[interpreter] Don't keep postfix result if only visited for effect

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/scopes.h"
 #include "src/code-stubs.h"
 #include "src/compiler.h"
 #include "src/interpreter/bytecode-flags.h"
@@ skipping 2764 matching lines @@
   execution_result()->SetResultInAccumulator();
 }
 
 void BytecodeGenerator::VisitCountOperation(CountOperation* expr) {
   DCHECK(expr->expression()->IsValidReferenceExpressionOrThis());
 
   // Left-hand side can only be a property, a global or a variable slot.
   Property* property = expr->expression()->AsProperty();
   LhsKind assign_type = Property::GetAssignType(property);
 
-  // TODO(rmcilroy): Set is_postfix to false if visiting for effect.
-  bool is_postfix = expr->is_postfix();
+  bool is_postfix = expr->is_postfix() && !execution_result()->IsEffect();
 
   // Evaluate LHS expression and get old value.
   Register object, home_object, key, old_value, value;
   Handle<String> name;
   switch (assign_type) {
     case VARIABLE: {
       VariableProxy* proxy = expr->expression()->AsVariableProxy();
       VisitVariableLoadForAccumulatorValue(proxy->var(),
                                            proxy->VariableFeedbackSlot());
       break;
@@ skipping 443 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