computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...

, register transfer language (RTL) is a kind of

intermediate representation An intermediate representation (IR) is the data structure or code used internally by a compiler or virtual machine to represent source code. An IR is designed to be conducive to further processing, such as optimization and translation. A "good" ...

(IR) that is very close to

assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...

, such as that which is used in a

compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...

. It is used to describe data flow at the

register-transfer level In digital circuit design, register-transfer level (RTL) is a design abstraction which models a synchronous digital circuit in terms of the flow of digital signals (data) between hardware registers, and the logical operations performed on those ...

of an

architecture Architecture is the art and technique of designing and building, as distinguished from the skills associated with construction. It is both the process and the product of sketching, conceiving, planning, designing, and constructing building ...

. Academic papers and textbooks often use a form of RTL as an architecture-neutral assembly language. RTL is used as the name of a specific intermediate representation in several compilers, including the

GNU Compiler Collection The GNU Compiler Collection (GCC) is an optimizing compiler produced by the GNU Project supporting various programming languages, hardware architectures and operating systems. The Free Software Foundation (FSF) distributes GCC as free software ...

(GCC), Zephyr, and the European compiler projects CerCo and

CompCert CompCert is a formally verified optimizing compiler for a large subset of the C99 programming language (known as Clight) which currently targets PowerPC, ARM, RISC-V, x86 and x86-64 architectures. This project, led by Xavier Leroy, started o ...

History

The idea behind RTL was first described in ''The Design and Application of a Retargetable Peephole Optimizer''.

GCC

In GCC, RTL is generated from the

GIMPLE The GNU Compiler Collection (GCC) is an optimizing compiler produced by the GNU Project supporting various programming languages, hardware architectures and operating systems. The Free Software Foundation (FSF) distributes GCC as free software ...

representation, transformed by various passes in the GCC middle-end, and then converted to assembly language. GCC's RTL is usually written in a form which looks like a

Lisp A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech. Types * A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lisping ...

S-expression In computer programming, an S-expression (or symbolic expression, abbreviated as sexpr or sexp) is an expression in a like-named notation for nested list (tree-structured) data. S-expressions were invented for and popularized by the programming la ...

: (set (reg:SI 140) (plus:SI (reg:SI 138) (reg:SI 139))) This 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 original high-level language of the program. 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 concise and precise manner. It can also be used to facilitate the design process of digital systems.

History

GCC

References

External links