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.
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.