What is computer compiler?

   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).[1] The most common reason for wanting to transform 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 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: 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 term compiler-compiler is sometimes used to refer to a parser generator, a tool often used to help create the lexer and parser.

Computer compiler types?
  A compiler may produce code 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 code 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). 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). 

• One-pass compiler, like early compilers for Pascal
The compilation is done in one pass, hence it is very fast.

• Threaded code compiler (or interpreter), like most implementations of FORTH
This kind of compiler can be thought of as a database lookup program. It just replaces given strings in the source with given binary code. The level of this binary code can vary; in fact, some FORTH compilers can compile programs that don't even need an operating system.

• Incremental compiler, like many Lisp systems
Individual functions can be compiled in a run-time environment that also includes interpreted functions. Incremental compilation dates back to 1962 and the first Lisp compiler, and is still used in Common Lisp systems.

• 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
Applications are delivered in bytecode, which is compiled to native machine code just prior to execution

A retargetable compiler is a compiler that can relatively easily be modified to generate code for different CPU architectures. The object code produced by these is frequently of lesser quality than that produced by a compiler developed specifically for a processor. Retargetable compilers are often also cross compilers. GCC is an example of a retargetable compiler.

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