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Compiler

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A diagram of the operation of a typical multi-language compiler.

A compiler is a computer program (or set of programs) that translates text written in a computer language (the source language) into another computer language (the target language). The original sequence is usually called the source code and the output called object code. Commonly the output has a form suitable for processing by other programs (e.g., a linker), but it may be a human readable text file.

The most common reason for wanting to translate source code is to create an executable program. The name "compiler" is primarily used for programs that translate source code from a high level language to a lower level language (e.g., assembly language or machine language). A program that translates from a low level language to a higher level one is a decompiler. A program that translates between high-level languages is usually called a language translator, source to source translator, or language converter. A language rewriter is usually a program that translates the form of expressions without a change of language.

A compiler is likely to perform many or all of the following operations: lexing, preprocessing, parsing, semantic analysis, code optimizations, and code generation.

History

Early computers did not use compilers, because they had just a few opcodes and little memory and users entered binary machine code directly by toggling switches on the computer console/front panel.

In late 1940s, programmers found that the tedious machine code could be denoted using some mnemonics (assembly language) and computers could translate those mnemonics into machine code. The primitive compiler, assembler, emerged.

During the 1950s, machine-dependent assembly languages were still not ideal for programmers and high level, machine-independent programming languages evolved. Subsequently, several experimental compilers were developed then (see, for example, the seminal work by Grace Hopper on the A-0 language), but the FORTRAN team led by John Backus at IBM is generally credited as having introduced the first complete compiler, in 1957. COBOL was an early language to be compiled on multiple architectures, in 1960. [1]

The idea of compilation quickly caught on, and most of the principles of compiler design were developed during the 1960s.

With the evolution of programming languages and the increasing power of computers, compilers are becoming more and more complex to bridge the gap between problem-solving modern programming languages and the various computer systems, aiming at getting the highest performance out of the target machines.

A compiler is itself a computer program written in some implementation language. Early compilers were written in assembly language. The first self-hosting compiler — capable of compiling its own source code in a high-level language — was created for Lisp by Hart and Levin at MIT in 1962 [2]. The use of high-level languages for writing compilers gained added impetus in the early 1970s when Pascal and C compilers were written in their own languages. Building a self-hosting compiler is a bootstrapping problem -- the first such compiler for a language must be compiled either by a compiler written in a different language, or (as in Hart and Levin's Lisp compiler) compiled by running the compiler in an interpreter.

Compiler construction and compiler optimization are taught at universities as part of the computer science curriculum. Such courses are usually supplemented with the implementation of a compiler for an educational programming language. A well documented example is the PL/0 compiler, which was originally used by Niklaus Wirth for teaching compiler construction in the 1970s. In spite of its simplicity, the PL/0 compiler introduced several concepts to the field which have since become established educational standards:

  1. The use of Program Development by Stepwise Refinement
  2. The use of a Recursive descent parser
  3. The use of EBNF to specify the syntax of a language
  4. The use of P-Code during generation of portable output code
  5. The use of T-diagrams for the formal description of the bootstrapping problem

Types of compilers

There are many ways to classify compilers according to the input and output, internal structure, and runtime behavior. For example,

  • A program that translates from a low level language to a higher level one is a decompiler.
  • A program that translates between high-level languages is usually called a language translator, source to source translator, language converter, or language rewriter (this last term is usually applied to translations that do not involve a change of language)

Native versus cross compiler

Most compilers are classified as either native compilers or cross compilers.

A compiler may produce binary output intended to run on the same type of computer and operating system ("platform") as the compiler itself runs on. This is sometimes called a native-code compiler. Alternatively, it might produce binary output designed to run on a different platform. This is known as a cross compiler. Cross compilers are very useful when bringing up a new hardware platform for the first time (see bootstrapping). Cross compilers are also necessary when developing software for microcontroller systems that have barely enough storage for the final machine code, much less a compiler. Compilers which are capable of producing both native and foreign binary output may be called either a cross compiler or a native compiler depending on a specific use, although it would be more correct to classify them as cross compilers.

Interpreters are never classified as native or cross compilers, because they do not output a binary representation of their input code.

Virtual machine (VM) compilers are typically not classified as either native or cross compilers. However, if need be, they can be classified as one or the other, especially in the less usual cases where a compiler is running inside the same VM (making it a native compiler), or where a compiler is capable of producing an output for several different platforms, including a VM (making it a cross compiler).

One-pass versus multi-pass compilers

Classifying compilers by number of passes has its background in the hardware resource limitations of computers. Compiling involves performing lots of 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 abillity to compile in a single pass is often seen as a benefit because it simplifies the job of writing a compiler and one pass compilers are generally faster than multi-pass compilers. Many languages were designed so that they could be compiled in a single pass (e.g., the Pascal programming language).

