The 2-pass compilers

Two-pass compilers:

  • A 2-pass compiler reads the source code twice to compile (= translate) a program

  • In the 1st pass, the compiler will:

      • Gather the type information of all variables and functions

  • In the 2nd pass, the compiler will:

      • Use the type information on the variables and functions to translate the computer program

  • Advantage of a 2-pass compiler:

    • Programmers can define variables after the statements that use the variable

Example of a two-pass compiler: Java
 

  • The Java compiler is a 2-pass compiler

Evidence: the Java compiler can compile this code: 

  public class demo
  {
     public static void main(String[] args)
     {
       System.out.println(x); // How can Java compiler know    
                              // which overloaded method to call ? 
     }                // Answer: read input file twice !

     static int x = 1234;
  }

DEMO:   /home/cs255001/demo/C/set1/java2pass.java

The 1-pass compilers

One-pass compilers:

  • A 1-pass compiler reads the source code once to compile (= translate) the program

  • In the one-pass compiler, the compiler will:

      • Gather the definition of all variables and functions while it is translating the program !!!

Notable fact about 1 pass compilers:

  • All variables must be defined before a statement that uses the variables

Example of a one-pass compiler: C
 

  • The C compiler is a 1-pass compiler

Evidence: the C compiler can compile this code: 

  #include <stdio.h>

  int x = 1234;

  int main( )
  {
     printf("%d\n", x);
  }

DEMO:   /home/cs255001/demo/c/set1/c-1pass.c

Example of a one-pass compiler: C
 

  • The C compiler is a 1-pass compiler

Evidence: the C compiler can not compile this code: 

  #include <stdio.h>

  int main( )
  {
     printf("%d\n", x);  // <-- error !
  }

  int x = 1234;

Reason: the data type of x is not (yet) known when the C compiler processes the highlight line !!

Pros and cons of 1-pass and 2-pass compilers
 

  • 2-pass compilers (e.g.: Java):

      • Pro:   convenience for programmers
      • Con:   longer compilation time

  • 1-pass compilers (e.g.: C):

      • Pro:   shorter compilation time
      • Con:   clumbersome:

        • Specifically: the C language needs to use declarations to convey the data type information of (global) variables and function prototypes