It's worst aspect maybe.
I don’t know enough about high performance microarchitecture design to evaluate that argument confidently, but it seems to make sense to me.
By the time you have an out-of-order core, there is already so much shared state you have to synchronise, and you have a bunch of complex mechanisms for dealing with it. Adding a flags register doesn't really add any more complexity, it's just a small bit of extra state attached to it.
And we already have the solution, it's register renaming. We are already renaming all the GPRs and FPRs, and we are probably also renaming part of fscr (because turns out, RISC-V does have flags for floating point operations), maybe a few other bits of state. So we just use the existing renaming mechanism to rename a bank of flags registers; That single logical shared flags register is actually backed with a bank of non-shared physical flags registers, neatly solving all concerns.
Sure, the renamed flags do take up a bit of die space. But IMO they don't add any extra design complexity, and shouldn't have any performance impact on maximum clock speed.
RISC-V isn't quite as disadvantaged by the lack of flags as some might suggest (and I wouldn’t say the lack of flags is RISC-V’s worst aspect), but there are a few sequences (add-with-carry, some conditional-moves, detecting overflow) where RISC-V is forced to burn an extra instruction or two to deal with the lack of flags, and IMO eliminating that would be worth the cost of slightly more die space.
Also, avoiding the need for dedicated compare-and-branch instructions would free up encoding space for other things (including larger range on branches)
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.
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".
By the time you have an out-of-order core
Doing the deliberate choice of leaving flags out of the core and then using them in the fp ops ext will nudge designers towards "this is probably the point you should think about out-of-order execution"
When you read their stuff, they're constantly overestimating the value of ultra-minimalist CPU designs in the modern context. In fact, they often show little understanding of the real impact of ISA design decisions on implementation complexity, so some of their decisions don't even make sense as minimalist decisions.
To expand on the low value of minimalism: even in trailing edge process nodes, if you're designing something on the scale of a simple single-issue in-order 32-bit RISC core targeting no particular frequency, gates are essentially free. The RISC-V guys are badly out of touch. If minimum gate count mattered as much as they think it does, there would still be a thriving market for 8-bit microcontrollers. Instead, they're steadily losing market share to 32-bitters, even in applications where an 8-bit µC would be more than enough. It's not the 1980s, you don't have to struggle to fit a featureful 32-bit core into a single die anymore, but they're hellbent on relitigating that era's debates.
But the argument in the spec explicitly uses the "added complexity to out-of-order microarchitectures" as a part of the justification for not having conditional move (and flags). It's the most commonly parroted part of the argument (see above) and the part of the argument I'm responding to.
I actually agree with much of the spec's argument. The cost of not having flags is pretty low, the MIPS approach does work pretty well, and it does simply things.
I'm just not sure it was the right trade off, and I strongly disagree with its attempt to use OoO cores as part of the justification.
I regard it as a mistake of RISC-V. The flag register was invented for good reasons, and dropping it is a trade-off I personally do not think is worth the downside.