Compiler-compiler

Not to be confused withself-hosting compilerorsource-to-source compiler.

Incomputer science,acompiler-compilerorcompiler generatoris a programming tool that creates aparser,interpreter,orcompilerfrom some form of formal description of aprogramming languageand machine.

The most common type of compiler-compiler is called aparser generator.It handles onlysyntactic analysis.

A formal description of a language is usually agrammarused as an input to a parser generator. It often resemblesBackus–Naur form(BNF),extended Backus–Naur form(EBNF), or has its own syntax. Grammar files describe asyntaxof a generated compiler's target programming language and actions that should be taken against its specific constructs.

Source codefor a parser of the programming language is returned as the parser generator's output. This source code can then be compiled into a parser, which may be either standalone or embedded. The compiled parser then accepts the source code of the target programming language as an input and performs an action or outputs anabstract syntax tree(AST).

Parser generators do not handle thesemanticsof the AST, or thegeneration of machine codefor the target machine.

Ametacompileris a software development tool used mainly in the construction ofcompilers,translators,andinterpretersfor other programming languages.The input to a metacompiler is acomputer programwritten in aspecializedprogrammingmetalanguagedesigned mainly for the purpose of constructing compilers.The language of the compiler produced is called the object language. The minimal input producing a compiler is ametaprogramspecifying the object language grammar andsemantictransformations into anobject program.

Variants

A typical parser generator associates executable code with each of the rules of the grammar that should be executed when these rules are applied by the parser. These pieces of code are sometimes referred to as semantic action routines since they define the semantics of the syntactic structure that is analyzed by the parser. Depending upon the type of parser that should be generated, these routines may construct aparse tree(orabstract syntax tree), or generate executable code directly.

One of the earliest (1964), surprisingly powerful, versions of compiler-compilers isMETA II,which accepted an analytical grammar with output facilitiesthat produce stack machinecode, and is able to compile its own source code and other languages.

Among the earliest programs of the originalUnixversions being built atBell Labswas the two-partlexandyaccsystem, which was normally used to outputC programming languagecode, but had a flexible output system that could be used for everything from programming languages to text file conversion. Their modernGNUversions areflexandbison.

Some experimental compiler-compilers take as input a formal description of programming language semantics, typically usingdenotational semantics.This approach is often called 'semantics-based compiling', and was pioneered byPeter Mosses' Semantic Implementation System (SIS) in 1978.However, both the generated compiler and the code it produced were inefficient in time and space. No production compilers are currently built in this way, but research continues.

The Production Quality Compiler-Compiler (PQCC) project atCarnegie Mellon Universitydoes not formalize semantics, but does have a semi-formal framework for machine description.

Compiler-compilers exist in many flavors, including bottom-up rewrite machine generators (seeJBurg) used to tile syntax trees according to a rewrite grammar for code generation, andattribute grammarparser generators (e.g.ANTLRcan be used for simultaneous type checking, constant propagation, and more during the parsing stage).

Metacompilers

Metacompilers reduce the task of writing compilers by automating the aspects that are the same regardless of the object language. This makes possible the design ofdomain-specific languageswhich are appropriate to the specification of a particular problem. A metacompiler reduces the cost of producingtranslatorsfor suchdomain-specificobject languages to a point where it becomes economically feasible to include in the solution of a problem adomain-specific languagedesign.

As a metacompiler'smetalanguagewill usually be a powerful string and symbol processing language, they often have strong applications for general-purpose applications, including generating a wide range of other software engineering and analysis tools.

Besides being useful fordomain-specific languagedevelopment, a metacompiler is a prime example of a domain-specific language, designed for the domain of compiler writing.

