How do I programmatically return the max of two integers without using any comparison operators and without using if, else, etc?

Question

How do I programmatically return the maximum of two integers without using any comparison operators and without using if, else, etc?

Answer 1

This is kind of cheating, using assembly language, but it's interesting nonetheless:

// GCC inline assembly
int max(int a, int b)
{
  __asm__("movl %0, %%eax\n\t"   // %eax = a
          "cmpl %%eax, %1\n\t"   // compare a to b
          "cmovg %1, %%eax"      // %eax = b if b>a
         :: "r"(a), "r"(b));
}

If you want to be strict about the rules and say that the cmpl instruction is illegal for this, then the following (less efficient) sequence will work:

int max(int a, int b)
{
  __asm__("movl %0, %%eax\n\t"
      "subl %1, %%eax\n\t"
          "cmovge %0, %%eax\n\t"
          "cmovl %1, %%eax"
         :: "r"(a), "r"(b)
         :"%eax");
}