I'll still need to keep some Intels around for VTune and optimizing deeply for AMD is going to be a chore... But given how the market is moving, I don't think I can avoid making AMD the primary development focus.
Ryzen < 2(!) has a much narrower AVX unit which could easily cause a 50% slowdown.
Branchy code like that of compilers tends to run somewhat slower on Ryzen, though closer to 15% than 50% (comparing AMD and Intel CPUs with otherwise same performance in mixed benchmarks). The latter really depends. Core and Ryzen have different branch predictors, both so complicated that they can't be fully described in documentation anymore. Ryzen 2's first level cache configuration also moved closer to Core's, presumably for software that has been fine-tuned for Core.
Specifically, in both a physics engine solver and some machine learning backwards passes I was working on, there are areas where a little false sharing is likely. It's relatively uncommon and a negligible concern on the smaller Intel chips I tested on, but Zen cores will sometimes have to synchronize with another CCX's caches and it seems to come with a penalty vastly larger than naively expected latency or IF bandwidth.
A fully loaded 2950x ended up getting beat by a 1700x in the solver despite having virtually the same architecture, twice the memory bandwidth, and an observed clocks advantage. Cutting the used thread count down to the same as the 1700x helped a little, but it looked like Windows was scheduling the threads on as many CCXs as possible and it ended up still being slower.
On the upside, the 2950x blasts through friendlier workloads like collision detection and inference with perfectly reasonable scaling.
I'm hoping that the dramatic redesign in Zen 2's memory architecture (unified IO die and whatnot) will help things a little. If not, I'll probably have to rework some stuff.