A compiler is a computer program (or set of programs) that transforms source code written in a programming language (the source language) into another computer language (the target language, often having a binary form known as object code). The most common reason for converting a source code is to create an executable program.
The name “compiler” is primarily used for programs that translate source code from a high-level programming language to a lower level language (e.g., assembly language or machine code). If the compiled program can run on a computer whose CPU or operating system is different from the one on which the compiler runs, the compiler is known as a cross-compiler.
A compiler is likely to perform many or all of the following operations: lexical analysis, preprocessing,parsing, semantic analysis (Syntax-directed translation), code generation, and code optimization. Program faults caused by incorrect compiler behavior can be very difficult to track down and work around; therefore, compiler implementors invest significant effort to ensure compiler correctness.
The first compiler was developed by Grace Hopper while working on the Harvard Mark I computer. Today, most high-level languages will include their own compiler or have toolkits available that can be used to compile the program. A good example of a compiler used withJava is Eclipse and an example of a compiler used with C and C++ is the gcc command. Depending on how big the program is it should take a few seconds or minutes to compile and if no errors are encountered while being compiled an executable file is created.
Note: Scripting languages like Perl and PHP do not need to be compiled.
Structure of a compiler
Compilers bridge source programs in high-level languages with the underlying hardware. A compiler verifies code syntax, generates efficient object code, performs run-time organization, and formats the output according to assembler and linker conventions. A compiler consists of:
- The front end: Verifies syntax and semantics, and generates an intermediate representation or IR of the source code for processing by the middle-end. Performs type checking by collecting type information. Generates errors and warning, if any, in a useful way. Aspects of the front end include lexical analysis, syntax analysis, and semantic analysis.
- The middle end: Performs optimizations, including removal of useless or unreachable code, discovery and propagation of constant values, relocation of computation to a less frequently executed place (e.g., out of a loop), or specialization of computation based on the context. Generates another IR for the backend.
- The back end: Generates the assembly code, performing register allocation in process. (Assigns processor registers for the program variables where possible.) Optimizes target code utilization of the hardware by figuring out how to keep parallel execution units busy, filling delay slots. Although most algorithms for optimization are in NP, heuristic techniques are well-developed.
The output of some compilers may target computer hardware at a very low level, for example a Field Programmable Gate Array (FPGA) or structuredApplication-specific integrated circuit (ASIC). Such compilers are said to be hardware compilers or synthesis tools because the source code they compile effectively controls the final configuration of the hardware and how it operates; the output of the compilation is not instructions that are executed in sequence – only an interconnection of transistors or lookup tables. For example, XST is the Xilinx Synthesis Tool used for configuring FPGAs. Similar tools are available from Altera, Synplicity, Synopsys and other vendors.
Courtesy: Wikipedia , computerhope.com