Heuristic optimization in expression classifiers

src/parsing/expression-classifier.h

 // 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.
 
 #ifndef V8_PARSING_EXPRESSION_CLASSIFIER_H
 #define V8_PARSING_EXPRESSION_CLASSIFIER_H
 
 #include "src/messages.h"
 #include "src/parsing/scanner.h"
 #include "src/parsing/token.h"
@@ skipping 376 matching lines @@
         // parameter.
         function_properties_ |= inner->function_properties_;
 
         if (!inner->is_valid_binding_pattern())
           errors |= ArrowFormalParametersProduction;
       }
 
       if (errors != 0) {
         invalid_productions_ |= errors;
         int arrow_index = inner->reported_errors_end_;
+        int hole_index = inner->reported_errors_end_;
         for (int i = inner->reported_errors_begin_;
              i < inner->reported_errors_end_; i++) {
-          if (reported_errors_->at(i).kind == kUnusedError ||
-              reported_errors_->at(i).kind == kArrowFormalParametersProduction)
+          if (reported_errors_->at(i).kind == kUnusedError) continue;
+          if (reported_errors_->at(i).kind ==
+              kArrowFormalParametersProduction) {
+            // Accumulate an ArrowFormalParametersProduction error as a hole in
+            // the list, to avoid having to shift all the remaining errors.
+            // This is an optimization for a frequently occurring case.
+            if (reported_errors_end_ == i) {
+              hole_index = reported_errors_end_++;
+              reported_errors_->at(i).kind = kUnusedError;
+            }
             continue;
+          }
           if (errors & (1 << reported_errors_->at(i).kind))
             Move(i);
           if (reported_errors_->at(i).kind == kBindingPatternProduction &&
               errors & ArrowFormalParametersProduction) {
             if (reported_errors_end_ <= i) {
               Move(i);
               reported_errors_->at(reported_errors_end_-1).kind =
                   kArrowFormalParametersProduction;
             } else {
               DCHECK_EQ(reported_errors_end_, i+1);
               arrow_index = i;
             }
           }
         }
         if (arrow_index < inner->reported_errors_end_) {
-          if (reported_errors_end_ < inner->reported_errors_end_) {
+          if (hole_index < inner->reported_errors_end_) {
+            // Use a hole in the list (if any exists) to store the additional
+            // ArrowFormalParametersProduction error.
+            Replace(hole_index, arrow_index);
+            reported_errors_->at(hole_index).kind =
+                kArrowFormalParametersProduction;
+          } else if (reported_errors_end_ < inner->reported_errors_end_) {
             Replace(reported_errors_end_, arrow_index);
             reported_errors_->at(reported_errors_end_++).kind =
                 kArrowFormalParametersProduction;
           } else {
             Add(reported_errors_->at(arrow_index));
             reported_errors_->at(reported_errors_end_-1).kind =
                 kArrowFormalParametersProduction;
           }
         }
       }
@@ skipping 69 matching lines @@
 };
 
 
 #undef ERROR_CODES
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_PARSING_EXPRESSION_CLASSIFIER_H