It's complicated yes, but it doesn't approach the level of thinking about all 20 AVX-512 extensions individually and testing for them.

Basically, on the ground, it is about as complicated as say the few 128 and 256 but extensions: there are only a few sets of functionality you have to care about (2 if you don't care about AI).

It's just that within those groups Intel decided to be very fine grained about the functionality, dividing the instructions among many flags (still, in a logical way).

So instead of the new generation just supporting SSE2, say, it supports 6 new flavors of AVX-512. My claim then is that this doesn't matter much: you can just think of all of those 6 as a unit, AVX-512-ICELAKE or whatever, because there are no CPUs that support a proper subset and there probably never will be (if there is, that's fine - you'll evaluate then if it makes sense for a new codepath).

Maybe I'm not making a good case that this is same :).

Nah, you're making a good case. I think what you're trying to say is to test the CPUID bits for a consistent subset of AVX512 flags and treat it all as one or two clumps. There was always going to be a fallback path for older subsets (SSE2-SSE4.2, AVX1-AVX2) anyways, so punt if it doesn't have all the features in a clump.

Exactly.

I wouldn't start with the CPUID testing though.

It's more like "Why do you care about ISA features"? Usually because you are trying to choose how many code paths to support for runtime ISA-based dispatching, or how many binaries to build when you build multiple versions of a binary (which may include compile-time dispatching).

So for that planning process, you only care about a few clumps. Then your CPUID testing strategy should still test all the required extensions, for completeness, and fall back as usual. Or something like that.