Learning the GCC AST structure

Howdy all,

I'd like to do my best to contribute to GCC, both because I'd like to 
give back to the compiler I've used for so long as well as to learn 
more about writing compiled languages. I figured the best place to 
start would be to learn how the AST is set up, as that's effectively 
what everything builds off of, and I'm wondering where I could look to 
learn how it's structured and how that affects the way it's used 
throughout the codebase. I recognize that a general overview won't 
provide the full story, but I figure it's a better place to start than 
trying to grasp the entire compiler at once.

- October