A metacompiler is ametaprogramusually written in its own metalanguageor an existing computer programming language. The process of a metacompiler, written in its own metalanguage, compiling itself is equivalent toself-hosting compiler.Most common compilers written today are self-hosting compilers. Self-hosting is a powerful tool, of many metacompilers, allowing the easy extension of their own metaprogramming metalanguage. The feature that separates a metacompiler apart from other compiler compilers is that it takes as input a specializedmetaprogramminglanguage that describes all aspects of the compiler's operation. A metaprogram produced by a metacompiler is as complete a program as aprogramwritten inC++,BASICor any other generalprogramming language.The metaprogrammingmetalanguageis a powerful attribute allowing easier development of computer programming languages and other computer tools. Command line processors, text string transforming and analysis are easily coded using metaprogramming metalanguages of metacompilers.

A full featured development package includes alinkerand arun timesupportlibrary.Usually, a machine-orientedsystem programming language,such asCor C++, is needed to write the support library. A library consisting of support functions needed for the compiling process usually completes the full metacompiler package.

The meaning of metacompiler

In computer science, the prefixmetais commonly used to meanabout (its own category).For example,metadataare data that describe other data. A language that is used to describe other languages is ametalanguage.Meta may also meanon a higher level of abstraction.Ametalanguageoperates on a higher level of abstraction in order to describe properties of a language.Backus–Naur form(BNF) is a formalmetalanguageoriginally used to defineALGOL 60.BNF is a weakmetalanguage,for it describes only thesyntaxand says nothing about thesemanticsor meaning. Metaprogramming is the writing ofcomputer programswith the ability to treatprogramsas their data. A metacompiler takes as input ametaprogramwritten in aspecialized metalanguages(a higher level abstraction) specifically designed for the purpose of metaprogramming.The output is an executable object program.

An analogy can be drawn: That as aC++compiler takes as input aC++programming language program, ametacompiler takes as input ametaprogrammingmetalanguageprogram.

Forth metacompiler

Many advocates of the languageForthcall the process of creating a new implementation of Forth a meta-compilation and that it constitutes a metacompiler. The Forth definition of metacompiler is:

"A metacompiler is a compiler which processes its own source code, resulting in an executable version of itself."

This Forth use of the term metacompiler is disputed in mainstream computer science. SeeForth (programming language)andHistory of compiler construction.The actual Forth process of compiling itself is a combination of a Forth being aself-hostingextensible programminglanguage and sometimescross compilation,long established terminology in computer science. Metacompilers are a general compiler writing system. Besides the Forth metacompiler concept being indistinguishable from self-hosting and extensible language. The actual process acts at a lower level defining a minimum subset of forthwords,that can be used to define additional forth words, A full Forth implementation can then be defined from the base set. This sounds like a bootstrap process. The problem is that almost every general purpose language compiler also fits the Forth metacompiler description.

When (self-hosting compiler) X processes its own source code, resulting in an executable version of itself, X is a metacompiler.

Just replace X with any common language, C, C++,Pascal,COBOL,Fortran,Ada,Modula-2,etc. And X would be a metacompiler according to the Forth usage of metacompiler. A metacompiler operates at an abstraction level above the compiler it compiles. It only operates at the same (self-hosting compiler) level when compiling itself. One has to see the problem with this definition of metacompiler. It can be applied to most any language.

However, on examining the concept of programming in Forth, adding new words to the dictionary, extending the language in this way is metaprogramming. It is this metaprogramming in Forth that makes it a metacompiler.

Programming in Forth is adding new words to the language. Changing the language in this way ismetaprogramming.Forth is a metacompiler, because Forth is a language specifically designed for metaprogramming. Programming in Forth is extending Forth adding words to the Forth vocabulary creates a new Forthdialect.Forth is a specialized metacompiler for Forth language dialects.

History

Design of the original Compiler Compiler was started byTony Brookerand Derrick Morris in 1959, with initial testing beginning in March 1962.The Brooker Morris Compiler Compiler (BMCC) was used to create compilers for the newAtlascomputer at theUniversity of Manchester,for several languages:Mercury Autocode,Extended Mercury Autocode,Atlas Autocode,ALGOL 60and ASAFortran.At roughly the same time, related work was being done by E. T. (Ned) Irons at Princeton, and Alick Glennie at the Atomic Weapons Research Establishment at Aldermaston whose "Syntax Machine" paper (declassified in 1977) inspired the META series of translator writing systems mentioned below.

