[phire]

The question is not so much when the first CPU ships with the instruction, but when the last CPU without it stops being relevant.

It varies from instruction to instruction, but alternative code paths are expensive, and not well supported by compilers, so new instructions tend to go unused (unless you are compiling code with -march=native).

In one way, RISC-V is lucky. It's not that currently widely deployed anywhere, so RVA23 should be picked up as the default target, and anything included in it will have widespread support.

But RVA23 is kind of pulling the door closed after itself. It will probably become the default target that all binary distributions will target for the next decade, and anything that didn't make it into RVA23 will have a hard time gaining adoption.

[brucehoult]

I'm confused. You appear to be against adding new instructions, but also against picking a baseline such as RVA23 and sticking with it for a long time.

Every ISA adds new instructions over time. Exactly the same considerations apply to all of them.

Some Linux distros are still built for original AMD64 spec published in August 2000, while some now require the x86-64-v2 spec defined in 2020 but actually met by CPUs from Nehalem and Jaguar on.

The ARMv8-A ecosystem (other than Apple) seems to have been very reluctant to move past the 8.2 spec published in January 2016, even on the hardware side, and no Linux distro I'm aware of requires anything past original October 2011 ARMv8.0-A spec.

[phire]

I'm not against adding new instructions. I love new instructions, even considered trying to push for a few myself.

What I'm against is the idea that it's easy to add instructions. Or more the idea that it's a good idea to start with the minimum subset of instructions and add them later as needed.

It seems like a good idea; Save yourself some upfront work. Be able to respond to actual real-world needs rather than trying to predict them all in advance. But IMO it just doesn't work in the real world.

The fact that distros get stuck on the older spec is the exact problem that drives me mad, and it's not even their fault. For example, compilers are forced generate some absolute horrid ARMv8.0-A exclusive load/store loops when it comes to atomics, yet there are some excellent atomic instructions right there in ARMv8.1-A, which most ARM SoCs support.

But they can't emit them because that code would then fail on the (substantial) minority of SoCs that are stuck on ARMv8.0-A. So those wonderful instructions end up largely unused on ARMv8 android/linux, simply because they arrived 11 years ago instead of 14 years ago.

At least I can use them on my Mac, or any linux code I compile myself.

-------

There isn't really a solution. Ecosystems getting stuck on increasingly outdated baseline is a necessary evil. It has happened to every single ecosystem to some extent or another, and it will happen to the various RISC-V ecosystems too.

I just disagree with the implication that the RISC-V approach was the right approach [1]. I think ARMv8.0-A did a much better job, including almost all the instructions you need in the very first version, if only they had included proper atomics.

[1] That is, not the right approach for creating a modern, commercially relevant ISA. RISC-V was originally intended as more of an academic ISA, so focusing on minimalism and "RISCness" was probably the best approach for that field.

[brucehoult]

It takes a heck of a lot longer if you wait until all the advanced features are ready before you publish anything at all.

I think RISC-V did pretty well to get everything in RVA23 -- which is more equivalent to ARMv9.0-A than to ARMv8.0-A -- out after RV64GC aka RVA20 in the 2nd half of 2019.

We don't know how long Arm was cooking up ARMv8 in secret before they announced it in 2011. Was it five years? Was it 10? More? It would not surprise me at all if it was kicked off when AMD demonstrated that Itanium was not going to be the only 64 bit future by starting to talk about AMD64 in 1999, publishing the spec in 2001, and shipping Opteron in April 2003 and Athlon64 five months later.

It's pretty hard to do that with an open and community-developed specification. By which I mean impossible.

I can't even imagine the mess if everyone knew RISC-V was being developed from 2015 but no official spec was published until late 2024.

I am sure it would not have the momentum that it has now.

[klelatti]

> We don't know how long Arm was cooking up ARMv8 in secret before they announced it in 2011. Was it five years? Was it 10? More?

Arm says "We started work on ARMv8-A in 2007"

Source: Arm plc Strategic Report 2014