Oct 07

Rewriting JavaScript

I’m having a very painful time rewriting Mozilla source code to switch it to garbage collection. So I took a little break to think about Myk’s JavaScript rewriting idea.Then I found excellent info on parsing JS with SpiderMonkey. The downside to SpiderMonkey is that it is in C and thus hard to reuse, the upside is that will be able to parse all valid JavaScript.

I think I shall find some free time to prototype a little tool to take a .js file and produce a JSON representation of it such that one could write transformation passes in JavaScript. This will be useful as it will complete DeHydra by enabling it to process JS in addition to C++ and it will save a lot of time for various JavaScript refactorings.

I expect developing a JS refactoring tool to be relatively trivial compared to refactoring C++ as there is no source-code mangling going on due to lack of preprocessing (but JS is dynamic and can be embedded in various document types, so that could complicate things). Perhaps it would even be of use to mozpad people.

Looking forward to playing more with this idea.

Oct 07

Rewriting Tools for Mozilla 2: Moving Forward as Planned

In the Beginning There Was a Void

Approximately a year ago, Brendan discussed with me the crazy possibility of rewriting most of the Mozilla code automatically to modernize the codebase. The benefits were huge. Gecko would use the C++ standard library to improve code readability and reducing size, XPCOM would be ripped out of the core to improve performance and decrease footprint, etc.

It seemed like a good idea, but in reality no other giant C++ project has attempted this before so we were not sure of how realistic it was. I spent a year in a lonely corner of Mozilla trying to materialize the idea.

Brendan & Graydon pointed me to elsa, the C++ parser that supposedly could parse Mozilla. However, it turned out that it was only able to parse an old version of Mozilla and rejected the new source. One of the elsa maintainers even tried to convince us to it was not designed for source-to-source transformations and wouldn’t work that way.

After I patched up elsa and started devising ways to use it for source rewriting I ran into more pain. After a few false starts, I realized that C++ in Mozilla is actually a mix of CPP and C++ and one can not rewrite C++ without dealing with the mess that is macro expansion. MCPP was pointed out to me as a good starting point for hacking on a preprocessor. So I designed an inline log for macro expansion. To my surprise the maintainer of MCPP, Kiyoshi MATSUI, volunteered to implement the spec and thus saved me from a world of pain. (For which I am eternally grateful as I can’t imagine a more depressing pastime than working on the root of all evil: the C preprocessor).

In parallel with Kiyoshi’s work I modified elkhound & elsa to make the C++ parser a lot more suitable for source transformations. I learned about LR & GLR parsing and confirmed my suspicion that I don’t want to write parser generators for a living.

Happy Conclusion

All this work finally got us what we discussed last September: a framework for doing lots of boring code rewrites.

The first big Moz2 task is switching from reference counting to garbage collection. Today, garburator produced a gigantic patch for subset of the content/ module and all of the affected files compiled. Hopefully next week I’ll have a multi-megabyte patch for the whole of Mozilla that compiles and possibly runs.

Oct 07

Multiple Degrees of Correctness


The trouble with prcheck and the automated prbool validation is that one can’t attach the giant patch it produces to bugzilla and expect it to get committed. So I am spending this week combing through prcheck output  and patch-bombing bugzilla with per-module patches.

I find going through the errors manually to be a lot of fun than I expected. I am finding types of errors that I was not considering when I was writing prcheck. For example, I expected the biggest gains to come at runtime from making all prbool values 0/1, but it seems that most of the cool errors are due to PRBool & PRInt32 resolving to the same type. That results in code mayhem ranging from wrong method overloads being called to method signatures claiming to return PRBool where method bodies act like the function returns nsresults.

The prbool check is just an incredibly minor restriction of the C++ system, yet it resulted in hundreds of errors(almost all of which are typos). In my mind this reinforces the importance of static typing (which C++ doesn’t do enough of).

The main lesson I learned today is that code doesn’t have to be correct in order to work correctly.


While on vacation in Ukraine I finally got to try out GPRS Internet through my cellphone. Sure EDGE is slow, but the convenience of having internet everywhere I go while traveling is unparalleled. It’s just too bad that I had to go to a developing country to be able to afford mobile internet. In Canada I would’ve paid over $750 for the $10 worth of Internet in Ukraine. So I am very excited that governments are starting to regulate mobile pricing. Looks like EU is first. I hope the local cellular oligopoly gets a kick to the head soon.