Please read this post by CatalystMaker here
Something very important that he says.
The second comment I would like to relate that to is from Tim Sweeney, the lead developer for the UT games. (also found on Beyond3d)
".. any code optimization performed on a function that does not change the resulting value of the function for any argument, is uncontroversially considered a valid optimization. "
That guys is exactly what we did. We optimized the code without changing the end resutl.
Notice that he is quoting Tim Sweeney. Notice that Sweeney says that any function that has the same result as the original for any argument
is a valid optimization. Then notice that CatalystMaker says that is exactly what they did.
Now look at your example. It does not have the same output for every argument. If c = false then you are skipping instructions.