MC97 HW 2: Activation Records (Spring 1999)

Due: March 17, 1999

• Do your experiments on one of the department's SGI Indys, and do each experiment twice, once with SGI's own C compiler and once with the GNU C compiler. These compilers are called cc and gcc respectively. For both compilers, you should use the -S option (as mentioned by Appel) to get assembly language output. (This will go in a file named like your C input file but with the ending changed from .c to .s, unless you specify a different output filename using the -o option.) For both compilers, you should use the option -O2 to enable optimization. For gcc, you should additionally use -mgp32 to specify 32-bit code, rather than the default of 64. (This isn't really essential, but will make the code from the two compilers more directly comparable and also make the code more familiar relative to MC48.)
• The code Appel gives for exercise 6.1(c) has the problem that any half-decent compiler (such as these two) will recognize that the variable x isn't used at all, and hence eliminate it entirely. To avoid this, we can pass x rather than y in the first call to f. We can also make the use of x prior to the call more exciting. This results in the following alternative code:

  void g(void);
  void h(int);
  
  int f(int a){
    int b;
    b = a + 1;
    g();
    h(b);
    return b + 2;
  }
  
  void h(int y){
    int x;
    for(x = y + 1; x < 100; x += x)
      ;
    f(x);
    f(2);
  }
  

• In addition to answering the questions Appel poses for each of the two compilers, remark upon any other differences you see in how they use the stack frame and registers to hold values.

Instructor: Max Hailperin