补码运算是否溢出的规则的实现

最近在看CSAPP/3e

判断补码加法是否溢出

/*Determine whether arguments can be added without overflow*/
int tadd_ok(int x, int y){
 int sum = x+y;
 int neg_over = x < 0 && y < 0 && sum >= 0;
 int pos_over = x >= 0 && y >= && sum < 0;
 return !neg_over && !pos_over;
}

当且仅当x=y= -2^w-1 < 0 时，x+y=0；
当且仅当x+y>2^w-1-1 时，x+y<0.

判断补码乘法是否溢出

/*Determine whether arguments can be multiplied without overflow*/
int tmult_ok(int x, int y){
int p = x*y;
/*Either x is zero, or dividing p by x gives y*/
return !x || p/x == y;
}