// Assume a hot loop worth optimizing.
while (...)
{
  ...
  // Protect access to x if multithreaded.
  if (bThreaded) oLock.lock ();

  // Do a lot of work with x here.
  x = f (x);

  // Protect access to x if multithreaded.
  if (bThreaded) oLock.unlock ();
  ...
}

// Since x is used a lot it is kept in some register.
register = x;
while (...)
{
  ...
  if (bThreaded)
  {
    // External call may use x so it is written back.
    x = register;
    oLock.lock ();
    register = x;
  }

  // A lot of work done efficiently with x in register.
  register = f (register);

  if (bThreaded)
  {
    // External call may use x so it is written back.
    x = register;
    oLock.unlock ();
    register = x;
  }
  ...
}
x = register;

Example adjusted from literature, see references.