The early history of metacompilers is closely tied with the history of SIG/PLAN Working group 1 on Syntax Driven Compilers. The group was started primarily through the effort of Howard Metcalfe in the Los Angeles area.In the fall of 1962, Howard Metcalfe designed two compiler-writing interpreters. One used a bottom-to-top analysis technique based on a method described by Ledley and Wilson.The other used a top-to-bottom approach based on work by Glennie to generate random English sentences from a context-free grammar.

At the same time, Val Schorre described two "meta machines", one generative and one analytic. The generative machine was implemented and produced random algebraic expressions. Meta I the first metacompiler was implemented by Schorre on an IBM 1401 at UCLA in January 1963. His original interpreters and metamachines were written directly in a pseudo-machine language.META II,however, was written in a higher-level metalanguage able to describe its own compilation into the pseudo-machine language.

Lee Schmidt at Bolt, Beranek, and Newman wrote a metacompiler in March 1963 that utilized a CRT display on the time-sharing PDP-l.This compiler produced actual machine code rather than interpretive code and was partially bootstrapped from Meta I.

Schorre bootstrapped Meta II from Meta I during the spring of 1963. The paper on the refined metacompiler system presented at the 1964 Philadelphia ACM conference is the first paper on a metacompiler available as a general reference. The syntax and implementation technique of Schorre's system laid the foundation for most of the systems that followed. The system was implemented on a small 1401, and was used to implement a smallALGOL-like language.

Many similar systems immediately followed.

Roger Rutman ofAC Delcodeveloped and implemented LOGIK, a language for logical design simulation, on the IBM 7090 in January 1964.This compiler used an algorithm that produced efficient code for Boolean expressions.

Another paper in the 1964 ACM proceedings describesMeta III,developed bySchneiderand Johnson at UCLA for the IBM 7090.Meta III represents an attempt to produce efficient machine code, for a large class of languages. Meta III was implemented completely in assembly language. Two compilers were written in Meta III, CODOL, a compiler-writing demonstration compiler, and PUREGOL, a dialect of ALGOL 60. (It was pure gall to call it ALGOL).

Late in 1964, Lee Schmidt bootstrapped the metacompiler EQGEN, from the PDP-l to the Beckman 420. EQGEN was a logic equation generating language.

In 1964, System Development Corporation began a major effort in the development of metacompilers. This effort includes powerful metacompilers, Bookl, and Book2 written inLispwhich have extensive tree-searching and backup ability. An outgrowth of one of theQ-32systems at SDC is Meta 5.The Meta 5 system incorporates backup of the input stream and enough other facilities to parse any context-sensitive language. This system was successfully released to a wide number of users and had many string-manipulation applications other than compiling. It has many elaborate push-down stacks, attribute setting and testing facilities, and output mechanisms. That Meta 5 successfully translatesJOVIALprograms toPL/Iprograms demonstrates its power and flexibility.

Robert McClure atTexas Instrumentsinvented a compiler-compiler calledTMG(presented in 1965). TMG was used to create early compilers for programming languages likeB,PL/IandALTRAN.Together with metacompiler of Val Schorre, it was an early inspiration for the last chapter ofDonald Knuth'sThe Art of Computer Programming.

The LOT system was developed during 1966 at Stanford Research Institute and was modeled very closely after Meta II.It had new special-purpose constructs allowing it to generate a compiler which could in turn, compile a subset of PL/I. This system had extensive statistic-gathering facilities and was used to study the characteristics of top-down analysis.

SIMPLE is a specialized translator system designed to aid the writing of pre-processors for PL/I, SIMPLE, written in PL/I, is composed of three components: An executive, a syntax analyzer and a semantic constructor.

