Seeking some advice on reading C++ front-end source code

[Gwen Fu]

Dear community：
I am currently trying to fix some bugs on the front-end.
I would like to ask you, what are some exquisite data structures or
algorithms worth savoring?
Best regards!

At the same time, this document https://gcc.gnu.org/onlinedocs/ is very
rich in content. Can it be used as an important resource for understanding
the code?