What Is Compilier

Posted by Tushar

A program that translates an executable

program in one language into an executable program in another language.

we expect the program produced by the

compiler to be better, in some way, than the original

Role of Compilers:

High-level programming languages provides following advantages -

increase programmer productivity

better maintenance

portable

but we require following Low-level machine details too!

instruction selection

addressing modes

pipelines

registers & cache

instruction-level parallelism

The Structure of a Compiler:

Multiple Phases

Modern compilers contain two (large) parts, each of which is often subdivided. These two parts are the front end, shown in green on the right and the back end, shown in pink.

The front end analyzes the source program, determines its constituent parts, and constructs an intermediate representation of the program. Typically the front end is independent of the target language.

The back end synthesizes the target program from the intermediate representation produced by the front end. Typically the back end is independent of the source language.

Advantage of front/back end division

This front/back division very much reduces the work for a compiling system that can handle several (N) source languages and several (M) target languages. Instead of NM compilers, we need N front ends and M back ends. For gcc (originally standing for Gnu C Compiler, but now standing for Gnu Compiler Collection), N=7 and M~30 so the savings is considerable.

The front and back end are themselves each divided into multiple phases. The input to each phase is the output of the previous.

Sometime a phase changes the representation of the input.

For example, the lexical analyser converts a character stream input into a token stream output.

Sometimes the representation is unchanged.

For example, the machine-dependent optimizer transforms target-machine code into (hopefully improved) target-machine code.

Conceptually, there are three phases of analysis with the output of one phase the input of the next. The phases are called lexical analysis or scanning, syntax analysis or parsing, and semantic analysis