seek advice about GCC learning

[hmsjwzb via Gcc]

Hi GCC developers,

        I am learning GCC. But the GCC code is hard to understand.
        I'm reading the c compiler of GCC. It seems the understanding of 
AST/GENERIC representation is very important.

        Is there a tool can visualize the AST/GENERIC representation?

        Do you have some advice for GCC beginner? Is there some documentations 
can help in the learning of GCC?

Thanks,
Flint