One-pass vs. two-pass compiler

One-pass and two-pass compilers

There are 2 kinds of compilers:

Two-pass compilers:

The compiler reads the source code once to compile (= translate) the program
In the 1st pass, the compiler will:

In the 2nd pass, the compiler will:

Use the gathered information on the variables and functions to translate the function calls
(E.g., insert the necessary conversion instructions if the data types are mismatched)

One-pass compilers:

The compiler reads the source code once to compile (= translate) the program
In an one-pass compiler, the compiler will:

Advantages and disadvantages of one-pass and two-pass compilers

Two-pass compilers:

The compiler will have available information on all defined functions before the compiler starts translation the function calls
However: the compilation time is longer (because the compiler will read the source program twice)

One-pass compilers:

The compilation time is shorter (because the compiler will read the source program only once)

However: there is one sticky problem with one-pass compilers:

A one-pass compiler will not have the any information on functions that are defined after its function call !!!

Example:

int main( int argc, char *argv[] ) { int a, b; a = 2; b = f( a ); // <---- ?????? // 1. What is the data type of the input parameter ?? // 2. What is the data type of the return value ??? } float f( float x ) // ONLY NOW does a ONE-PASS compiler find out // about the data types !!!!!! { float r; // Define a local variable r = x * x; // Statement return ( r ); // Return statement }

Observe that:

When the one-pass compiler encounters the statement:

the one-pass compiler has not yet processed the corrsponding function definition:

float f( float x ) // ONLY NOW does a ONE-PASS compiler find out // about the data types !!!!!! { float r; // Define a local variable r = x * x; // Statement return ( r ); // Return statement }

Consequently:

The one-pass compiler has no information on:

This is a grave problem that we have to address next !!!

Examples of one-pass and two-pass compilers
- Two-pass compiler:
- One-pass compiler: