What is the purpose of the "tree-***" family of GCC optimization options?

Question

Reading gcc's manual, I see a family of -ftree-*** optimization options. What is the purpose of these options? What are these "trees"?

Answer 1

They are optimizer passes that work on the (SSA) trees inside the compiler, rather than on the RTL (register transfer language) level.

For an overview what this is about, see the GCC internals documentation: http://gcc.gnu.org/onlinedocs/gccint/

For an introduction about SSA, see http://en.wikipedia.org/wiki/Static_single_assignment_form