Why why why? hmm. Some other way of delivering the same value that fits better with their overall strategy? like pushing GPU capabilities instead? Or… something else? Strange.
Four possibilities: 1) Intel wants to make AVX-512 a Xeon extension only. 2) Intel has figured out that the complexity (time and silicon area) cost of AVX-512 isn’t worth it. That’s what AMD says, but it may be an effect of cache sizes data pathway widths. 3) Intel has a new set of SIMD extensions they want to field, which are incompatible with AVX-512. 4) It actually doesn’t work, but Intel doesn’t want to do a massive recall.
The first seems possible. The second? Whether or not AVX-512 as such is a benefit is a close call. Yes, there are other instructions within AVX-512, but the headline operation is multiply and accumulate (MAC or FMAC). Using multiple registers to do Ay+Z where A and Y are two or four wide times the SIMD factor, so up to 16 rows of your actual (double precision) arguments. (Complex number arrays are something I won’t go into here.) If you are doing trigonometric or exponential computations fine. But simple arithmetic, you jam up against the bandwidth from L1D to registers. AMD just upped it from two reads and one write to three reads and two writes except with 256-bit wide values. I expect AMD to eliminate that restriction with Zen 4. Intel could be planning to increase that even further than in Alder Lake.
Option three is unlikely. Not the new instructions part, but the incompatibility. That can always be finessed with a feature bit, that when enabled, disables AVX-512. Option four is probably the winner. Imagine that the last bits of the second argument occasionally get trashed. It could happen only when an interrupt occurs or with denormalized numbers. Or just for certain bit patterns. Since most floating multiplies round away any effect of the last bytes of either argument, it would take work to find, and not affect most computations.
If that case is true, Intel had two choices, one a huge recall, and possibly some SKUs that couldn’t be easily replaced. The other choice? Say Alder Lake doesn’t support AVX-512 and make it clear that it takes unsupported BIOS revisions or whatever to activate it.
That last is my bet. If anyone with access to an Alder Lake CPU in a system with AVX-512 implemented, I’d be happy to tell you how to test it.