GCC uses three main intermediate languages to represent the program during compilation: GENERIC, GIMPLE and RTL. GENERIC is a language-independent representation generated by each front end. It is used to serve as an interface between the parser and optimizer. GENERIC is a common representation that is able to represent programs written in all the languages supported by GCC.
GIMPLE and RTL are used to optimize the program. GIMPLE is used for target and language independent optimizations (e.g., inlining, constant propagation, tail call elimination, redundancy elimination, etc). Much like GENERIC, GIMPLE is a language independent, tree based representation. However, it differs from GENERIC in that the GIMPLE grammar is more restrictive: expressions contain no more than 3 operands (except function calls), it has no control flow structures and expressions with side-effects are only allowed on the right hand side of assignments. See the chapter describing GENERIC and GIMPLE for more details.
This chapter describes the data structures and functions used in the GIMPLE optimizers (also known as “tree optimizers” or “middle end”). In particular, it focuses on all the macros, data structures, functions and programming constructs needed to implement optimization passes for GIMPLE.