The trees are built up by the GCC Language Front Ends. They provide a relatively high level description of the original source program.
The GCC 'middle-end' performs some transformations on the trees, and then generates RTL from them.
The tree representation of a program is more-or-less independent of the processor being targeted. However, the meaning of a tree is somewhat different for different language front-ends, and front-ends can provide their own tree codes.
The tree is a C language union, a concept related to the set theoretic union of the various abstract syntax tree types that are subdivided by codes.
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