Heuristic optimization in expression classifiers

src/parsing/expression-classifier.h

Index: src/parsing/expression-classifier.h
diff --git a/src/parsing/expression-classifier.h b/src/parsing/expression-classifier.h
index 7b31a2143f281149e3751564ee5c0f8adcfbe6e2..1b50204b6e4910c7a3db76a2a74c86bfee120ec4 100644
--- a/src/parsing/expression-classifier.h
+++ b/src/parsing/expression-classifier.h
@@ -394,11 +394,21 @@ class ExpressionClassifier {
       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 &&
@@ -414,7 +424,13 @@ class ExpressionClassifier {
           }
         }
         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;