In some cases the design of a language feature may require a compiler to perform more than one pass over the source. For instance, when a declaration appearing on line 20 of the source affects the translation of the statement appearing on line 10; the first pass needs to gather information about declarations appearing after statements that they affect, with the actual translation happening during a second 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 requiring less effort than proving the correctness of a larger, single, equivalent program.

While the typical multi-pass compiler outputs machine code from its final pass, there are several other types:

  • A "source-to-source compiler" is a type of compiler that takes a high level language as its input and outputs a high level language. For example, an automatic parallelizing compiler will frequently take in a high level language program as an input and then transform the code and annotate it with parallel code annotations (e.g. OpenMP) or language constructs (e.g. Fortran's DOALL statements).
  • Stage compiler that compiles to assembly language of a theoretical machine, like some Prolog implementations
    • This Prolog machine is also known as the Warren Abstract Machine (or WAM). Byte-code compilers for Java, Python (and many more) are also a subtype of this.
  • Just-in-time compiler, used by Smalltalk and Java systems, and also by Microsoft .Net's Common Intermediate Language (CIL)
    • Applications are delivered in bytecode, which is compiled to native machine code just prior to execution.

Compiled versus interpreted languages

Many people divide higher-level programming languages into compiled languages and interpreted languages. However, there is rarely anything about a language that requires it to be compiled or interpreted. Compilers and interpreters are implementations of languages, not languages themselves. The categorization usually reflects the most popular or widespread implementations of a language -- for instance, BASIC is thought of as an interpreted language, and C a compiled one, despite the existence of BASIC compilers and C interpreters.

There are exceptions; some language specifications assume the use of a compiler (as with C), or spell out that implementations must include a compilation facility (as with Common Lisp). Some languages have features that are very easy to implement in an interpreter, but make writing a compiler much harder; for example, SNOBOL4, and many scripting languages are capable of constructing arbitrary source code at runtime with regular string operations, and then executing that code by passing it to a special evaluation function. To implement these features in a compiled language, programs must usually be shipped with a runtime environment that includes the compiler itself.

Compiler design

The approach taken to compiler design is affected by the complexity of the processing that needs to be done, the experience of the person(s) designing it, and the resources (eg, people and tools) available.

A compiler for a relatively simple language written by one person might be a single, monolithic, piece of software. When the source language is large and complex, and high quality output is required the design may be split into a number of relatively independent phases, or passes. Having separate phases means development can be parcelled up into small parts and given to different people. It also becomes much easier to replace a single phase by an improved one, or to insert new phases later (eg, additional optimizations).

The division of the compilation processes in phases (or passes) was championed by the Production Quality Compiler-Compiler Project (PQCC) at Carnegie Mellon University. This project introduced the terms front end, middle end (rarely heard today), and back end.

All but the smallest of compilers have more than two phases. However, these phases are usually regarded as being part of the front end or the back end. The point at where these two ends meet is always open to debate. The front end is generally considered to be where syntactic and semantic processing takes place, along with translation to a lower level of representation (than source code).

The middle (or 'analysis stage') performs optimizations on a more convenient form than either the source code (the input) or machine language (the output). Many optimizations can be performed without knowing or caring the exact architecture of the computer. In a compiler system, middle optimizations minimize the new programming needed to get a pretty good compiler for either a new computer or computer language.

The back end takes the output from the middle. It may perform more analysis, transformations and optimizations that are for a particular computer. Then, it generates code for a particular computer.

This front-end/analysis/back-end approach makes it possible to combine front ends for different languages with back ends for different CPUs.

This was actually done both in GCC and the Amsterdam compiler kit, which have multiple front-ends, shared analysis and multiple back-ends.

Front end

The front end analyses the source code to build an internal representation of the program, called the intermediate representation or 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. This is done over several phases:

  1. Preprocessing. Some languages, e.g., C, require a preprocessing phase to do things such as conditional compilation and macro substitution. In the case of C the preprocessing phase includes lexical analysis.
  2. Lexical analysis breaks the source code text into small pieces called tokens. Each token is a single atomic unit of the language, for instance a keyword, identifier or symbol name. The token syntax is typically a regular language, so a finite state automaton constructed from a regular expression can be used to recognize it. This phase is also called lexing or scanning, and the software doing lexical analysis is called a lexical analyzer or scanner.
  3. Syntax analysis involves parsing the token sequence to identify the syntactic structure of the program.
  4. Semantic analysis is the phase that checks the meaning of the program to ensure it obeys the rules of the language. One example is type checking. The compiler emits most diagnostics during semantic analysis, and frequently combines it with syntax analysis.

Back end

The term of Back end is sometime confused with code generator for the overlapped functionality of generating assembly code. Some literature use Middle end to distinguish the generic analysis and optimization phases in the back end from the machine dependent code generators.