TheTREE-METAcompiler was developed at Stanford Research Institute in Menlo Park, California. April 1968. The early metacompiler history is well documented in theTREE META manual.TREE META paralleled some of the SDC developments. Unlike earlier metacompilers it separated the semantics processing from the syntax processing. The syntax rules containedtreebuilding operations that combined recognized language elements with tree nodes. The tree structure representation of the input was then processed by a simple form of unparse rules. The unparse rules used node recognition and attribute testing that when matched resulted in the associated action being performed. In addition like tree element could also be tested in an unparse rule. Unparse rules were also a recursive language being able to call unparse rules passing elements of thee tree before the action of the unparse rule was performed.

The concept of the metamachine originally put forth by Glennie is so simple that three hardware versions have been designed and one actually implemented. The latter at Washington University in St. Louis. This machine was built from macro-modular components and has for instructions the codes described by Schorre.

CWIC (Compiler for Writing and Implementing Compilers) is the last known Schorre metacompiler. It was developed at Systems Development Corporation by Erwin Book, Dewey Val Schorre and Steven J. Sherman With the full power of (lisp 2) a list processing language optimizing algorithms could operate on syntax generated lists and trees before code generation. CWIC also had a symbol table built into the language.

With the resurgence of domain-specific languages and the need for parser generators which are easy to use, easy to understand, and easy to maintain, metacompilers are becoming a valuable tool for advanced software engineering projects.

Other examples of parser generators in the yacc vein areANTLR,Coco/R,CUP,GNU Bison,Eli,FSL,SableCC,SID (Syntax Improving Device),andJavaCC.While useful, pure parser generators only address the parsing part of the problem of building a compiler. Tools with broader scope, such asPQCC,Coco/RandDMS Software Reengineering Toolkitprovide considerable support for more difficult post-parsing activities such as semantic analysis, code optimization and generation.

Schorre metalanguages

The earliest Schorre metacompilers, META I and META II, were developed by D. Val Schorre at UCLA. Other Schorre based metacompilers followed. Each adding improvements to language analysis and/or code generation.

In programming it is common to use the programming language name to refer to both the compiler and the programming language, the context distinguishing the meaning. A C++ program is compiled using a C++ compiler. That also applies in the following. For example, META II is both the compiler and the language.

The metalanguages in the Schorre line of metacompilers are functional programming languages that use top down grammar analyzing syntax equations having embedded output transformation constructs.

A syntax equation:

<name> = <body>;

is a compiledtestfunction returningsuccessorfailure.<name> is the function name. <body> is a form of logical expression consisting of tests that may be grouped, have alternates, and output productions. Atestis like aboolin other languages,successbeingtrueandfailurebeingfalse.

Defining a programming language analytically top down is natural. For example, a program could be defined as:

program = $declaration;

Defining a program as a sequence of zero or more declaration(s).

In the Schorre METAXlanguages there is a driving rule. The program rule above is an example of a driving rule. The program rule is atestfunction that calls declaration, atestrule, that returnssuccessorfailure.The $ loop operator repeatedly calling declaration untilfailureis returned. The $ operator is always successful, even when there are zero declaration. Above program would always return success. (In CWIC a long fail can bypass declaration. A long-fail is part of the backtracking system of CWIC)

The character sets of these early compilers were limited. The character/was used for the alternant (or) operator. "A or B" is written as A / B. Parentheses ( ) are used for grouping.

A (B / C)

Describes a construct of A followed by B or C. As a boolean expression it would be

Aand(BorC)

A sequence X Y has an implied XandY meaning.( )are grouping and/theoroperator. The order of evaluation is always left to right as an input character sequence is being specified by the ordering of the tests.

Special operator words whose first character is a "." are used for clarity..EMPTY is used as the last alternate when no previous alternant need be present.

