Class fields, part 3 (backends)

src/interpreter/bytecode-generator.cc

 // Copyright 2015 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/interpreter/bytecode-generator.h"
 
 #include "src/ast/compile-time-value.h"
 #include "src/ast/scopes.h"
 #include "src/code-stubs.h"
 #include "src/compilation-info.h"
@@ skipping 1474 matching lines @@
   // Load the "prototype" from the constructor.
   register_allocator()->PrepareForConsecutiveAllocations(2);
   Register literal = register_allocator()->NextConsecutiveRegister();
   Register prototype = register_allocator()->NextConsecutiveRegister();
   FeedbackVectorSlot slot = expr->PrototypeSlot();
   builder()
       ->StoreAccumulatorInRegister(literal)
       .LoadNamedProperty(literal, prototype_string(), feedback_index(slot))
       .StoreAccumulatorInRegister(prototype);
 
+  // The below proxy exists to allow static field initializers to have the
+  // correct home object and receiver. It's only necessary if the initializers
+  // are called as a part of class definition instead of immediately after it,
+  // in which case they could simply refer to the class just constructed. The
+  // latter is actually what's currently specified, and this will need to be
+  // changed if that behavior is settled upon. See also
+  // https://github.com/tc39/proposal-class-public-fields/issues/50
+  VariableProxy* static_initializer_proxy = expr->static_initializer_proxy();
+  if (static_initializer_proxy != nullptr) {
+    Variable* variable = static_initializer_proxy->var();
+    builder()->LoadAccumulatorWithRegister(literal);
+    VisitVariableAssignment(variable, Token::INIT,
+                            FeedbackVectorSlot::Invalid());
+  }
+
   VisitClassLiteralProperties(expr, literal, prototype);
   builder()->CallRuntime(Runtime::kToFastProperties, literal, 1);
   // Assign to class variable.
   if (expr->class_variable_proxy() != nullptr) {
     Variable* var = expr->class_variable_proxy()->var();
     FeedbackVectorSlot slot = expr->NeedsProxySlot()
                                   ? expr->ProxySlot()
                                   : FeedbackVectorSlot::Invalid();
     VisitVariableAssignment(var, Token::INIT, slot);
   }
@@ skipping 34 matching lines @@
   Register attr = register_allocator()->NextConsecutiveRegister();
   Register set_function_name = register_allocator()->NextConsecutiveRegister();
 
   bool attr_assigned = false;
   Register old_receiver = Register::invalid_value();
 
   // Create nodes to store method values into the literal.
   for (int i = 0; i < expr->properties()->length(); i++) {
     ClassLiteral::Property* property = expr->properties()->at(i);
 
+    if (property->kind() == ClassLiteral::Property::FIELD &&
+        !property->is_static()) {
+      // Non-static properties produced by the parser have as their 'key' an
+      // expression producing their name and as their 'value' a variable which
+      // is refered to by the synthetic initializer function in order to
+      // determine the name during class instantiation. This is necessary
+      // because computed names must only be evaluated once, at class definition
+      // time.
+      // That is, code which looks like `class C { [f()] = 1; }` is desugared
+      // into something like
+      // class C { constructor(){ this.[.class-field-0-name] = 1; } };
+      // let .class-field-0-name = f();
+      // except that the assignment to .class-field-name-0 occurs interleaved
+      // with the rest of the class body; it is performed by the block in which
+      // this comment appears.
+      DCHECK(property->value()->IsVariableProxy());
+      Variable* variable = property->value()->AsVariableProxy()->var();
+      VisitForAccumulatorValue(property->key());
+      VisitVariableAssignment(variable, Token::INIT,
+                              FeedbackVectorSlot::Invalid());
+      continue;

[rmcilroy, 2016/09/16 09:33:35]

Drive-by - could you add an example of class fields to test-bytecode-generator
so that we have expected bytecode for these patterns.

[bakkot, 2016/09/16 18:59:53]

Done.

[rmcilroy, 2016/09/19 10:06:08]

Thanks!

+    }
+
     // Set-up receiver.
     Register new_receiver = property->is_static() ? literal : prototype;
     if (new_receiver != old_receiver) {
       builder()->MoveRegister(new_receiver, receiver);
       old_receiver = new_receiver;
     }
 
     VisitForAccumulatorValue(property->key());
     builder()->ConvertAccumulatorToName(key);
     // The static prototype property is read only. We handle the non computed
@@ skipping 28 matching lines @@
         builder()->CallRuntime(Runtime::kDefineGetterPropertyUnchecked,
                                receiver, 4);
         break;
       }
       case ClassLiteral::Property::SETTER: {
         builder()->CallRuntime(Runtime::kDefineSetterPropertyUnchecked,
                                receiver, 4);
         break;
       }
       case ClassLiteral::Property::FIELD: {
-        UNREACHABLE();
+        DCHECK(property->is_static());
+        builder()
+            ->LoadLiteral(Smi::FromInt(property->NeedsSetFunctionName()))
+            .StoreAccumulatorInRegister(set_function_name);
+        builder()->CallRuntime(Runtime::kDefineDataPropertyInLiteral, receiver,
+                               5);
         break;
       }
     }
   }
 }
 
 void BytecodeGenerator::VisitClassLiteralStaticPrototypeWithComputedName(
     Register key) {
   BytecodeLabel done;
   builder()
@@ skipping 1783 matching lines @@
   return execution_context()->scope()->language_mode();
 }
 
 int BytecodeGenerator::feedback_index(FeedbackVectorSlot slot) const {
   return TypeFeedbackVector::GetIndex(slot);
 }
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8