[antlr-interest] Exception tests eat performance?

[Karl Meissner]

It seems that antlr uses exceptions in a lot of the generated parsers where simple test will do. 
Everything that I have read is that an exceptions have a very high overhead and it is undesirable 
to  generate them in a tight loop where they happen a lot. 

Exceptions are used so extensivly it would require a big rewrite to change but I wanted to 
raise it as an issue

A very common piece of code in a parser is 

try {
  {
    match( SYM1 );
    match( SYM2 );
    match( SYM3 );
 }
} catch (RecognitionException)	{
  synPredMatched84 = false;
}

where match throws an exception if the next token is not the parameter. 
Since match is just
public virtual void  match(int t)
{
    if (LA(1) != t)
	throw new MismatchedTokenException(tokenNames, LT(1), t, false, getFilename());
    else
	consume();
}


Using if based tests would be faster I think...

Something like this

bool Rule10() { 
  if( !Rule22() ) goto 	_RecognitionException_123:

  currentSym = SYM1; if ( LA( 1 ) != currentSym ) goto _RecognitionException_123; consume(); 
  currentSym = SYM2; if ( LA( 1 ) != currentSym ) goto _RecognitionException_123; consume(); 
  currentSym = SYM3; if ( LA( 1 ) != currentSym ) goto _RecognitionException_123; consume(); 
    
  return true; 

  _RecognitionException_123:
    if (0 == inputState.guessing)
    {
      reportError(currentSym);
      consume();
      consumeUntil(tokenSet_27_);

      return true;  //attempt to recover in the calling rule
    }
    else //guessing
    {
	return false; 
    }
  }

I probably missed some the the code paths with predicate guessing and error handling but 
I still think you can get the same behavior as the exception with a combination of if, goto and
storing information into stack.   

And get a performance improvement....

Karl 
















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