CS 208 s21 — Learning Block #8

Table of Contents

1 Review

  • Before we dive in, a quick review of some C concepts1 
void swap(long* xp, long* yp) {
    long t0 = *xp;
    long t1 = *yp;
    *xp = t1;
    *yp = t0;
}
  • what type of data are these parameters, what size?
  • what is the effect of this function?
  • How would we call? 
long a = 10;
long b = 42;
swap(???);

2 swap example

  • see the slides here: swap-slides.pdf
  • what happens when you only use one temporary variable in the C implementation of swap?
    • i.e., *xp = *yp instead of lines 2 and 3

3 Practice

  • CSPP practice problems 3.1 (p. 182) and 3.5 (p. 189)
  • Write C code for mystery() 2 
void mystery(long* xp, long* yp, long *zp)
// xp in %rdi, yp in %rsi, zp in %rdx

mystery:
    movq (%rdi), %r8
    movq (%rsi), %rcx
    movq (%rdx), %rax
    movq %r8, (%rsi)
    movq %rcx, (%rdx)
    movq %rax, (%rdi)
    ret

Footnotes:

1
  • what type of data are these parameters, what size? xp and yp are pointers to long. As pointers, they each take 8 bytes.
  • what is the effect of this function? swap switches the values that xp and yp point to. They still point to the same locations in memory, but the values at those locations have been switched.
  • How would we call swap? Use the & operator to get the address of a and b. After the call to swap, a would be 42 and b would be 10. 
long a = 10;
long b = 42;
swap(&a, &b);
2

There are many different C implementations of mystery that would be consistent with the given assembly, here is one possibility: 

void mystery(long *xp, long *yp, long *zp) {
    long t0 = *xp;
    long t1 = *yp;
    long t2 = *zp;
    *yp = t0;
    *zp = t1;
    *xp = t2;
}