Using and Porting GNU CC
Chapter 14: RTL Representation
Most of the work of the compiler is done on an intermediate representation
called register transfer language. In this language, the instructions to be
output are described, pretty much one by one, in an algebraic form that
describes what the instruction does.
RTL is inspired by Lisp lists. It has both an internal form, made up of
structures that point at other structures, and a textual form that is used
in the machine description and in printed debugging dumps. The textual
form uses nested parentheses to indicate the pointers in the internal form.