alt Hacker News> Adding a flags register doesn't really add any more complexity, it's just a small bit of extra state attached to it.
I agree in general, we do however see that the cost of flags isn't free by the fact that most modern Arm processors only support ADCS on half of the ALUs supporting ADD. If it was free/negligible, you would see ADCS support on all ALUs.
Yeah... I more mean that it shouldn't add much design and verification complexity. You are mostly just reusing mechanisms you already need. Nor should it negatively impact FMAX. And I suspect the area cost is reasonably low (but not zero)
The lack of flags on those ALUs probably tells us more about the lengths CPU designers will go for reasonably small savings than it tells us about how much flags actually costs. And it's probably more of a "our metrics never gave us enough justification to even consider adding flags to the extra three ALUs" than "we considered it, but the cost was too high".