Phases of a Compiler

Prerequisite – Introduction of Compiler design

We basically have two phases of compilers, namely Analysis phase and Synthesis phase. Analysis phase creates an intermediate representation from the given source code. Synthesis phase creates an equivalent target program from the intermediate representation.

Symbol Table – It is a data structure being used and maintained by the compiler, consists all the identifier’s name along with their types. It helps the compiler to function smoothly by finding the identifiers quickly.

The analysis of a source program is divided into mainly three phases. They are:



  1. Linear Analysis-
    This involves a scanning phase where the stream of characters are read from left to right. It is then then grouped into various tokens having a collective meaning.
  2. Hierarchical Analysis-
    In this analysis phase,based on a collective meaning, the tokens are categorized hierarchically into nested groups.
  3. Semantic Analysis-
    This phase is used to check whether the components of the source program are meaningful or not.

The compiler has two modules namely front end and back end. Front-end constitutes of the Lexical analyzer, semantic analyzer, syntax analyzer and intermediate code generator. And the rest are assembled to form the back end.

  1. Lexical Analyzer
    It is also called scanner. It takes the output of preprocessor (which performs file inclusion and macro expansion) as the input which is in pure high level language. It reads the characters from source program and groups them into lexemes (sequence of characters that “go together”). Each lexeme corresponds to a token. Tokens are defined by regular expressions which are understood by the lexical analyzer. It also removes lexical errors (for e.g., erroneous characters), comments and white space.
  2. Syntax Analyzer It is sometimes called as parser. It constructs the parse tree. It takes all the tokens one by one and uses Context Free Grammar to construct the parse tree.

    Why Grammar ?
    The rules of programming can be entirely represented in some few productions. Using these productions we can represent what the program actually is. The input has to be checked whether it is in the desired format or not.
    The parse tree is also called the derivation tree.Parse trees are generally constructed to check for ambiguity in the given grammar. There are certain rules associated with the derivation tree.

    • Any identifier is an expression
    • Any number can be called an expression
    • Performing any operations in the given expression will always result in an expression. For example,the sum of two expressions is also an expression.
    • The parse tree can be compressed to form a syntax tree
    • Syntax error can be detected at this level if the input is not in accordance with the grammar.

    • Semantic Analyzer – It verifies the parse tree, whether it’s meaningful or not. It furthermore produces a verified parse tree. It also does type checking, Label checking and Flow control checking.
    • Intermediate Code Generator It generates intermediate code, that is a form which can be readily executed by machine We have many popular intermediate codes. Example – Three address code etc. Intermediate code is converted to machine language using the last two phases which are platform dependent.

      Till intermediate code, it is same for every compiler out there, but after that, it depends on the platform. To build a new compiler we don’t need to build it from scratch. We can take the intermediate code from the already existing compiler and build the last two parts.

    • Code Optimizer It transforms the code so that it consumes fewer resources and produces more speed. The meaning of the code being transformed is not altered. Optimisation can be categorized into two types: machine dependent and machine independent.
    • Target Code Generator – The main purpose of Target Code generator is to write a code that the machine can understand and also register allocation, instruction selection etc. The output is dependent on the type of assembler. This is the final stage of compilation. The optimized code is converted into relocatable machine code which then forms the input to the linker and loader.

All these six phases are associated with the symbol table manager and error handler as shown in the above block diagram.

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