[brucehoult]

Rotate and popcount are very specialised instructions. The vast majority of software doesn't use them at all, or uses them so infrequently that a software implementation is fine.

You are confusing embedded applications, which have huge flexibility with RISC-V, and standard operating systems with packaged software.

For the next few years (5?) standard operating systems have to support exactly two choices:

- RV64GC

- RVA22

RVA22 includes all the bit manipulation instructions, vectors, cache management, scalar crypto, and some other stuff. You can't pick and choose -- you have to support it all.

If you are making an embedded appliance on the other hand you can pick and choose exactly what extensions you need (a huge number of combinations, as you say), specify a core with exactly those extensions, build a chip around that with the other IP blocks you need, and tell your compiler which extensions you have. You compile all your software yourself, whether bare metal, using an RTOS, or a minimal Linux such as builtroot or yocto. There is zero confusion because you know what you have and you have what you need -- no more and no less.

No one who knows what they are talking about is talking about fusing five-instruction sequences. That's a total red herring.

[ncmncm]

You appear to have an idealized view of embedded development. High-profile and high-volume products may get custom silicon, but most designs are driven by per-part price, which means in practice piggybacking on manufacturing runs for highest-volume products of other companies, with effectively no choices.

The assertion that rotate and popcount instructions are unimportant is false. All compilers peephole-optimize to generate rotate instructions where supported, and not because nobody needs that. There is a long history of mis-estimating instructions and their importance, going back to optimizing an instruction used only in a kernel idle loop.

A more objective measure is to note how often a neglected instruction has needed to be added after the first ISA version shipped, because its lack handicapped the chips on the market. Popcount wins that race everywhere: always neglected, always added. Its neglect reveals the blinders of the CS academics who do the initial ISA designs, and the need to patch reveals the reality.

The importance of an instruction is poorly represented by both its static frequency and by its total execution frequency for the same reason as that idle-loop instruction was miscounted: the importance of lines of code varies by many orders of magnitude, and there is no way to measure importance when counting. It is easy to prioritize instructions used in signature benchmarks, but they are a cracked mirror.

The market is another cracked mirror: it takes a very large signal to penetrate it. Any that does merits attention.

[brucehoult]

The embedded market using custom silicon is by far the shipment volume and dollar value leader. That's why that's what RISC-V vendors have been concentrating on at first.

The part of the embedded market that uses off the shelf chips may have more designs, but they are each low volume enough that they are dominated by engineering time (and thus employ more engineers, who are vocal online) not by saving a few cents on an MCU that doesn't have the instruction you want.

The packaging and testing and stocking costs of a chip with nothing more than a generic MCU inside it are such a high proportion of the cost compared to the actual die that it would be silly to leave any available low silicon cost extensions out, unless done for monopolistic market segmentation reasons e.g. you can't buy a Cortex M0+ with an FPU at any price because ARM would prefer to sell you an M4F for much more money.

The RISC-V market with many vendors with many cores is not prone to such artificial market segmentation.

If you want the equivalent of an M0 (short pipe, no cache, no branch prediction etc) but with an FPU or with 64 bits or with a vector unit then RISC-V vendors say "sure, no problem".

[ncmncm]

Idealizing, again.

All of that is fine for somebody designing for a million-unit SSD, who doesn't need to read any of this, and for chip vendors selling to that person.

But for each such somebody, literally thousands are stuck with whatever chip purchasing says they can get cheap enough off the shelf. Those chips will be exactly the ones that somebody else ordered 100M of without considering for even a second what the thousands of others whose experience they dictate need.

And, it remains a fact that none of the RISC-V MCU chips I can buy off the shelf have any of the B extension instructions implemented.

[brucehoult]

It seems to be you who has an idealised and unrealistic idea of the time and complexity of producing specifications and going from specification to a taped-out chip to a chip you can buy at Digikey. Most companies do this whole process behind closed doors and the first you hear about new features is when it is announced that the products incorporating them are in stores today or next month.

As RISC-V extensions are developed using a cooperative process between domain experts at dozens of different companies and educational or scientific institutions it is obviously impossible and unproductive to attempt to do this in secret.

You can't buy a chip with the B extension because the B extension isn't ratified yet so anyone who claimed to make a B-compatible chip would be taking a risk that the spec might change incompatibly before ratification. The spec was frozen in June and the 45 day Public Comment phase was held in June/July. As far as I know, no issues were raised. The extension (actually three of them, covering different areas) will be ratified before the end of the year, along with several others including V.

That's obviously a very short time scale compared to making a chip. Any chip you can buy now would have been taped out in 2019 or the first part of 2020.

RISC-V is very new. If you want to ignore it until 2023 then feel free. Others find it useful how it is now.