A proper implementation uses the correct compiler intrinsics and hints to cause such operations to be constant-time. Timing problems are a fairly well-known problem in cryptography and constant-time implementations are often required for safe cryptography.

While downvoted at the time of my response, this answer is correct. The optimizing compiler makes no timing guarantees, so it's not required to use any of the constructs in your second answer at all. If you need guaranteed timing, you cannot use standard C (instead use assembly or compiler extensions).