MCS-388 Lab 5: Procedures (Spring 2009)

Due: May 4, 2009


You will extend your compiler to allow procedures to be more flexibly defined and called. Rather than your program containing only a single zero-argument procedure (ordinarily main), it will be able to contain any number of procedures, each expecting any number of arguments. Rather than only being able to call one-argument procedures (such as println), you will be able to call procedures with any number of arguments. Thus, you will be able to write interesting multi-procedure programs. As a side effect, the library won't be needed any more.

The basics

Remember to test your work frequently. You don't need to complete this whole section in order to conduct useful tests; each paragraph can be tested.

Change the syntax of a program so that there can be any number of procedure definitions.

Because each procedure call still passes in one argument, it would be nice if each procedure were expecting an argument. The syntax of a procedure definition should no longer have an empty pair of parentheses. Instead, include the keyword int and a variable name. Your LLVM code should allocate stack space, just like for any other local variable, and store the argument into that variable. Here is an example of how the LLVM code for a typical procedure definition might start:

define i32 @factorial(i32 %t0) {
    %t1 = alloca i32 ; parameter n
    store i32 %t0, i32* %t1

If you didn't take Lab 2's invitation to add return statements, now would be a good time to do so. With your own procedures, having them always return 0 would be rather boring.

At this point, you can eliminate the library, because the definitions of print and println are now within the language your compiler can process. (For that to be literally true, you need to have taken Lab 2's suggestion of allowing character constants. If you skipped that, you can give your compiler modified definitions of print and println in which each character constant is replaced with the corresponding decimal code number. The command man ascii will let you find those numbers.) If you need help with the build.xml aspects of eliminating the library, I'd be glad to assist. For the moment you'll still need the LibraryDeclarations class to provide a declaration of the standard putchar procedure.

Allowing procedure declarations, not just definitions

Languages like C conventionally provide procedure declarations, such as

int putchar(int c);

as well as procure definitions, such as

int square(int n){return n*n;}

By making your compiler accept any list of zero or more items, each of which is either a procedure declaration or a procedure definition, you'll be able to totally eliminate LibraryDeclarations. If someone wants to use putchar, they can jolly well declare it themself.

Allowing more or fewer arguments

Change procedure calls, procedure definitions, and procedure declarations to allow any number of arguments. In addition to allowing you to write more interesting procedures yourself, this opens up more of the standard C library for your use. If you find yourself wanting to use a test program that reads input, you'll need to modify build.xml. If you don't find ant's documentation readable, I can give you a hand with this.

Checking calls, declarations, and definitions

It would be nice if your compiler enforced the following rules, which help prevent programming errors:


Write a lab report highlighting the differences from the code for your previous compiler. You should also describe the tests you ran and any problems uncovered.

Course web site:
Instructor: Max Hailperin <>