Intel Architecture Day 2021: Alder Lake, Golden Cove, and Gracemont Detailedby Dr. Ian Cutress & Andrei Frumusanu on August 19, 2021 9:00 AM EST
Instruction Sets: Alder Lake Dumps AVX-512 in a BIG Way
One of the big questions we should address here is how the P-cores and E-cores have been adapted to work inside a hybrid design. One of the critical aspects in a hybrid design is if both cores support different levels of instructions. It is possible to build a processor with an unbalanced instruction support, however that requires hardware to trap unsupported instructions and do core migration mid-execution. The simple way to get around this is to ensure that both types of cores have the same level of instruction support. This is what Intel has done in Alder Lake.
In order to get to this point, Intel had to cut down some of the features of its P-core, and improve some features on the E-core. The biggest thing that gets the cut is that Intel is losing AVX-512 support inside Alder Lake. When we say losing support, we mean that the AVX-512 is going to be physically fused off, so even if you ran the processor with the E-cores disabled at boot time, AVX-512 is still disabled.
Intel’s journey with AVX-512 has been long and fragmented. Some workloads can be vectorised – multiple bits of consecutive data all require the same operation, so you can pack them into a single register and perform it all at once with a single instruction. Designed as its third generation of vector instructions (AVX is 128-bit, AVX2 is 256-bit, AVX512 is 512-bit), AVX-512 was initially found on server processors, then mobile, and we found it in the previous version of desktop processors. At the time, Intel stated that by enabling AVX-512 on its processor line from top to bottom, it would encourage greater adoption, and they were leaning hard into this missive.
But that all changes with Alder Lake. Both desktop processors and mobile processors will now have AVX-512 disabled in all scenarios. But the silicon will still be physically present in the core, only because Intel uses the same core in its next generation server processors called Sapphire Rapids. One could argue that if the AVX-512 unit was removed from the desktop cores that they would be a lot smaller, however Intel has disagreed on this point in previous launches. What it means is that for the consumer parts we have some extra dark silicon in the design, which ultimately might help thermals, or absorb defects.
But it does mean that AVX-512 is probably dead for consumers.
Intel isn’t even supporting AVX-512 with a dual-issue AVX2 mode over multiple operations - it simply won’t work on Alder Lake. If AMD’s Zen 4 processors plan to support some form of AVX-512 as has been theorized, even as dual-issue AVX2 operations, we might be in some dystopian processor environment where AMD is the only consumer processor on the market to support AVX-512.
On the E-core side, Gracemont will be Intel’s first Atom processor to support AVX2. In testing with the previous generation Tremont Atom core, at 2.9 GHz it performed similarly to a Haswell 2.9 GHz Celeron processor, i.e. identical in non-AVX2 situations. By adding AVX2, plus fundamental performance increases, we’re told to expect ‘Skylake-like performance’ from the new E-cores. Intel also stated that both the P-core and E-core will be at ‘Haswell-level’ AVX2 support.
By enabling AVX2 on the E-cores, Intel is also integrating support for VNNI instructions for neural network calculations. In the past VNNI (and VNNI2) were built for AVX-512, however this time around Intel has done a version of AVX2-VNNI for both the P-core and E-core designs in Alder Lake. So while AVX-512 might be dead here, at least some of those AI acceleration features are carrying over, albeit in AVX2 form.
For the data center versions of these big cores, Intel does have AVX-512 support and new features for matrix extensions, which we will cover in that section.