Introduction

Basics

At the most basic definition, the compiler takes as input a program written in some language and produces as its output an equivalent program.
An interpreter takes as input an executable specification and produces as output the result of executing the specification.

Front End: focuses on understanding the source-language program
Back End: focuses on mapping programs to the target machine
The front end encodes the source program in a structure for the later use by the back end. This is called intermediate representation or IR.
- At each point, one representation will have a definite IR.
- IR allows us to have more phases between the front end and the back end
Optimizer: Takes in an IR, and produces a semantically equivalent IR as it’s output. It’s aim is to improve the IR in some way

Aim: to generate a translation for the following code

$a \leftarrow a \times 2 \times b \times c \times d$

The front end determines if the input code is well formed, in terms of both syntax and semantics.

The compiler compares the input program’s structure against a formal definition of the language
This requires a formal and appropriate definition, a mechanism to test the input against the definition and a plan to handle illegal input
Grammar: the source language is a set, usually infinite, of strings defined by some finite set of rules.
Scanner: Scanner takes a stream of characters and converts it into a stream of classified words.
Next, the compiler tries to match the stream of categorized words against the rules that specify syntax for the input language.
Parsing: The process of automatically finding derivations (proof that a statement is valid) is called parsing.

The final task of the front end is to output an IR. For our statement above, the IR becomes

$t_{0} \leftarrow a \times 2$

$t_{1} \leftarrow t_{0} \times b$

$t_{2} \leftarrow t_{1} \times c$

$t_{3} \leftarrow t_{2} \times d$

$t_{3} \leftarrow a$