corresponding to the initialization expressions.
    // While unclear from the spec (ECMA-262, 3rd., 12.2), the value of
    // a variable declaration with initialization expression is 'undefined'
    // with some JS VMs: For instance, using smjs, print(eval('var x = 7'))
    // returns 'undefined'. To obtain the same behavior with v8, we need
    // to prevent rewriting in that case.
    if (!node->ignore_completion_value()) {
      BreakableScope scope(this, node->is_breakable());
      Process(node->statements());
    }
    replacement_ = node;
  }
  
  
  void Processor::VisitExpressionStatement(ExpressionStatement* node) {
    // Rewrite : <x>; -> .result = <x>;
    if (!is_set_) {
      node->set_expression(SetResult(node->expression()));
      is_set_ = true;
    }
    replacement_ = node;
  }
  
  
  void Processor::VisitIfStatement(IfStatement* node) {
    // Rewrite both branches.
    bool set_after = is_set_;
  
    Visit(node->then_statement());
    node->set_then_statement(replacement_);
    bool set_in_then = is_set_;
  
    is_set_ = set_after;
    Visit(node->else_statement());
    node->set_else_statement(replacement_);
  
    replacement_ = set_in_then && is_set_ ? node : AssignUndefinedBefore(node);
    is_set_ = true;
  }
  
  
  void Processor::VisitIterationStatement(IterationStatement* node) {
    // The statement may have to produce a value, so always assign undefined
    // before.
    // TODO(verwaest): Omit it if we know that there's no break/continue leaving
    // it early.
    DCHECK(breakable_ || !is_set_);
    BreakableScope scope(this);
  
    Visit(node->body());
    node->set_body(replacement_);
  
    replacement_ = AssignUndefinedBefore(node);
    is_set_ = true;
  }
  
  
  void Processor::VisitDoWhileStatement(DoWhileStatement* node) {
    VisitIterationStatement(node);
  }
  
  
  void Processor::VisitWhileStatement(WhileStatement* node) {
    VisitIterationStatement(node);
  }
  
  
  void Processor::VisitForStatement(ForStatement* node) {
    VisitIterationStatement(node);
  }
  
  
  void Processor::VisitForInStatement(ForInStatement* node) {
    VisitIterationStatement(node);
  }
  
  
  void Processor::VisitForOfStatement(ForOfStatement* node) {
    VisitIterationStatement(node);
  }
  
  
  void Processor::VisitTryCatchStatement(TryCatchStatement* node) {
    // Rewrite both try and catch block.
    bool set_after = is_set_;
  
    Visit(node->try_block());
    node->set_try_block(static_cast<Block*>(replacement_));
    bool set_in_try = is_set_;
  
    is_set_ = set_after;
    Visit(node->catch_block());
    node->set_catch_block(static_cast<Block*>(replacement_));
  
    replacement_ = is_set_ && set_in_try ? node : AssignUndefinedBefore(node);
    is_set_ = true;
  }
  
  
  void Processor::VisitTryFinallyStatement(TryFinallyStatement* node) {
    // Only rewrite finally if it could contain 'break' or 'continue'. Always
    // rewrite try.
    if (breakable_) {
      // Only set result before a 'break' or 'continue'.
      is_set_ = true;
      Visit(node->finally_block());
      node->set_finally_block(replacement_->AsBlock());
      CHECK_NOT_NULL(closure_scope());
      if (is_set_) {
        // Save .result value at the beginning of the finally block and restore it
        // at the end again: ".backup = .result; ...; .result = .backup" This is
        // necessary because the finally block does not normally contribute to the
        // completion value.
        Variable* backup = closure_scope()->NewTemporary(
            factory()->ast_value_factory()->dot_result_string());
        Expression* backup_proxy = factory()->NewVariableProxy(backup);
        Expression* result_proxy = factory()->NewVariableProxy(result_);
        Expression* save = factory()->NewAssignment(
            Token::ASSIGN, backup_proxy, result_proxy, kNoSourcePosition);
        Expression* restore = factory()->NewAssignment(
            Token::ASSIGN, result_proxy, backup_proxy, kNoSourcePosition);
        node->finally_block()->statements()->InsertAt(
            0, factory()->NewExpressionStatement(save, kNoSourcePosition),
            zone());
        node->finally_block()->statements()->Add(
            factory()->NewExpressionStatement(restore, kNoSourcePosition),
            zone());
      } else {
        // If is_set_ is false, it means the finally block has a 'break' or a
        // 'continue' and was not preceded by a statement that assigned to
        // .result. Try-finally statements return the abrupt completions from the
        // finally block, meaning this case should get an undefined.
        //
        // Since the finally block will definitely result in an abrupt completion,
        // there's no need to save and restore the .result.
        Expression* undef = factory()->NewUndefinedLiteral(kNoSourcePosition);
        Expression* assignment = SetResult(undef);
        node->finally_block()->statements()->InsertAt(
            0, factory()->NewExpressionStatement(assignment, kNoSourcePosition),
            zone());
      }
      // We can't tell whether the finally-block is guaranteed to set .result, so
      // reset is_set_ before visiting the try-block.
      is_set_ = false;
    }
    Visit(node->try_block());
    node->set_try_block(replacement_->AsBlock());
  
