Semantic Analysis in Compiler Design

Semantic Analysis is the third phase of Compiler. Semantic Analysis makes sure that declarations and statements of program are semantically correct. It is a collection of procedures which is called by parser as and when required by grammar. Both syntax tree of previous phase and symbol table are used to check the consistency of the given code. Type checking is an important part of semantic analysis where compiler makes sure that each operator has matching operands.

Semantic Analyzer:
It uses syntax tree and symbol table to check whether the given program is semantically consistent with language definition. It gathers type information and stores it in either syntax tree or symbol table. This type information is subsequently used by compiler during intermediate-code generation.

Semantic Errors:
Errors recognized by semantic analyzer are as follows:



  • Type mismatch
  • Undeclared variables
  • Reserved identifier misuse

Functions of Semantic Analysis:

  1. Type Checking –
    Ensures that data types are used in a way consistent with their definition.
  2. Label Checking –
    A program should contain labels references.
  3. Flow Control Check –
    Keeps a check that control structures are used in a proper manner.(example: no break statement outside a loop)

Example:

float x = 10.1;
float y = x*20; 

In the above example integer 30 will be typecasted to float 30.0 before multiplication, by semantic analyzer.

Static and Dynamic Semantics:

  1. Static Semantics –
    It is named so because of the fact that these are checked at compile time. The static semantics and meaning of program during execution, are indirectly related.
  2. Dynamic Semantic Analysis –
    It defines the meaning of different units of program like expressions and statements. These are checked at runtime unlike static semantics.



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