[antlr-interest] Exception tests eat performance?

[Ric Klaren]

Hi,

On Wed, Jan 07, 2004 at 08:54:41AM -0800, Karl Meissner wrote:
> 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.

You are right, yet the whole ANTLR 2 codegeneration is based around this
(in a sense it's a easy way to deal with some of the errors and with the
backtracking). Also makes ports to languages without exception handling
awkward. Another thing is that it increases codesize for C++ (need RTTI)
embedded software folks would probably love to have that turned off.

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

It's been raised before. Maybe it can be addressed in ANTLR 3 since the
whole codegen gets an overhaul then.

> 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();
> }

That one is particularly ugly indeed. Yet it allows to do some tricks via
overloading of match. Bit of a tradeoff between extendability and
performance.

Cheers,

Ric
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