Multi-pass compiler

A multi-pass compiler is a type of compiler that processes the source code or abstract syntax tree of a program several times. This is in contrast to a one-pass compiler, which traverses the program only once. Each pass takes the result of the previous pass as the input, and creates an intermediate output. In this way, the (intermediate) code is improved pass by pass, until the final pass produces the final code.

Multi-pass compilers are sometimes called wide compilers, referring to the greater scope of the passes: they can "see" the entire program being compiled, instead of just a small portion of it. The wider scope thus available to these compilers allows better code generation (e.g. smaller code size, faster code) compared to the output of one-pass compilers, at the cost of higher compiler time and memory consumption. In addition, some languages cannot be compiled in a single pass, as a result of their design.

This stage of a multi-pass compiler is to remove irrelevant information from the source program that syntax analysis will not be able to use or interpret. Irrelevant information could include things like comments and white space. In addition to removing the irrelevant information, the lexical analysis determines the lexical tokens of the language. This step means that forward declaration is generally not necessary if a multi-pass compiler is used. This phase is focused on breaking a sequence of characters into tokens with attributes such as kind, type, value, and potentially others, as well.

Syntax analysis is responsible for looking at syntax rules of the language (often as a context-free grammar), and building some intermediate representation of the language. An example of this intermediate representation could be something like an Abstract Syntax Tree or a Directed Acyclic Graph.

Semantic analysis takes the representation made from syntax analysis and applies semantic rules to the representation to make sure that the program meets the semantic rules requirements of the language. For example, in the example below at the stage of semantic analysis if the language required that conditions on if statements were boolean expressions the cond would be type-checked to make sure it would be a valid boolean expression.

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