Btw this is a nice example how this stuff is subtle. My first answer was going to be that the transformation is not allowed, but then I thought about it more.

Btw this is a nice example how this stuff is subtle. My first answer was going to be that the transformation is not allowed, but then I thought about it more. I couldn't get gcc to break it either though. Either it misses the optimization or tries to be conservative with reordering UB with side effects.