This is question 3.67 (pg 318) from Book: Computer Systems: A Programmers Perspective (3rd Edition):

For this exercise, we will examine the code generated by GCC for functions that have structures as arguments and return values, and from this see how these language features are typically implemented. The following C code has a function process having structures as argument and return values, and a function eval that calls process:

typedef struct {

int a[2];

int *p;

} strA;

typedef struct {

long u [2];

long q;

} strB;

strB process(strA s) {

strB r;

r.u[0] = s.a [1];

r.u [1] = s.a [0];

r.q = *s.p;

return r;

}

long eval(long x, long y, long z) {

strA s;

s.a [0] = x;

s.a [1] = y;

s.p = &z;

strB r = process(s);

return r.u [0] + r.u[1] + r.9;

}

GCC generates the following code for these two functions:

strB process (strA s)

process:

movq %rdi, %rax

movq 24(%rsp), %rdx

movq (%rdx), %rdx

movq 16(%rsp), %rcx

movq %rcx, (%rdi)

movq 8(%rsp), %rcx

movq %rcx, 8(%rdi)

movq %rdx, 16(%rdi)

ret

long eval (long x, long y, long z)

x in %rdi, y in %rsi, z in %rdx

subq $104, %rsp

movq %rdx, 24(%rsp)

leaq 24(%rsp), %rax

movq %rdi, (%rsp)

movq %rsi, 8(%rsp)

movq %rax, 16(%rsp)

leaq 64(%rsp), %rdi

call process

movq 72(%rsp), %rax

addq 64(%rsp), %rax

addq 80(%rsp), %rax

addq $104, %rsp

ret

A. We can see on line 2 of function eval that it allocates 104 bytes on the stack. Diagram the stack frame for eval. showing the values that it stores on the stack prior to calling process.

B. What value does eval pass in it's call to process ?

C. How does the code for process access the elements of structure argument s ?

D. How does the code for process set the fields of results structure r ?

E. Complete your diagram of the stack frame for eval, showing how eval accesses the elements of structure r following the return from process.

F. What general principles can you discern about how structure values are passed as function arguments and how they are returned as function results ?