Hacker News Reader

Comment by mtklein | Hacker News Reader

IIRC what you’re describing here is indeed what shipped. All the AVX-512 instructions are available for 128-bit, 256-bit and 512-bit registers (xmm, ymm, zmm). If not strictly all, essentially all?

View on HN

Base avx512, on the first phis, did in fact only work for 512-bit registers (it also used a slightly different encoding). A later extension is avx512vl (included on all 'normal' cpus implementing avx512), which adds support for the instructions on smaller vector sizes. But there is no standard mode which allows the hardware to support the instructions only for 256- or 128-bit registers; they must be supported at least for 512-bit registers or else not at all.

Oh! Yeah! I do forget about “base” avx-512 sometimes. No one but HPC folks really ever had any proximity to that. In practice if you’ve got a computer that supports avx-512, it supports VL.

You can microcode them on top of high density SRAM if need be.

The Phi’s inherited that from Larrabee: the processor simply didn’t have any 128/256 or even “real” x87 (fpu). It did have a lot more FMA variants, including an instruction that did “single cycle next step rasterization” (faddsets).

Right, but the issue is you can't "just" offer AVX-512 with a 256-bit vector length. You have to also offer the 512-bit options too, which has costs that your processor vendor may not be willing to pay. So you end up only getting AVX2 support.

Hmmm, there are only two desktop processor vendors. Is AVX-512 at all available on mobile?

From the two desktop vendors AMD has AVX-512 support on all their AM5 CPUs. Intel has support of AVX-512 on all 11th gen CPUs and on some 12th gen CPUs. The supports is there in the silicon on all P-cores in 12/13th gen CPUs, just disabled in microcode.

So AMD and Intel have already paid the cost.

AMD - until AM5 - has not been willing to pay the cost. Intel was not willing to pay the cost for their E-cores. AVX-512 is almost 10 years old at this point and because of adoption issues still can't be relied upon.

Previously Intel had AVX-512 support in 3 generations of mobile CPUs, Cannon Lake U, Ice Lake U, Tiger Lake H/U, but only the last generation had widespread availability.

Starting with Alder Lake, Intel has dropped the AVX-512 support in non-server CPUs.

On the other hand, AMD has just launched their Phoenix mobile CPUs (Ryzen x 7x40 HS or U), which have excellent AVX-512 support.

Reply on news.ycombinator.com