C/C++ most efficient if statement evaluation

Question

X is true nearly 99.9% of the time but I need to handle Y and Z as well. Although the body of the X condition is empty, I'm thinking it should be faster than potentially checking 2 other conditions Y and Z if the X condition is omitted. What do you think?

if (likely(X))
{
}
else if (unlikely(Y))
{
...
}
else if (unlikely(Z))
{
...
}

Answer 1

You might want to know what exactly happens when you use likely or unlikely:
http://kerneltrap.org/node/4705

I would personally write

if (unlikely(!X))
{
  if (unlikely(Y))
  {
  ...
  }
  else if (unlikely(Z))
  {
   ...
  }
}

Which means if x, continue execution, else jump to if body.