Only rewrite all statements in a block if we're in a breakable scope

src/parsing/rewriter.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/parsing/rewriter.h"
 
 #include "src/ast/ast.h"
 #include "src/ast/scopes.h"
 #include "src/parsing/parse-info.h"
 #include "src/parsing/parser.h"
 
 namespace v8 {
 namespace internal {
 
 class Processor final : public AstVisitor<Processor> {
  public:
   Processor(Isolate* isolate, DeclarationScope* closure_scope, Variable* result,
             AstValueFactory* ast_value_factory)
       : result_(result),
         result_assigned_(false),
         replacement_(nullptr),
         is_set_(false),
+        breakable_(false),
         zone_(ast_value_factory->zone()),
         closure_scope_(closure_scope),
         factory_(ast_value_factory) {
     DCHECK_EQ(closure_scope, closure_scope->GetClosureScope());
     InitializeAstVisitor(isolate);
   }
 
   Processor(Parser* parser, DeclarationScope* closure_scope, Variable* result,
             AstValueFactory* ast_value_factory)
       : result_(result),
         result_assigned_(false),
         replacement_(nullptr),
         is_set_(false),
+        breakable_(false),
         zone_(ast_value_factory->zone()),
         closure_scope_(closure_scope),
         factory_(ast_value_factory) {
     DCHECK_EQ(closure_scope, closure_scope->GetClosureScope());
     InitializeAstVisitor(parser->stack_limit());
   }
 
   void Process(ZoneList<Statement*>* statements);
   bool result_assigned() const { return result_assigned_; }
 
@@ skipping 24 matching lines @@
   // 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_;
 
   // 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_;
+
+  class 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();
 };
 
 
 Statement* Processor::AssignUndefinedBefore(Statement* s) {
   Expression* result_proxy = factory()->NewVariableProxy(result_);
   Expression* undef = factory()->NewUndefinedLiteral(kNoSourcePosition);
   Expression* assignment = factory()->NewAssignment(Token::ASSIGN, result_proxy,
                                                     undef, kNoSourcePosition);
   Block* b = factory()->NewBlock(NULL, 2, false, kNoSourcePosition);
   b->statements()->Add(
       factory()->NewExpressionStatement(assignment, kNoSourcePosition), zone());
   b->statements()->Add(s, zone());
   return b;
 }
 
 
 void Processor::Process(ZoneList<Statement*>* statements) {
-  for (int i = statements->length() - 1; i >= 0; --i) {
+  // 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()) Process(node->statements());
+  if (!node->ignore_completion_value()) {
+    BreakableScope scope(this, node->labels() != nullptr);
+    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;
   }
@@ skipping 14 matching lines @@
   replacement_ = node;
 
   if (!is_set_) {
     is_set_ = true;
     replacement_ = AssignUndefinedBefore(node);
   }
 }
 
 
 void Processor::VisitIterationStatement(IterationStatement* node) {
+  BreakableScope scope(this);
   // Rewrite the body.
   bool set_after = is_set_;
   is_set_ = false;  // We are in a loop, so we can't rely on [set_after].
   Visit(node->body());
   node->set_body(replacement_);
   is_set_ = is_set_ && set_after;
   replacement_ = node;
 
   if (!is_set_) {
     is_set_ = true;
@@ skipping 76 matching lines @@
   replacement_ = node;
 
   if (!is_set_) {
     is_set_ = true;
     replacement_ = AssignUndefinedBefore(node);
   }
 }
 
 
 void Processor::VisitSwitchStatement(SwitchStatement* node) {
+  BreakableScope scope(this);
   // Rewrite statements in all case clauses (in reverse order).
   ZoneList<CaseClause*>* clauses = node->cases();
   bool set_after = is_set_;
   for (int i = clauses->length() - 1; i >= 0; --i) {
     CaseClause* clause = clauses->at(i);
     Process(clause->statements());
   }
   is_set_ = is_set_ && set_after;
   replacement_ = node;
 
@@ skipping 125 matching lines @@
           processor.SetResult(undef), expr->position());
       body->Add(completion, factory->zone());
     }
   }
   return true;
 }
 
 
 }  // namespace internal
 }  // namespace v8