Initial implementation of optional chaining

Each LHS expression that contains an optional chain of some form is
wrapped in an OptionalChain node. This root node allows us to use a
single jump location for every individual item in the chain,
improving the performance and simplifying the implementation.

Bug: v8:9553
Change-Id: I678563928b2dbfd6200bff55801919d4fd816962
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1723359
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63120}

src/ast/ast-traversal-visitor.h

@@ -382,6 +382,12 @@ void AstTraversalVisitor<Subclass>::VisitThrow(Throw* expr) {
   RECURSE_EXPRESSION(Visit(expr->exception()));
 }
 
+template <class Subclass>
+void AstTraversalVisitor<Subclass>::VisitOptionalChain(OptionalChain* expr) {
+  PROCESS_EXPRESSION(expr);
+  RECURSE_EXPRESSION(Visit(expr->expression()));
+}
+
 template <class Subclass>
 void AstTraversalVisitor<Subclass>::VisitProperty(Property* expr) {
   PROCESS_EXPRESSION(expr);

src/ast/ast.h

@@ -94,6 +94,7 @@ namespace internal {
   V(ImportCallExpression)       \
   V(Literal)                    \
   V(NativeFunctionLiteral)      \
+  V(OptionalChain)              \
   V(Property)                   \
   V(ResolvedProperty)           \
   V(Spread)                     \
@@ -1595,6 +1596,20 @@ class VariableProxy final : public Expression {
   friend base::ThreadedListTraits<VariableProxy>;
 };
 
+// Wraps an optional chain to provide a wrapper for jump labels.
+class OptionalChain final : public Expression {
+ public:
+  Expression* expression() const { return expression_; }
+
+ private:
+  friend class AstNodeFactory;
+
+  explicit OptionalChain(Expression* expression)
+      : Expression(0, kOptionalChain), expression_(expression) {}
+
+  Expression* expression_;
+};
+
 // Assignments to a property will use one of several types of property access.
 // Otherwise, the assignment is to a non-property (a global, a local slot, a
 // parameter slot, or a destructuring pattern).
@@ -1612,6 +1627,10 @@ enum AssignType {
 
 class Property final : public Expression {
  public:
+  bool is_optional_chain_link() const {
+    return IsOptionalChainLinkField::decode(bit_field_);
+  }
+
   bool IsValidReferenceExpression() const { return true; }
 
   Expression* obj() const { return obj_; }
@@ -1653,10 +1672,13 @@ class Property final : public Expression {
  private:
   friend class AstNodeFactory;
 
-  Property(Expression* obj, Expression* key, int pos)
+  Property(Expression* obj, Expression* key, int pos, bool optional_chain)
       : Expression(pos, kProperty), obj_(obj), key_(key) {
+    bit_field_ |= IsOptionalChainLinkField::encode(optional_chain);
   }
 
+  using IsOptionalChainLinkField = Expression::NextBitField<bool, 1>;
+
   Expression* obj_;
   Expression* key_;
 };
@@ -1694,6 +1716,10 @@ class Call final : public Expression {
     return IsTaggedTemplateField::decode(bit_field_);
   }
 
+  bool is_optional_chain_link() const {
+    return IsOptionalChainLinkField::decode(bit_field_);
+  }
+
   bool only_last_arg_is_spread() {
     return !arguments_.is_empty() && arguments_.last()->IsSpread();
   }
@@ -1726,13 +1752,14 @@ class Call final : public Expression {
 
   Call(Zone* zone, Expression* expression,
        const ScopedPtrList<Expression>& arguments, int pos,
-       PossiblyEval possibly_eval)
+       PossiblyEval possibly_eval, bool optional_chain)
       : Expression(pos, kCall),
         expression_(expression),
         arguments_(0, nullptr) {
     bit_field_ |=
         IsPossiblyEvalField::encode(possibly_eval == IS_POSSIBLY_EVAL) |
-        IsTaggedTemplateField::encode(false);
+        IsTaggedTemplateField::encode(false) |
+        IsOptionalChainLinkField::encode(optional_chain);
     arguments.CopyTo(&arguments_, zone);
   }
 
