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This isn't an architecture, though. It's an instruction set.

What if the JIT generates new instructions dynamically and initially there are no AVX512 instructions but later on in the process' lifetime there are?

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The instruction set is the major part of an architecture spec from the point of view of an userspace application. You can design the binary header flags to express instruction sub/set when you find that reasonable. In any case, regardless if the instructions are hardcoded to the text or generated during the runtime, you need to communicate the intention to use the specific instruction set to the kernel, so it can schedule your process on appropriate cores, and check if there are such cores present in the first place.

Then it crashes with an illegal instruction exception.

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