Editing Register transfer language (section)

=== GCC ===
In GCC, RTL is generated from the [[GIMPLE]] representation, transformed by various passes in the GCC [[compiler middle end|middle-end]], and then converted to assembly language.

GCC's RTL is usually written in a form that looks like a [[Lisp (programming language)|Lisp]] [[S-expression]]:
<syntaxhighlight lang="scheme">
(set (reg: SI 140)
     (plus:SI (reg:SI 138)
              (reg: SI 139)))
</syntaxhighlight>
This [[side effect (computer science)|side-effect expression]] says "sum the contents of register 138 with the contents of register 139 and store the result in register 140". The SI specifies the access mode for each register. In the example, it is "SImode", i.e. "access the register as 32-bit integer".

The sequence of RTL generated has some dependency on the characteristics of the processor for which GCC is generating code. However, the meaning of the RTL is more or less independent of the target: it would 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 program's original high-level language.

A register transfer language is a system for expressing in symbolic form the microoperation sequences among the registers of a digital module. It is a convenient tool for describing the internal organization of digital computers in a concise and precise manner. It can also be used to facilitate the design process of digital systems.<ref>{{Cite book|last=Mano|first=Morris M.|title=Computer System Architecture|publisher=Prentice Hall|year=1992|isbn=0131755633|pages=94|edition=3rd}}</ref>