@@ -1743,12 +1770,14 @@ class Call final : public Expression {
         expression_(expression),
         arguments_(0, nullptr) {
     bit_field_ |= IsPossiblyEvalField::encode(false) |
-                  IsTaggedTemplateField::encode(true);
+                  IsTaggedTemplateField::encode(true) |
+                  IsOptionalChainLinkField::encode(false);
     arguments.CopyTo(&arguments_, zone);
   }
 
   using IsPossiblyEvalField = Expression::NextBitField<bool, 1>;
   using IsTaggedTemplateField = IsPossiblyEvalField::Next<bool, 1>;
+  using IsOptionalChainLinkField = IsTaggedTemplateField::Next<bool, 1>;
 
   Expression* expression_;
   ZonePtrList<Expression> arguments_;
@@ -3040,8 +3069,13 @@ class AstNodeFactory final {
     return new (zone_) Variable(variable);
   }
 
-  Property* NewProperty(Expression* obj, Expression* key, int pos) {
-    return new (zone_) Property(obj, key, pos);
+  OptionalChain* NewOptionalChain(Expression* expression) {
+    return new (zone_) OptionalChain(expression);
+  }
+
+  Property* NewProperty(Expression* obj, Expression* key, int pos,
+                        bool optional_chain = false) {
+    return new (zone_) Property(obj, key, pos, optional_chain);
   }
 
   ResolvedProperty* NewResolvedProperty(VariableProxy* obj,
@@ -3052,8 +3086,10 @@ class AstNodeFactory final {
 
   Call* NewCall(Expression* expression,
                 const ScopedPtrList<Expression>& arguments, int pos,
-                Call::PossiblyEval possibly_eval = Call::NOT_EVAL) {
-    return new (zone_) Call(zone_, expression, arguments, pos, possibly_eval);
+                Call::PossiblyEval possibly_eval = Call::NOT_EVAL,
+                bool optional_chain = false) {
+    return new (zone_)
+        Call(zone_, expression, arguments, pos, possibly_eval, optional_chain);
   }
 
   Call* NewTaggedTemplate(Expression* expression,

src/ast/prettyprinter.cc

@@ -342,19 +342,28 @@ void CallPrinter::VisitAwait(Await* node) { Find(node->expression()); }
 
 void CallPrinter::VisitThrow(Throw* node) { Find(node->exception()); }
 
+void CallPrinter::VisitOptionalChain(OptionalChain* node) {
+  Find(node->expression());
+}
 
 void CallPrinter::VisitProperty(Property* node) {
   Expression* key = node->key();
   Literal* literal = key->AsLiteral();
   if (literal != nullptr &&
       literal->BuildValue(isolate_)->IsInternalizedString()) {
     Find(node->obj(), true);
+    if (node->is_optional_chain_link()) {
+      Print("?");
+    }
     Print(".");
     // TODO(adamk): Teach Literal how to print its values without
     // allocating on the heap.
     PrintLiteral(literal->BuildValue(isolate_), false);
   } else {
     Find(node->obj(), true);
+    if (node->is_optional_chain_link()) {
+      Print("?.");
+    }
     Print("[");
     Find(key, true);
     Print("]");
@@ -1272,6 +1281,11 @@ void AstPrinter::VisitThrow(Throw* node) {
   Visit(node->exception());
 }
 
+void AstPrinter::VisitOptionalChain(OptionalChain* node) {
+  IndentedScope indent(this, "OPTIONAL_CHAIN", node->position());
+  Visit(node->expression());
+}
+
 void AstPrinter::VisitProperty(Property* node) {
   EmbeddedVector<char, 128> buf;
   SNPrintF(buf, "PROPERTY");

src/common/message-template.h

@@ -575,7 +575,10 @@ namespace internal {
     "FinalizationGroup.prototype.register: target and holdings must not be "   \
     "same")                                                                    \
   T(WeakRefsWeakRefConstructorTargetMustBeObject,                              \
-    "WeakRef: target must be an object")
+    "WeakRef: target must be an object")                                       \
+  T(OptionalChainingNoNew, "Invalid optional chain from new expression")       \
+  T(OptionalChainingNoSuper, "Invalid optional chain from super property")     \
+  T(OptionalChainingNoTemplate, "Invalid tagged template on optional chain")
 
 enum class MessageTemplate {
 #define TEMPLATE(NAME, STRING) k##NAME,

src/flags/flag-definitions.h

@@ -206,7 +206,8 @@ DEFINE_IMPLICATION(harmony_import_meta, harmony_dynamic_import)
 #define HARMONY_INPROGRESS_BASE(V)                                        \
   V(harmony_private_methods, "harmony private methods in class literals") \
   V(harmony_regexp_sequence, "RegExp Unicode sequence properties")        \
-  V(harmony_weak_refs, "harmony weak references")
+  V(harmony_weak_refs, "harmony weak references")                         \
+  V(harmony_optional_chaining, "harmony optional chaining syntax")
 
