Intel buying SiFive would be one of the best things to ever happen to RISC-V.
They bought Alpha to strip it for parts (Sandy bridge looks an awful lot like ev8) and because nobody else could make alpha chips which killed a competitor to x86.
The game has changed.
If they buy SiFive, that encourages even more investment into the ISA. They can’t kill it by doing that and even if that were the real goal, M1 competition would just convince everyone they were lying and working in secret.
What Intel brings to the table is talent and money. They have a ton of amazing developers worth decades of experience. They would think nothing of throwing two or three teams at the problem even if most of that money would be wasted.
Then there’s software. Intel realizes more than anyone just how important good software is to an ecosystem. A couple billion dollars for a thousand man years of developer time is also not much to them in the grand scheme of things. They’ve wasted way more money chasing actually bad ideas (see all their GPU attempts through the years).
ARM is wary of RISC-V, but they’d be actually scared if they knew Intel was about to throw massive amounts money into R&D.
I think Intel would develop extensions and variants on top of RISC-V, and clever techniques for implementing RISC-V, that they would then patent heavily.
This would be terrible for everyone else currently having fun developing RISC-V cores, because they would find the next obvious RISC-V-specific improvements that they think of on their journey would tend to be patented, as Intel got there just before.
After a few of those, the fear would start to discourage people in the open source world from even trying to make big improvements to RISC-V designs - the same way people without funds steer clear of their own x86 and ARM processor designs just to keep safe, even though patents on 20+ year old designs should be all expired by now.
The core of RISC-V is already out there. It includes all the most critical parts and they aren't likely to be patented.
Intel wouldn't own the ISA and be (effectively) the only game in town. If they refused to license at reasonably costs, other companies wouldn't add the feature and it simply wouldn't get much use by developers. This has already happened to some extent with the current duopoly.
Patents also cut both ways. Intel hardly has a monopoly on good ideas. For any idea they want to patent, they have to be very sure that one of the myriad other companies, students, and hobbyists haven't already had the same idea too otherwise their patent will simply be invalidated.
Finally, if Intel has come up with a good ISA extension and is willing to license it to others for a reasonable cost, I think that's a great boon for everyone.