X (A / B /.EMPTY)

Indicates that X is optionally followed by AorB. This is a specific characteristic of these metalanguages being programming languages. Backtracking is avoided by the above. Other compiler constructor systems may have declared the three possible sequences and left it up to the parser to figure it out.

The characteristics of the metaprogramming metalanguages above are common to all Schorre metacompilers and those derived from them.

META I

META I was a hand compiled metacompiler used to compile META II. Little else is known of META I except that the initial compilation of META II produced nearly identical code to that of the hand coded META I compiler.

META II

Main article:META II

Each rule consists optionally of tests, operators, and output productions. A rule attempts to match some part of the input program source character stream returning success or failure. On success the input is advanced over matched characters. On failure the input is not advanced.

Output productions produced a form of assembly code directly from a syntax rule.

TREE-META

Main article:TREE-META

TREE-META introduced tree building operators:<node_name> and[<number>]moving the output production transforms to unparsed rules. The tree building operators were used in the grammar rules directly transforming the input into anabstract syntax tree.Unparse rules are also test functions that matched tree patterns. Unparse rules are called from a grammar rule when anabstract syntax treeis to be transformed into output code. The building of anabstract syntax treeand unparse rules allowed local optimizations to be performed by analyzing the parse tree.

Moving of output productions to the unparse rules made a clear separation of grammar analysis and code production. This made the programming easier to read and understand.

CWIC

In 1968–1970, Erwin Book, Dewey Val Schorre, and Steven J. Sherman developed CWIC.(Compiler for Writing and Implementing Compilers) atSystem Development CorporationCharles Babbage Institute Center for the History of Information Technology (Box 12, folder 21),

CWIC is a compiler development system composed of three special-purpose, domain specific, languages, each intended to permit the description of certain aspects of translation in a straight forward manner. The syntax language is used to describe the recognition of source text and the construction from it to an intermediatetreestructure. The generator language is used to describe the transformation of thetreeinto appropriate object language.

The syntax language follows Dewey Val Schorre's previous line of metacompilers. It most resembles TREE-META havingtreebuilding operators in the syntax language. The unparse rules of TREE-META are extended to work with the object based generator language based onLISP 2.

CWIC includes three languages:

• Syntax:Transforms the source program input, into list structures using grammar transformation formula. A parsed expression structure is passed to a generator by placement of a generator call in a rule. Atreeis represented by a list whose first element is a node object. The language has operators,<and>,specifically for making lists. The colon:operator is used to create node objects.:ADDcreates an ADD node. The exclamation!operator combines a number of parsed entries with a node to make atree.Trees created by syntax rules are passed to generator functions, returning success or failure. The syntax language is very close to TREE-META. Both use a colon to create a node. CWIC'streebuilding exclamation!<number> functions the same as TREE-META's [<number>].
• Generator:a named series of transforming rules, each consisting of an unparse, pattern matching, rule. and an output production written in a LISP 2 like language. the translation was to IBM 360 binary machine code. Other facilities of the generator language generalized output.
• MOL-360:an independentmid level implementation languagefor the IBM System/360 family of computers developed in 1968 and used for writing the underlying support library.

Generators language

Generators Language had semantics similar toLisp.The parsetreewas thought of as a recursive list. The general form of a Generator Language function is:

function-name(first-unparse_rule) => first-production_code_generator
(second-unparse_rule) => second-production_code_generator
(third-unparse_rule) => third-production_code_generator
...

The code to process a giventreeincluded the features of a general purpose programming language, plus a form: <stuff>, which would emit (stuff) onto the output file. A generator call may be used in the unparse_rule. The generator is passed the element of unparse_rule pattern in which it is placed and its return values are listed in (). For example:

expr_gen(ADD[expr_gen(x),expr_gen(y)]) =>
<AR + (x*16)+y;>
releasereg(y);
return x;
(SUB[expr_gen(x),expr_gen(y)])=>
<SR + (x*16)+y;>
releasereg(y);
return x;
(MUL[expr_gen(x),expr_gen(y)])=>
.
.
.
(x)=> r1 = getreg();
load(r1, x);
return r1;
...

