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It is usually more productive for a programmer to use a high-level language, so the development of high-level languages followed naturally from the capabilities offered by digital computers. High-level languages are formal languages that are strictly defined by their syntax and semantics which form the high-level language architecture.
Elements of these formal languages include:. While no actual implementation occurred until the s, it presented concepts later seen in APL designed by Ken Iverson in the late s. High-level language design during the formative years of digital computing provided useful programming tools for a variety of applications:.
Compiler technology evolved from the need for a strictly defined transformation of the high-level source program into a low-level target program for the digital computer. The compiler could be viewed as a front end to deal with the analysis of the source code and a back end to synthesize the analysis into the target code. Optimization between the front end and back end could produce more efficient target code. Early operating systems and software were written in assembly language.
In the s and early s, the use of high-level languages for system programming was still controversial due to resource limitations. Unics eventually became spelled Unix.
In , a new PDP provided the resource to define extensions to B and rewrite the compiler. Object-oriented programming OOP offered some interesting possibilities for application development and maintenance. The initial design leveraged C language systems programming capabilities with Simula concepts.
Object-oriented facilities were added in In many application domains, the idea of using a higher-level language quickly caught on. Because of the expanding functionality supported by newer programming languages and the increasing complexity of computer architectures, compilers became more complex. PQCC might more properly be referred to as a compiler generator. PQCC research into code generation process sought to build a truly automatic compiler-writing system.
The effort discovered and designed the phase structure of the PQC. The PQCC project investigated techniques of automated compiler construction. The design concepts proved useful in optimizing compilers and compilers for the since , object-oriented programming language Ada.
Initial Ada compiler development by the U. While the projects did not provide the desired results, they did contribute to the overall effort on Ada development.
In the U. VADS provided a set of development tools including a compiler. GNAT is free but there is also commercial support, for example, AdaCore, was founded in to provide commercial software solutions for Ada.
High-level languages continued to drive compiler research and development. Focus areas included optimization and automatic code generation. Trends in programming languages and development environments influenced compiler technology. The interrelationship and interdependence of technologies grew.
The advent of web services promoted growth of web languages and scripting languages. Scripts trace back to the early days of Command Line Interfaces CLI where the user could enter commands to be executed by the system. User Shell concepts developed with languages to write shell programs. Early Windows designs offered a simple batch programming capability.
The conventional transformation of these language used an interpreter. While not widely used, Bash and Batch compilers have been written. More recently sophisticated interpreted languages became part of the developers tool kit. Lua is widely used in game development. All of these have interpreter and compiler support. The compiler field is increasingly intertwined with other disciplines including computer architecture, programming languages, formal methods, software engineering, and computer security.
Security and parallel computing were cited among the future research targets. A compiler implements a formal transformation from a high-level source program to a low-level target program. Compiler design can define an end-to-end solution or tackle a defined subset that interfaces with other compilation tools e.
Design requirements include rigorously defined interfaces both internally between compiler components and externally between supporting toolsets. In the early days, the approach taken to compiler design was directly affected by the complexity of the computer language to be processed, the experience of the person s designing it, and the resources available. Resource limitations led to the need to pass through the source code more than once.
A compiler for a relatively simple language written by one person might be a single, monolithic piece of software. However, as the source language grows in complexity the design may be split into a number of interdependent phases. Separate phases provide design improvements that focus development on the functions in the compilation process. Classifying compilers by number of passes has its background in the hardware resource limitations of computers.
Compiling involves performing much work and early computers did not have enough memory to contain one program that did all of this work.
So compilers were split up into smaller programs which each made a pass over the source or some representation of it performing some of the required analysis and translations. The ability to compile in a single pass has classically been seen as a benefit because it simplifies the job of writing a compiler and one-pass compilers generally perform compilations faster than multi-pass compilers.
Thus, partly driven by the resource limitations of early systems, many early languages were specifically designed so that they could be compiled in a single pass e. In some cases, the design of a language feature may require a compiler to perform more than one pass over the source. For instance, consider a declaration appearing on line 20 of the source which affects the translation of a statement appearing on line In this case, the first pass needs to gather information about declarations appearing after statements that they affect, with the actual translation happening during a subsequent pass.
The disadvantage of compiling in a single pass is that it is not possible to perform many of the sophisticated optimizations needed to generate high quality code. It can be difficult to count exactly how many passes an optimizing compiler makes. For instance, different phases of optimization may analyse one expression many times but only analyse another expression once.
Splitting a compiler up into small programs is a technique used by researchers interested in producing provably correct compilers. Proving the correctness of a set of small programs often requires less effort than proving the correctness of a larger, single, equivalent program. Regardless of the exact number of phases in the compiler design, the phases can be assigned to one of three stages. The stages include a front end, a middle end, and a back end. The front end analyzes the source code to build an internal representation of the program, called the intermediate representation IR.
It also manages the symbol table, a data structure mapping each symbol in the source code to associated information such as location, type and scope.
While the frontend can be a single monolithic function or program, as in a scannerless parser, it was traditionally implemented and analyzed as several phases, which may execute sequentially or concurrently.
This method is favored due to its modularity and separation of concerns. Most commonly today, the frontend is broken into three phases: lexical analysis also known as lexing or scanning , syntax analysis also known as scanning or parsing , and semantic analysis. Lexing and parsing comprise the syntactic analysis word syntax and phrase syntax, respectively , and in simple cases, these modules the lexer and parser can be automatically generated from a grammar for the language, though in more complex cases these require manual modification.
The lexical grammar and phrase grammar are usually context-free grammars, which simplifies analysis significantly, with context-sensitivity handled at the semantic analysis phase. The semantic analysis phase is generally more complex and written by hand, but can be partially or fully automated using attribute grammars. These phases themselves can be further broken down: lexing as scanning and evaluating, and parsing as building a concrete syntax tree CST, parse tree and then transforming it into an abstract syntax tree AST, syntax tree.
In some cases additional phases are used, notably line reconstruction and preprocessing, but these are rare.
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