    replacement_ = is_set_ ? node : AssignUndefinedBefore(node);
    is_set_ = true;
  }
  
  
  void Processor::VisitSwitchStatement(SwitchStatement* node) {
    // The statement may have to produce a value, so always assign undefined
    // before.
    // TODO(verwaest): Omit it if we know that there's no break/continue leaving
    // it early.
    DCHECK(breakable_ || !is_set_);
    BreakableScope scope(this);
    // Rewrite statements in all case clauses.
    ZonePtrList<CaseClause>* clauses = node->cases();
    for (int i = clauses->length() - 1; i >= 0; --i) {
      CaseClause* clause = clauses->at(i);
      Process(clause->statements());
    }
  
    replacement_ = AssignUndefinedBefore(node);
    is_set_ = true;
  }
  
  
  void Processor::VisitContinueStatement(ContinueStatement* node) {
    is_set_ = false;
    replacement_ = node;
  }
  
  
  void Processor::VisitBreakStatement(BreakStatement* node) {
    is_set_ = false;
    replacement_ = node;
  }
  
  
  void Processor::VisitWithStatement(WithStatement* node) {
    Visit(node->statement());
    node->set_statement(replacement_);
  
    replacement_ = is_set_ ? node : AssignUndefinedBefore(node);
    is_set_ = true;
  }
  
  
  void Processor::VisitSloppyBlockFunctionStatement(
      SloppyBlockFunctionStatement* node) {
    Visit(node->statement());
    node->set_statement(replacement_);
    replacement_ = node;
  }
  
  
  void Processor::VisitEmptyStatement(EmptyStatement* node) {
    replacement_ = node;
  }
  
  
  void Processor::VisitReturnStatement(ReturnStatement* node) {
    is_set_ = true;
    replacement_ = node;
  }
  
  
  void Processor::VisitDebuggerStatement(DebuggerStatement* node) {
    replacement_ = node;
  }
  
  void Processor::VisitInitializeClassMembersStatement(
      InitializeClassMembersStatement* node) {
    replacement_ = node;
  }
  
  void Processor::VisitInitializeClassStaticElementsStatement(
      InitializeClassStaticElementsStatement* node) {
    replacement_ = node;
  }
  
  // Expressions are never visited.
  #define DEF_VISIT(type)                                         \
    void Processor::Visit##type(type* expr) { UNREACHABLE(); }
  EXPRESSION_NODE_LIST(DEF_VISIT)
  #undef DEF_VISIT
  
  
  // Declarations are never visited.
  #define DEF_VISIT(type) \
    void Processor::Visit##type(type* expr) { UNREACHABLE(); }
  DECLARATION_NODE_LIST(DEF_VISIT)
  #undef DEF_VISIT
  
  
  // Assumes code has been parsed.  Mutates the AST, so the AST should not
  // continue to be used in the case of failure.
  bool Rewriter::Rewrite(ParseInfo* info) {
    RCS_SCOPE(info->runtime_call_stats(),
              RuntimeCallCounterId::kCompileRewriteReturnResult,
              RuntimeCallStats::kThreadSpecific);
  
    FunctionLiteral* function = info->literal();
    DCHECK_NOT_NULL(function);
    Scope* scope = function->scope();
    DCHECK_NOT_NULL(scope);
    DCHECK_EQ(scope, scope->GetClosureScope());
  
    if (scope->is_repl_mode_scope()) return true;
    if (!(scope->is_script_scope() || scope->is_eval_scope() ||
          scope->is_module_scope())) {
      return true;
    }
  
    ZonePtrList<Statement>* body = function->body();
    return RewriteBody(info, scope, body).has_value();
  }
  
  base::Optional<VariableProxy*> Rewriter::RewriteBody(
      ParseInfo* info, Scope* scope, ZonePtrList<Statement>* body) {
    DisallowGarbageCollection no_gc;
    DisallowHandleAllocation no_handles;
    DisallowHandleDereference no_deref;
  
    DCHECK_IMPLIES(scope->is_module_scope(), !body->is_empty());
    if (!body->is_empty()) {
      Variable* result = scope->AsDeclarationScope()->NewTemporary(
          info->ast_value_factory()->dot_result_string());
      Processor processor(info->stack_limit(), scope->AsDeclarationScope(),
                          result, info->ast_value_factory(), info->zone());
      processor.Process(body);
  
      DCHECK_IMPLIES(scope->is_module_scope(), processor.result_assigned());
      if (processor.result_assigned()) {
        int pos = kNoSourcePosition;
        VariableProxy* result_value =
            processor.factory()->NewVariableProxy(result, pos);
        if (!info->flags().is_repl_mode()) {
          Statement* result_statement =
              processor.factory()->NewReturnStatement(result_value, pos);
          body->Add(result_statement, info->zone());
        }
        return result_value;
      }
  
      if (processor.HasStackOverflow()) {
        info->pending_error_handler()->set_stack_overflow();
        return base::nullopt;
      }
    }
    return nullptr;
  }
  
  }  // namespace internal
  }  // namespace v8
  