I wonder if the issue is actually the cost of calculating the flags.
N and C are basically free (copy of bit 63, and it's carry out), V is an extra gate or two, but Z requires a full 64-bit wide NOR gate.
Or it might be about making the register file holding the flags smaller (only 3R3W, instead of 6R6W), along with simplifying the associated bypass network and routing (the first three ALUs are also the only units that can consume flags).
When I say ooo cpus get flag handling for "essentially free", I'm only actually talking about the complexity of tracking "implicit state", and that it can be done without extra latency. You still need to spend transistors to support renaming flags, and to actually calculate and storing the flags.
I can see an argument for an ISA that got rid of the N and Z flags, but kept C/V (potentially merged into a single flag). You can trivially reconstruct N/Z from the result register, but not C/V.
That seems like a weird thing to save on, for architectures that have expensive but rarely used things like CLZ on all ALUs.
But I hadn't considered the ISA design option of only carry flag, no seperate cmp and branch, before.
Huh, it does have CLZ on all ALUs (And CLZ can actually be used to reduce the cost of checking for all zeros).
So it must be something about limiting the complexity of the flags register file and/or its bypass network.
> But I hadn't considered the ISA design option of only carry flag, no seperate cmp and branch, before.
Well, RISC-V does have a decent point: Most of the time you simply don't need flags. The only area this approach falls apart is detecting overflow or add-with-carry.
So, why not just cover that flaw by adding a carry/overflow flag.
The counter-argument is that as soon has you "ruin the purity of the design" by adding one flag, you might as well add the complete set of all four. Then you can simply branch instruction encodings, and use the extra bits for something else (like seperate set-flags versions of all the ALU instructions)
Designing an ISA arround carry flags certainly gives you many goodies: cmov, adc, ccmp, larger branch range