 #ifdef V8_INTL_SUPPORT
 #define HARMONY_INPROGRESS(V)                              \

src/init/bootstrapper.cc

@@ -4260,6 +4260,7 @@ EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_private_methods)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_dynamic_import)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_import_meta)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_regexp_sequence)
+EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_optional_chaining)
 
 #ifdef V8_INTL_SUPPORT
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_intl_add_calendar_numbering_system)

src/interpreter/bytecode-generator.cc

@@ -915,6 +915,27 @@ class BytecodeGenerator::IteratorRecord final {
   Register next_;
 };
 
+class BytecodeGenerator::OptionalChainNullLabelScope final {
+ public:
+  explicit OptionalChainNullLabelScope(BytecodeGenerator* bytecode_generator)
+      : bytecode_generator_(bytecode_generator),
+        labels_(bytecode_generator->zone()) {
+    prev_ = bytecode_generator_->optional_chaining_null_labels_;
+    bytecode_generator_->optional_chaining_null_labels_ = &labels_;
+  }
+
+  ~OptionalChainNullLabelScope() {
+    bytecode_generator_->optional_chaining_null_labels_ = prev_;
+  }
+
+  BytecodeLabels* labels() { return &labels_; }
+
+ private:
+  BytecodeGenerator* bytecode_generator_;
+  BytecodeLabels labels_;
+  BytecodeLabels* prev_;
+};
+
 namespace {
 
 // A map from property names to getter/setter pairs allocated in the zone that
@@ -994,6 +1015,7 @@ BytecodeGenerator::BytecodeGenerator(
       execution_context_(nullptr),
       execution_result_(nullptr),
       incoming_new_target_or_generator_(),
+      optional_chaining_null_labels_(nullptr),
       dummy_feedback_slot_(feedback_spec(), FeedbackSlotKind::kCompareOp),
       generator_jump_table_(nullptr),
       suspend_count_(0),
@@ -4318,7 +4340,17 @@ void BytecodeGenerator::VisitThrow(Throw* expr) {
 }
 
 void BytecodeGenerator::VisitPropertyLoad(Register obj, Property* property) {
+  if (property->is_optional_chain_link()) {
+    DCHECK_NOT_NULL(optional_chaining_null_labels_);
+    // TODO(ignition): Add a single opcode for JumpIfNullOrUndefined
+    builder()
+        ->LoadAccumulatorWithRegister(obj)
+        .JumpIfUndefined(optional_chaining_null_labels_->New())
+        .JumpIfNull(optional_chaining_null_labels_->New());
+  }
+
   AssignType property_kind = Property::GetAssignType(property);
+
   switch (property_kind) {
     case NON_PROPERTY:
       UNREACHABLE();
@@ -4416,6 +4448,16 @@ void BytecodeGenerator::VisitKeyedSuperPropertyLoad(Property* property,
   }
 }
 
+void BytecodeGenerator::VisitOptionalChain(OptionalChain* expr) {
+  BytecodeLabel done;
+  OptionalChainNullLabelScope label_scope(this);
+  VisitForAccumulatorValue(expr->expression());
+  builder()->Jump(&done);
+  label_scope.labels()->Bind(builder());
+  builder()->LoadUndefined();
+  builder()->Bind(&done);
+}
+
 void BytecodeGenerator::VisitProperty(Property* expr) {
   AssignType property_kind = Property::GetAssignType(expr);
   if (property_kind != NAMED_SUPER_PROPERTY &&
@@ -4549,6 +4591,15 @@ void BytecodeGenerator::VisitCall(Call* expr) {
       UNREACHABLE();
   }
 
