[antlr-interest] Re: ANTLR Java Code Generation

tom at psy.unsw.edu.au tom at psy.unsw.edu.au
Tue Nov 13 15:47:44 PST 2001

Will there be a performance gain? There will be a bytecode size 
decrease, which will decrease memory usage on platforms like Unix 
which memory-map classes. But won't there be a performance decrease 
because instead of a nice super clean:
1. create Array long _tokenSet_0_data_[] with size n
2. put -549755813896L at 0
3. put -268435457L at 1
4. put -1L at 2
n. put -1L at n
with O[n] time you'll have:
1. create Array long _tokenSet_0_data_[] with size n
2. put -549755813896L at 0
6. put -268435457L at 1
10. put -1L at 2
n*4. put -1L at n
With all the extra loop overhead. Thus your time is now O[x*n] (where 
x is number of instructions to do for each entry). Is this actually 
going to be a gain? You still have to add the same number of entries. 
Just trades off class size vs. speed doesn't it. And I would have 
thought for many cases speed was more important than class size. You 
only store\load the class once, you have to do the BitSet creation 
every parse (if you did 2 passes with one init, then you'd double the 
amount of computation (and thus overhead) but same memory overhead).

And surely the JDK's gonna be better at optimizing the initializer 
cause it knows everything that's going on, where as the second one 
must deal with any side effects.

Still might be worth offering it as an option, but I would have 
thought it would be best to check performance before using only this 

It doesn't cut down the intialization does it? Still got to 
initialize every value, in fact makes it worse because it's x times 
more ops. Just cuts down class size.

--- In antlr-interest at y..., Christian Ernst <christian.ernst at p...> 
> Hy !
> Stdiobe wrote:
> > Seems to me like a good suggestion. Any drawbacks performance-
> We didn't do any performance measures.
> But there should be a performance gain.
> For example for the Java Lexer 1.3.
> It cuts down the needed initializing of 6 Bitsets
> with about 11.000 Long Values
> to only about 4.000 Long Values
> mfg
> Christian


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