test __builtin_expect in GCC

test_builtin_expect_asm.cpp

// Use the following command to compile, -O2 is necessary
// g++ test_builtin_expect_asm.cpp -O2 -S -o test_builtin_expect_asm.S
// You can see that the branch expected will not jump.


#include <cstdlib>
#include <cstring>
#include <cstdio>

#define likely(x)    __builtin_expect(!!(x), 1)
#define unlikely(x)  __builtin_expect(!!(x), 0)

int main(int argc, char *argv[])
{
   int a;

   /* Get the value from somewhere GCC can't optimize */
   a = atoi (argv[1]);

   if (likely (a == 2))
   {
      a++;
      a += argc + atoi(argv[1]);
   }
   else
   {
      a--;
      a += argc + 5;
      printf ("%d\n", a);
   }

   printf ("%d\n", a);

   return 0;
}