用算术右移, 完成逻辑右移; 用逻辑右移, 完成算术右移

main.c

#include <stdio.h>

/*
 * CSAPP exercise 2.63
 * */

/*
 * 用算术右移, 完成逻辑右移
 * */
unsigned srl(unsigned x, int k);

/*
 * 用逻辑右移, 完成算术右移
 * */
int sra(int x, int k);

int main() {
  printf("%x \n", sra(-1, 3));
  printf("%x \n", sra(0x40000000, 3));
  printf("%x \n", sra(0x80000000, 3));

  return 0;
}

unsigned srl(unsigned x, int k) {
  unsigned xsra = (int) x >> k;
  int w = sizeof(int) << 3;
  int right_value = -1 << (w - k);
  return xsra & (~right_value);
}

int sra(int x, int k) {
  int xsrl = (unsigned) x >> k;

  int w = sizeof(int) << 3;
  int mask = -1 << (w - k);

  int m = 1 << (w - 1);
  int most_high_bit = m & x;
  mask &= !most_high_bit - 1;
  return xsrl | mask;
}