Yeah, it's all fun and games until you realize that you compiler may invent writes in order to eliminate a branch. Which is rather nonsensical, when you stop to think about it — surely an additional store to global can't be faster than a mispredicted branch?

Branch mispredictions on deep machines can be devastating, whereas a store can hit in your L1. Storing can definitely be faster than a mispredicted branch if you think you'll hit in your L1/L2

store to a global is typically 0 cycles (although subsequent reads may be slowed down)