AMD's Zen 6 Family: Feeding the Beast

The instruction level parallelism in the 2-ahead branch prediction is super cool stuff, I know zen5 had this feature but it's kind of crazy that memory is so slow relative to the cores that this complexity is beneficial and worthwhile.

> AMD has reduced the number of integer multiply units from three to two, with PDEP and PEXT throughput also falling from three instructions per cycle to two.

Are PDEP and PEXT using the same (parts of) circuits as multiply (they are very different operations...), or are this just two independent drops of circuitry and the wording is just a bit unclear?

32 cores is a lot of cores in a CCD. Is there a there any smaller logical unit between a CCD and a core, or are you expected to wrangle 32 cores in your lowest NUMA level?

What looks like a lot of small balanced improvements to the core itself. Four vector loads per cycle at 256b of less, I wonder if the gather performance is finally acceptable? Another balance example, they actually reduced the number of integer multipliers down to two. I wonder if that was to make diespace for the vnni pipes?

The real star looks like the c cores though. 32 C cores per chiplet with increased l3 cache is impressive

Seems like they are starting to optimize the C core architecture already. Wonder how long it'll be before it becomes its own line up and branding.

I was going to be sad than Zen 6c was divergent, but they increased a bunch of OoO structures on Zen 6c? To absorb more potential latency, since the memory is going to be heavier loaded? Ok, that I can appreciate!

It's absolutely wild that adjusting some l3 and using a smaller node goes from 12 to 32 cores for basically the same chip.

I wonder if the CCD will remain as monolithic as it does, for how long? The l3 latency being signficantly higher for 6c (10.3ns zen5 -> 10.7ns z6 -> 16.4ns z6c) feels like a symptom of a broader scaling challenge as the CCD hosts more and more. I can imagine dividing zen 7 or zen8 CCD into partitions, perhaps?

One thing I wonder about is that these CPUs are just so much faster than their memory controllers. So why not add more? Why don't they have a memory controller per core, for example?

It is hilarious that the SPEC2026 graph does not include apple's P-cores. And no, not because they are ARM and not x86, since Graviton is included. It is because their integer & FP scores of 9.8 & 13 respectively absolutely wipe out everything else on this chart.