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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. Compilers had not yet been invented because early computers had very little memory and programs were necessarily quite short. Users often entered the decimal or binary machine code for a program directly by turning or toggling switches on the computer console or entering digits via paper tape. Programmers soon began to create programs to help with the tedious task of entering machine code. Such a program which would translate an alphabetic letter (which were chosen to be mnemonic) into a corresponding numeric instruction which the machine could execute. In this manner, assembly languages and the primitive compiler, the assembler, emerged. Towards the end of the 1950s, 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 programming 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. * 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 *. 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: Types of compilers There are many ways to classify compilers according to the input and output, internal structure, and runtime behavior. For example, Native versus cross compiler Most compilers are classified as either native (or hosted) compilers or cross compilers. A native or host compiler is one whose output is intended to run on the same type of computer and operating system ("platform") as the compiler itself runs on. The output of a cross compiler is designed to run on a different platform. Cross compilers are often used when developing software for microcontroller systems that are not intended to support an environment intended for software development. Interpreters and Virtual machine (VM) compilers are not usually classified as native or cross compilers. 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 ability 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., Pascal). 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 10. 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 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: DOALL statements).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 spell out that implementations must include a compilation facility (eg, Common Lisp), while other 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. Hardware compilation The output of some compilers may target hardware at a very low level, eg a Field Programmable Gate Array (FPGA). Such compilers are said to be hardware compilers because the programs they compile effectively control the final configuration of the hardware and how it operates; there are no instructions that are executed in sequence. 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 parceled 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 end is usually designed to performs optimizations on a form other than the source code or machine code. This source code/machine code independence is intended to enable generic optimizations to be shared between versions of the compiler supporting different languages and target processors. 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 processor and OS. This front-end/middle/back-end approach makes it possible to combine front ends for different languages with back ends for different CPUs. A practical example of this approach is 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: 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 the back end is done in multiple steps: 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 optimizations 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 optimizations is common in modern commercial compilers from IBM, 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 optimizations, 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 represents a very simple one-pass compiler, written in C. This compiler compiles an expression defined in infix notation to postfix notation and also into an assembly-like machine language. This compiler uses the recursive descent strategy. This strategy is recognisable by the fact that each function corresponds to a non-terminal symbol in a language grammar. char lookahead; int pos; int compile_mode; char expression20+1; void error() void match( char t ) void digit() void term() void expr() int main ( int argc, char A possible execution of this simple compiler results in the following output: Please enter an infix-notated expression with single digits: Compiling to postfix-notated expression: 342 Compiling to assembly-notated machine code: PUSH 3 PUSH 4 PUSH 2 POP B POP A MUL A, B PUSH A POP B POP A SUB A, B PUSH A PUSH 2 POP B POP A ADD A, B PUSH A Notes See also | |||||||
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