[ama5322]

I understand for overflow exceptions, but I would have expected the cost of a register flag to be zero or near zero, for example, in an integer adder?

It also doesn't look like to me the cost of a software check can always be trivial. It can be for a single operation, but an advantage of an overflow register is that it allows to check for a group of operations as a whole (check/branch just once and abort), which is what is probably practical to do algorithmically. In such scenario switching to software checks for each op and/or bound check the inputs sounds by far not minimal.

[snvzz]

Flags aren't that simple. In a superscalar microarchitecture, then you have a lot more to track re: who set the flag, as the flag is a target of every instruction that can set it.