The work in back end is done in multiple steps:

  1. Compiler analysis - This is the process to gather program information from the intermediate representation of the input source files. Typical analysis are variable define-use and use-define chain, dependence analysis, alias analysis etc. Accurate analysis is the base for any compiler optimizations. The call graph and control flow graph are usually also built during the analysis phase.
  2. Optimization - the intermediate language representation is transformed into functionally equivalent but faster (or smaller) forms. Popular optimizations are inline expansion, dead code elimination, constant propagation, loop transformation, register allocation or even automatic parallelization.
  3. Code generation - the transformed intermediate language is translated into the output language, usually the native machine language of the system. This involves resource and storage decisions, such as deciding which variables to fit into registers and memory and the selection and scheduling of appropriate machine instructions along with their associated addressing modes (see also Sethi-Ullman algorithm).

Compiler analysis is the prerequisite for any compiler optimization and they tightly work together. For example, dependence analysis is crucial for loop transformation.

In addition, the scope of compiler analysis and optimization vary greatly, from as small as a basic block to the procedure/function level, or even over the whole program (interprocedural optimization). Obviously, a compiler can potentially do a better job using a broader view. But that broad view is not free: large scope analysis and optimizations are very costly in terms of compilation time and memory space; this is especially true for interprocedural analysis and optimizations.

The existence of interprocedural analysis and optimization is common in modern commercial compilers from SGI, Intel, Microsoft, and Sun Microsystems. The open source GCC was criticized for a long time for lacking powerful interprocedural optimizations, but it is changing in this respect. Another good open source compiler with full analysis and optimization infrastructure is Open64, which is used by many organizations for research and commercial purposes.

Due to the extra time and space needed for compiler analysis and optimization, some compilers skip them by default. Users have to use compilation options to explicitly tell the compiler which optimizations should be enabled.

A compiler example

The following program is a very simple one-pass compiler, written in the C programming language. This compiler compiles expression defined in infix notation to postfix notation. For example, the expression 9-5+2 in infix notation will be compiled into 95-2+, which is in postfix notation. 

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char	lookahead;
int	pos = 0;
char	expression[20+1];


void error()
{
	printf("Syntax error!\n");
}

void match( char t )
{
	if( lookahead == t )
	{
		pos++;
		lookahead = expression[pos];		
	}
	else
		error();
}

void term()
{
	switch( lookahead )
	{
		case '0':
		case '1':
		case '2':
		case '3':
		case '4':
		case '5':
		case '6':
		case '7':
		case '8':
		case '9':
			printf("%c", lookahead);
			match( lookahead );
			break;
		default:
			error();
			break;
	}
}

void expr()
{
	term();
	while(true)
	{
		switch( lookahead )
		{
			case '+':
				match('+');
				term();
				
				printf("+");
				break;
			case '-':
				match('-');
				term();

				printf("-");
				break;
			default:
				return;
		
		}
	}
}


int main ( int argc, char** argv )
{
	strcpy( expression, "9-5+2");
	lookahead = *expression;

	expr();
	printf("\n");
	getchar();

	return 0;
}

Notes

  1. a A pass has also been known as a parse in some textbooks. The idea is that the source code is parsed by gradual, iterative refinement to produce the completely translated object code at the end of the process. There is, however, some dispute over the general use of parse for all those phases (passes), since some of them, e.g. object code generation, are arguably not regarded to be parsing as such.

References

  • Compilers: Principles, Techniques and Tools by Alfred V. Aho, Ravi Sethi, and Jeffrey D. Ullman (ISBN 0201100886) is considered to be the standard authority on compiler basics(undergraudate level), and makes a good primer for the techniques mentioned above. (It is often called the Dragon Book because of the picture on its cover showing a Knight of Programming fighting the Dragon of Compiler Design.) link to publisher
  • Advanced Compiler Design and Implementation by Steven Muchnick (ISBN 1558603204). One of the widely-used text books for advanced compiler courses(graudate level).
  • Understanding and Writing Compilers: A Do It Yourself Guide (ISBN 0333217322) by Richard Bornat is an unusually helpful book, being one of the few that adequately explains the recursive generation of machine instructions from a parse-tree. Having learnt his subject in the early days of mainframes and minicomputers, the author has many useful insights that more recent books often fail to convey.
  • An Overview of the Production Quality Compiler-Compiler Project by Leverett, Cattel, Hobbs, Newcomer, Reiner, Schatz and Wulf. Computer 13(8):38-49 (August 1980)
  • Compiler Construction by Niklaus Wirth (ISBN 0-201-40353-6) Addison-Wesley 1996, 176 pages, also available at [3]. Step-by-step guide to using recursive descent parser. Describes a compiler for Oberon-0, a subset of the author's Oberon programming language.

See also

The Wikibook [[wikibooks:|]] has a page on the topic of: Compiler construction
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