Experimental parser: elide cursor reads in some cases

tools/lexer_generator/code_generator.py

 # Copyright 2013 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 81 matching lines @@
         class_keys += 1
       elif t == 'PRIMARY_RANGE':
         distinct_keys += r[1] - r[0] + 1
         ranges += 1
       else:
         raise Exception()
     return {
       'node_number' : None,
       'original_node_number' : state.node_number(),
       'transitions' : transitions,
+      'can_elide_read' : len(transitions) == 0,
       'switch_transitions' : [],
       'deferred_transitions' : [],
       'long_char_transitions' : [],
       'disjoint_keys' : disjoint_keys,
       'inline' : None,
       'action' : action,
       'entry_action' : entry_action,
       'match_action' : match_action,
       'class_keys' : class_keys,
       'distinct_keys' : distinct_keys,
@@ skipping 167 matching lines @@
       'goto_start'])
     for state in states:
       if not state['match_action']:
         continue
       if state['match_action'][0] in mapped_actions:
         value = state['match_action'][1]
         value = tuple(list(value[:-1]) + [goto_map[value[-1]]])
         state['match_action'] = (state['match_action'][0], value)
         if state['match_action'][0] != 'goto_start':
           states[value[-1]]['entry_points']['after_entry_code'] = True
+          state['can_elide_read'] = False
+        else:
+          states[value[-1]]['can_elide_read'] = False
 
   @staticmethod
   def __mark_entry_points(dfa_states):
     # inlined states can write no labels
     for state in dfa_states:
       entry_points = state['entry_points']
       if state['inline']:
         for k in entry_points.keys():
           entry_points[k] = False
 
@@ skipping 35 matching lines @@
     char_type = char_types[encoding.name()]
 
     return template.render(
       start_node_number = 0,
       debug_print = self.__debug_print,
       default_action = default_action,
       dfa_states = dfa_states,
       encoding = encoding.name(),
       char_type = char_type,
       upper_bound = encoding.primary_range()[1])