That is, if the parsetreelooks like (ADD[<something1>,<something2>]), expr_gen(x) would be called with <something1> and return x. A variable in the unparse rule is a local variable that can be used in the production_code_generator. expr_gen(y) is called with <something2> and returns y. Here is a generator call in an unparse rule is passed the element in the position it occupies. Hopefully in the above x and y will be registers on return. The last transforms is intended to load an atomic into a register and return the register. The first production would be used to generate the 360 "AR" (Add Register) instruction with the appropriate values in general registers. The above example is only a part of a generator. Every generator expression evaluates to a value that con then be further processed. The last transform could just as well have been written as:

(x)=> return load(getreg(), x);

In this case load returns its first parameter, the register returned by getreg(). the functions load and getreg are other CWIC generators.

CWIC addresseddomain-specific languagesbefore the termdomain-specific languageexisted

From the authors of CWIC:

"A metacompiler assists the task of compiler-building by automating its non creative aspects, those aspects that are the same regardless of the language which the produced compiler is to translate. This makes possible the design of languages which are appropriate to the specification of a particular problem. It reduces the cost of producing processors for such languages to a point where it becomes economically feasible to begin the solution of a problem with language design."

Examples

See also:Comparison of parser generators

• ANTLR
• GNU Bison
• Coco/R,Coco-2
• DMS Software Reengineering Toolkit,a program transformation system with parser generators
• Epsilon Grammar Studio
• Lemonparser generator
• LRSTAR: LR(*) parser generator
• META II
• parboiled,a Java library for building parsers.
• Packrat parser
• PQCC,a compiler-compiler that is more than a parser generator.
• Syntax Improving Device (SID)
• SYNTAX,an integrated toolset for compiler construction.
• TREE-META
• Yacc
• Xtext
• XPL
• JavaCC

See also

• Parsing expression grammar
• LL parser
• LR parser
• Simple LR parser
• LALR parser
• GLR parser
• Domain analysis
• Domain-specific language
• History of compiler construction
  • History of compiler construction#Self-hosting compilers
• Metacompilation
• Program transformation

Further reading

• Brooker, R. A.;MacCallum, I. R.; Morris, D.; Rohl, J. S. (1963)."The compiler-compiler"(PDF).Annual Review in Automatic Programming.3:229–275.doi:10.1016/S0066-4138(63)80009-9.
• Brooker, R.A.;Morris, D.; Rohl, J. S. (February 1967)."Experience with the Compiler Compiler"(PDF).Computer Journal.9:350.doi:10.1093/comjnl/9.4.350.Retrieved2021-05-07.
• Napper, R. B. E (December 1965).An Introduction To The Compiler Compiler(PDF).
• MacCallum, I. R. (January 1963).Some Aspects of the Implementation of the Compiler Compiler on ATLAS(PDF)(Thesis).
• Johnson, Stephen C.(July 1975).Yacc—yet another compiler-compiler.Murray Hill, New Jersey, USA:Bell Laboratories.Computer Science Technical Report 32.
• McKeeman, William M.;Horning, James J.;Wortman, David B. (1970).A Compiler Generator.Englewood Cliffs, New Jersey, USA:Prentice-Hall.ISBN978-0-13-155077-3.Retrieved2012-12-13.

External links

• Tony Brooker and the Atlas Compiler Compiler
• An Explanation of the Compiler Compiler listings (1963)
• Compiler Compiler source code(starts around PDF page 182)
• Original Compiler Compiler flowcharts
• Computer50.org,Brooker Autocodes
• Catalog.compilertools.netArchived2011-08-13 at theWayback Machine,The Catalog of Compiler Construction Tools
• Labraj.uni-mb.si,Lisa
• Skenz.it,Jflex and Cup resources(in Italian)