All the x86 extensions are freely implementable by any constructor. There's some stuff like SSE4a which is not available on Intel processors, and others that Intel has implemented and AMD chose not to. But they are published as extensions and part of the standard.
“x86-64/AMD64 was solely developed by AMD. AMD holds patents on techniques used in AMD64; those patents must be licensed from AMD in order to implement AMD64. Intel entered into a cross-licensing agreement with AMD, licensing to AMD their patents on existing x86 techniques, and licensing from AMD their patents on techniques used in x86-64. In 2009, AMD and Intel settled several lawsuits and cross-licensing disagreements, extending their cross-licensing agreements.”
⇒ I think only Intel and AMD currently can freely implement x64.
“However, there have been reports that some companies may try to emulate Intel’s proprietary x86 ISA without Intel’s authorization. Emulation is not a new technology, and Transmeta was notably the last company to claim to have produced a compatible x86 processor using emulation (“code morphing”) techniques. Intel enforced patents relating to SIMD instruction set enhancements against Transmeta’s x86 implementation even though it used emulation.”
It was a message to Microsoft in 2017. The patents for x86-64 and SSE2 should have all expired since then. I don't think most software needs SSE3/4/AVX.
I believe you misunderstood the comment. Intel also has undocumented extensions and functionality, that requires reverse engineering. It’s exactly the same as the vendor specific cases here. You were thinking of documented and well-specified vendor-specific extensions, but I don’t think that’s the main concern.
I don't see you complaining about ASML being the only way of getting photolithography machines, or TSMC being basically the only constructor at scale.
Some industries are awfully complex and getting into it requires insane amounts of work. x86 processor making is one of these. It's the same as making new browser engines/js JIT/etc. There is just so much work that catching up with the incumbents is almost impossible.
I'm not complaining about anything, just saying there are two companies that have licenses to make x86¹, so it's not true that anybody could implement a cpu with sse4 extensions.
¹Not sure what happened to that x86 license that Cyrix had.
