bh.cpp

//
// Branch hints benchmark: 30% faster with the right hint (LIKELY=0),
// 20% slower with the wrong one (LIKELY=1), compared to no hint.
//

#include <iostream>
#include <vector>

#ifdef LIKELY
#define hint(x) __builtin_expect((x), LIKELY)
#else
#define hint(x) (x)
#endif

int main() {
  // Fill a large vector with almost all zeroes.
  const int N = 1024 * 1024;
  std::vector<int> vec(N);
  vec[42] = 1;
  // Do some silly computation on this vector.
  int a = 0;
  int b = 0;
  for (int i = 0; i < 1000; i++) {
    for (int j = 1; j < N; j++) {
      if (hint(vec[j])) {
        // This vector entry is non-zero, which almost never happens.
        a += vec[j]; // Use the value to prevent optimizations.
        vec[j] = i; // Add a write to prevent optimizations.
      } else {
        b++;
      }
    }
  }
  std::cout << "a: " << a << '\n';
  std::cout << "b: " << b << '\n';
}