+  if (expr->is_optional_chain_link()) {
+    DCHECK_NOT_NULL(optional_chaining_null_labels_);
+    // TODO(ignition): Add a single opcode for JumpIfNullOrUndefined
+    builder()
+        ->LoadAccumulatorWithRegister(callee)
+        .JumpIfUndefined(optional_chaining_null_labels_->New())
+        .JumpIfNull(optional_chaining_null_labels_->New());
+  }
+
   // Evaluate all arguments to the function call and store in sequential args
   // registers.
   VisitArguments(expr->arguments(), &args);
@@ -4810,6 +4861,28 @@ void BytecodeGenerator::VisitDelete(UnaryOperation* unary) {
     Register object = VisitForRegisterValue(property->obj());
     VisitForAccumulatorValue(property->key());
     builder()->Delete(object, language_mode());
+  } else if (expr->IsOptionalChain()) {
+    Expression* expr_inner = expr->AsOptionalChain()->expression();
+    if (expr_inner->IsProperty()) {
+      Property* property = expr_inner->AsProperty();
+      DCHECK(!property->IsPrivateReference());
+      BytecodeLabel done;
+      OptionalChainNullLabelScope label_scope(this);
+      VisitForAccumulatorValue(property->obj());
+      builder()
+          ->JumpIfUndefined(label_scope.labels()->New())
+          .JumpIfNull(label_scope.labels()->New());
+      Register object = register_allocator()->NewRegister();
+      builder()->StoreAccumulatorInRegister(object);
+      VisitForAccumulatorValue(property->key());
+      builder()->Delete(object, language_mode()).Jump(&done);
+      label_scope.labels()->Bind(builder());
+      builder()->LoadTrue();
+      builder()->Bind(&done);
+    } else {
+      VisitForEffect(expr);
+      builder()->LoadTrue();
+    }
   } else if (expr->IsVariableProxy() &&
              !expr->AsVariableProxy()->is_new_target()) {
     // Delete of an unqualified identifier is allowed in sloppy mode but is

src/interpreter/bytecode-generator.h

@@ -66,6 +66,7 @@ class BytecodeGenerator final : public AstVisitor<BytecodeGenerator> {
   class AccumulatorPreservingScope;
   class TestResultScope;
   class ValueResultScope;
+  class OptionalChainNullLabelScope;
 
   using ToBooleanMode = BytecodeArrayBuilder::ToBooleanMode;
 
@@ -491,6 +492,8 @@ class BytecodeGenerator final : public AstVisitor<BytecodeGenerator> {
 
   Register incoming_new_target_or_generator_;
 
+  BytecodeLabels* optional_chaining_null_labels_;
+
   // Dummy feedback slot for compare operations, where we don't care about
   // feedback
   SharedFeedbackSlot dummy_feedback_slot_;

src/parsing/parse-info.cc

@@ -62,6 +62,7 @@ ParseInfo::ParseInfo(Isolate* isolate, AccountingAllocator* zone_allocator)
   set_allow_natives_syntax(FLAG_allow_natives_syntax);
   set_allow_harmony_dynamic_import(FLAG_harmony_dynamic_import);
   set_allow_harmony_import_meta(FLAG_harmony_import_meta);
+  set_allow_harmony_optional_chaining(FLAG_harmony_optional_chaining);
   set_allow_harmony_private_methods(FLAG_harmony_private_methods);
 }
 

src/parsing/parse-info.h

@@ -105,6 +105,8 @@ class V8_EXPORT_PRIVATE ParseInfo {
                 set_allow_harmony_dynamic_import)
   FLAG_ACCESSOR(kAllowHarmonyImportMeta, allow_harmony_import_meta,
                 set_allow_harmony_import_meta)
+  FLAG_ACCESSOR(kAllowHarmonyOptionalChaining, allow_harmony_optional_chaining,
+                set_allow_harmony_optional_chaining)
   FLAG_ACCESSOR(kAllowHarmonyPrivateMethods, allow_harmony_private_methods,
                 set_allow_harmony_private_methods)
   FLAG_ACCESSOR(kIsOneshotIIFE, is_oneshot_iife, set_is_oneshot_iife)
@@ -304,10 +306,11 @@ class V8_EXPORT_PRIVATE ParseInfo {
     kAllowHarmonyStaticFields = 1 << 24,
     kAllowHarmonyDynamicImport = 1 << 25,
     kAllowHarmonyImportMeta = 1 << 26,
-    kAllowHarmonyPrivateFields = 1 << 27,
-    kAllowHarmonyPrivateMethods = 1 << 28,
-    kIsOneshotIIFE = 1 << 29,
-    kCollectSourcePositions = 1 << 30,
+    kAllowHarmonyOptionalChaining = 1 << 27,
+    kAllowHarmonyPrivateFields = 1 << 28,
+    kAllowHarmonyPrivateMethods = 1 << 29,
+    kIsOneshotIIFE = 1 << 30,
+    kCollectSourcePositions = 1 << 31,
   };
 
   //------------- Inputs to parsing and scope analysis -----------------------