如何使用二进制运算有效地执行==？

Question

I am interested in finding a Boolean/arithmetic function that will return -1 if two values match, and 0 if they do not match, in other words the == operation. I know this can be done with a logical NOT, for example:

! (AB)

which will be -1 (= 111111111.... in binary) if A == B. But the problem is that this compiles to a big right shift, like >> 31 which is expensive. I was hoping to find short cycle binary instructions that could achieve the same effect.

How does == compile when used as an R-value, same way as !(AB)?

Answer 1

would this help?

#include <stdio.h>

int compare (int a, int b) {
char res[] = { 0, 0, -1};
char *p = &res[2];
return p[(a - b) ^ (b -a)];
}

main() {
 printf("\nans %d \n",  compare(12435, 12435));
 printf("\nans %d \n",  compare(221, 12435));
 printf("\nans %d \n",  compare(-43221, 12435));

}

BTW You can use a ^ b to get 0 for equal