本文共 4400 字，大约阅读时间需要 14 分钟。

P70

#include 
   
    int fits_bits(int x,int n){
       int w=sizeof(int)<<3;    return x == x<<(w-n)>>(w-n);}int main(){
       printf("%d\n",fits_bits(114514,19));}
   

P71

• A
  这个函数不能从word中提取一个负值的byte。
• B

#include 
   
    typedef unsigned packed_t;int xbyte(packed_t word,int bytenum){
       int w = sizeof(unsigned);    int left = (w-bytenum-1)<<3;    int right = (w-1)<<3;    return (int)(word<
    
     >right);}int main(){
       printf("%d\n",xbyte(-114514,0));}
    
   

P72

• A
  sizeof返回的值类型是unsigned，所以maxbytes-sizeof(val)的值也是unsigned类型，大于等于0恒成立。
• B

#include 
   
    void copy_int(int val,void* buf,int maxbytes){
       if(maxbytes-(int)sizeof(val)>=0)        memcpy(buf,(void*)&val,sizeof(val));}int main(){
       int max = 114;    void* buff = malloc(max);    copy_int(514,&buff,114);}
   

P73

#include 
   
    #include 
    
     int saturating_add(int x,int y){
       int sum = x+y;    int mask = INT_MIN;    int pos_over = !(x&mask)&&!(y&mask)&&(sum&mask);    int neg_over = (x&mask)&&(y&mask)&&!(sum&mask);    (pos_over&&(sum=INT_MAX))||(neg_over&&(sum=INT_MIN));//这段是求教大佬得到的，实在精妙    return sum;}int main(){
       printf("%x\n",saturating_add(0x10000000,-1));}
    
   

P74

#include 
   
    #include 
    
     int tsub_ok(int x,int y){
       int sub = x-y;    return !((x>0&&y<0&&sub<0)||(x<0&&y>0&&sub>0));}int main(){
       printf("%d\n",tsub_ok(114514,INT_MIN));}
    
   

P75

#include 
   
    #include 
    
     int signed_high_prod(int x, int y) {
     int64_t mul = (int64_t) x * y;  return mul >> 32;}unsigned unsigned_high_prod(unsigned x, unsigned y) {
     int sig_x = x >> 31;  int sig_y = y >> 31;  int signed_prod = signed_high_prod(x, y);  return signed_prod + x * sig_y + y * sig_x;}unsigned test(unsigned x, unsigned y) {
     uint64_t mul = (uint64_t) x * y;  return mul >> 32;}int main(){
       printf("%d\n",test(114514,1919810)==unsigned_high_prod(114514,1919810));}
    
   

P76

#include 
   
    #include 
    
     #include 
     
      #include 
      
       void* mcalloc(size_t nmemb,size_t size){
       if(nmemb==0||size==0)        return NULL;    int tmp = nmemb*size;        if(nmemb==tmp/size){
   //注意不能与上一行合并写成nmemb==(nmemb*size)/size        void* p = malloc(size);        if(p)            memset(p,0,nmemb);        return p;        }     return NULL; }int main(){
       printf("%d\n",mcalloc(INT_MAX,INT_MAX));}
      
     
    
   

P77

#include 
   
    int mul17(int x){
       return (x<<4)+x;}int mulm7(int x){
       return x-(x<<3);}int mul60(int x){
       return (x<<6)-(x<<2);}int mulm112(int x){
       return (x<<4)-(x<<7);}int main(){
       printf("%d\n",mul17(114514)==17*114514);    printf("%d\n",mulm7(114514)==(-7)*114514);    printf("%d\n",mul60(114514)==60*114514);    printf("%d\n",mulm112(114514)==(-112)*114514);}
   

P78

#include 
   
    #include 
    
     int divide_power2(int x,int k){
       int sig = x&INT_MIN;    (sig&&(x=x+(1<
     
      >k;}int main(){
       printf("%d\n",divide_power2(-114514,3)==(-114514)/8);}
     
    
   

P79

#include 
   
    #include 
    
     int divide_power2(int x,int k){
       int sig = x&INT_MIN;    (sig&&(x=x+(1<
     
      >k;}int mul3div4(int x){
       x = (x<<1)+x;    return divide_power2(x,2);}int main(){
       printf("%d\n",mul3div4(114514)==114514*3/4);}
     
    
   

P80

#include 
   
    #include 
    
     int threeforths(int x) {
     int sig = x & INT_MIN;  int head = x & ~0x3;  int tail = x & 0x3;  int hdd4 = head >> 2;  int hdd4m3 = (hdd4 << 1) + hdd4;  int tlm3 = (tail << 1) + tail;  int bias = (1 << 2) - 1;  (sig && (tlm3 += bias));  int tlm3d4 = tlm3 >> 2;  return hdd4m3 + tlm3d4;}int main(int argc, char* argv[]) {
       printf("%d\n",threeforths(-3)==-2);}
    
   

P81

• A

#include 
   
    int patternGenerator(int k){
       return (-1<
   

• B

#include 
   
    int patternGenerator(int k,int j){
       return (~((-1)<
    
     <
    
   

P82

• A
  不是。x=INT_MIN
• B
  是（即答
• C
  是
• D
  是
• E
  是

P83

• A
  $$\frac{Y}{2^k-1}$$
• B
  a. $$\frac{5}{7}$$
  b.$$\frac{2}{5}$$
  c.$$\frac{19}{63}$$

P84

#include 
   
    unsigned f2u(float x){
       return *(unsigned*)&x;}int float_le(float x,float y){
       unsigned ux = f2u(x);    unsigned uy = f2u(y);    unsigned sx = ux>>31;    unsigned sy = uy>>31;    return (ux<<1==0&&uy<<1==0)||        (sx&&!sy)||        (!sx&&!sy&&ux<=uy)||        (sx&&sy&&ux>=uy);}int main(){
       printf("%d\n",float_le(-0,+0));    printf("%d\n",float_le(+0,-0));}
   

此段代码非原创，参见。

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