Speed up parsing w/ grammar shortcut.

src/parsing/parser-base.h

 // Copyright 2012 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_PARSER_BASE_H
 #define V8_PARSING_PARSER_BASE_H
 
 #include "src/ast/scopes.h"
 #include "src/bailout-reason.h"
 #include "src/base/hashmap.h"
@@ skipping 1109 matching lines @@
 
   void ParseFormalParameter(FormalParametersT* parameters,
                             ExpressionClassifier* classifier, bool* ok);
   void ParseFormalParameterList(FormalParametersT* parameters,
                                 ExpressionClassifier* classifier, bool* ok);
   void CheckArityRestrictions(int param_count, FunctionKind function_type,
                               bool has_rest, int formals_start_pos,
                               int formals_end_pos, bool* ok);
 
   bool IsNextLetKeyword();
+  bool IsTrivialExpression();
 
   // Checks if the expression is a valid reference expression (e.g., on the
   // left-hand side of assignments). Although ruled out by ECMA as early errors,
   // we allow calls for web compatibility and rewrite them to a runtime throw.
   ExpressionT CheckAndRewriteReferenceExpression(
       ExpressionT expression, int beg_pos, int end_pos,
       MessageTemplate::Template message, bool* ok);
   ExpressionT CheckAndRewriteReferenceExpression(
       ExpressionT expression, int beg_pos, int end_pos,
       MessageTemplate::Template message, ParseErrorType type, bool* ok);
@@ skipping 406 matching lines @@
   switch (peek()) {
     case Token::THIS: {
       BindingPatternUnexpectedToken(classifier);
       Consume(Token::THIS);
       return this->ThisExpression(factory(), beg_pos);
     }
 
     case Token::NULL_LITERAL:
     case Token::TRUE_LITERAL:
     case Token::FALSE_LITERAL:
-      BindingPatternUnexpectedToken(classifier);
-      return this->ExpressionFromLiteral(Next(), beg_pos, scanner(), factory());
     case Token::SMI:
     case Token::NUMBER:
       BindingPatternUnexpectedToken(classifier);
       return this->ExpressionFromLiteral(Next(), beg_pos, scanner(), factory());
 
     case Token::ASYNC:
       if (allow_harmony_async_await() &&
           !scanner()->HasAnyLineTerminatorAfterNext() &&
           PeekAhead() == Token::FUNCTION) {
         Consume(Token::ASYNC);
@@ skipping 732 matching lines @@
   Scope::Snapshot scope_snapshot(scope());
 
   bool is_async = allow_harmony_async_await() && peek() == Token::ASYNC &&
                   !scanner()->HasAnyLineTerminatorAfterNext() &&
                   IsValidArrowFormalParametersStart(PeekAhead());
 
   bool parenthesized_formals = peek() == Token::LPAREN;
   if (!is_async && !parenthesized_formals) {
     ArrowFormalParametersUnexpectedToken(&arrow_formals_classifier);
   }
-  ExpressionT expression = this->ParseConditionalExpression(
-      accept_IN, &arrow_formals_classifier, CHECK_OK);
+
+  // Parse a simple, faster sub-grammar (primary expression) if it's evident
+  // that we have only a trivial expression to parse.
+  ExpressionT expression = IsTrivialExpression()
+                               ? this->ParsePrimaryExpression(
+                                     &arrow_formals_classifier, &is_async, ok)
+                               : this->ParseConditionalExpression(
+                                     accept_IN, &arrow_formals_classifier, ok);
+  if (!*ok) return this->EmptyExpression();

[adamk, 2016/08/11 17:11:05]

I'd rather you use an if/else above so you can keep using CHECK_OK, these manual
 if (!*ok) checks scare me.

[vogelheim, 2016/08/11 17:44:08]

I'm more scared by CHECK_OK-makes-control-flow-look-like-a-parameter, but ...
Done. :)

[adamk, 2016/08/11 17:50:15]

Heh, to each their own I suppose. But CHECK_OK is clearly the consistent style,
and nikolaos@ recently tweaked some macros to make it even more consistent, so
I'd rather stay on that road at least for now.

 
   if (is_async && peek_any_identifier() && PeekAhead() == Token::ARROW) {
     // async Identifier => AsyncConciseBody
     IdentifierT name =
         ParseAndClassifyIdentifier(&arrow_formals_classifier, CHECK_OK);
     expression = this->ExpressionFromIdentifier(
         name, position(), scanner()->location().end_pos, factory());
   }
 
   if (peek() == Token::ARROW) {
@@ skipping 1036 matching lines @@
     case Token::ASYNC:
       return true;
     case Token::FUTURE_STRICT_RESERVED_WORD:
       return is_sloppy(language_mode());
     default:
       return false;
   }
 }
 
 template <class Traits>
+bool ParserBase<Traits>::IsTrivialExpression() {
+  Token::Value peek_token = peek();
+  if (peek_token == Token::SMI || peek_token == Token::NUMBER ||
+      peek_token == Token::NULL_LITERAL || peek_token == Token::TRUE_LITERAL ||
+      peek_token == Token::FALSE_LITERAL || peek_token == Token::STRING ||
+      peek_token == Token::IDENTIFIER || peek_token == Token::THIS) {
+    // PeekAhead() is expensive & may not always be called, so we only call it
+    // after checking peek().
+    Token::Value peek_ahead = PeekAhead();
+    if (peek_ahead == Token::COMMA || peek_ahead == Token::RPAREN ||
+        peek_ahead == Token::SEMICOLON || peek_ahead == Token::RBRACK) {
+      return true;
+    }
+  }
+  return false;
+}
+
+template <class Traits>
 typename ParserBase<Traits>::ExpressionT
 ParserBase<Traits>::ParseArrowFunctionLiteral(
     bool accept_IN, const FormalParametersT& formal_parameters, bool is_async,
     const ExpressionClassifier& formals_classifier, bool* ok) {
   if (peek() == Token::ARROW && scanner_->HasAnyLineTerminatorBeforeNext()) {
     // ASI inserts `;` after arrow parameters if a line terminator is found.
     // `=> ...` is never a valid expression, so report as syntax error.
     // If next token is not `=>`, it's a syntax error anyways.
     ReportUnexpectedTokenAt(scanner_->peek_location(), Token::ARROW);
     *ok = false;
@@ skipping 315 matching lines @@
     has_seen_constructor_ = true;
     return;
   }
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_PARSING_PARSER_BASE_H