Register Transfer Language (RTL) is an intermediate representation used by the GCC compiler.
RTL is used to represent the code being generated, in a form closer to assembly language than to the high level languages which GCC compiles.
RTL is generated from the GCC Abstract Syntax Tree representation, transformed by various passes in the GCC 'middle-end', and then converted to assembly language. GCC currently uses the RTL form to do most of its optimisation work.
(set:SI (reg:SI 140) (plus:SI (reg:SI 138) (reg:SI 139)))
This 'side-effect expression' says 'add register 138 to register 139, and store the result in register 140'.
The RTL generated for a program is different when GCC generates code for different processors. However, the meaning of the RTL is more-or-less independent of the target: it would be usually be possible to read and understand a piece of RTL without knowing what processor it was generated for. Similarly, the meaning of the RTL doesn't usually depend on the original high-level language of the program.
Register Transfer Language (RTL) is also a language used to describe the operation of instructions within a processor. RTL describes the requirements of data and control units in terms of digital logic to execute an assembly language instruction. Each instuction from the architecture's instruction set is defined in RTL. The resulting modules are sufficiently defined to allow the actual wiring of processor circuits to be derived. NB: derived in terms of connections, you still need place and route algorithms to get a physical layout.