I don't really know Microsoft's C compiler well enough to answer why your specific example fails so spectacularly, but I can make a guess: You are using inline

I don't really know Microsoft's C compiler well enough to answer why your specific example fails so spectacularly, but I can make a guess: You are using inline assembly containing a computed jump instruction. In this situation the compiler no longer has a complete view of the control flow graph of your function, and it is likely that all optimizations which need a control flow graph will be disabled. Common subexpression elimination needs a CFG since it determines if two computations are the same for all paths in the program. Now I'm reasonably certain that the C standard does not allow inline assembly like this. However, given the decades of legacy software that Microsoft has to contend with I would not be too surprised if this was part of the reason. By the way, GCC's computed gotos are not actually much better. In this case you get a control flow graph where you cannot split all critical edges. There is a workaround for this, but it's probably easier to just disable all optimizations that depend on it.