Is AMD's inferior geometry throughput only affecting framerate in games with high amounts of tessellation or is it a universal framerate sink? I ask because I'm wondering if primitive shaders being enabled will only help certain games or all games?
High-end GCN GPUs are
generally limited by geometry performance in PC games.
At the same time, Vega 56/64 in particular seem to have an excess of compute performance for games, as seen by the fact that Vega 64 = 56 at ISO clocks, both cards have a hard time scaling >20% above Fiji despite up to 50% higher clocks and their performance-per-clock-per-ALU is below Polaris 10.
(Note: this last one may be because
Polaris 10 has a fixed-function primitive discard accelerator, which Vega doesn't have because supposedly this would be done on the ALUs/NCUs).
AFAICT, all things point to Vega having been designed to use a portion of this overkill ALU throughput to reduce geometry, fillrate and bandwidth demands from raw scene to "viewable" scene output through the use of primitive shaders. Even more if primitive shaders can even partially make use of FP16.
Polaris 10's slide about the effects of the primitive discard accelerator says it all: it reduces geometry bottlenecks, fillrate bottlenecks (MSAA intensive scenes) and "frees internal bandwidth resources" (to which I'd guess it leads to less cache hits)
Now what the primitive shaders could/will do when active is a completely different discussion on when / if ever they will be active.
Plus, just because enabling primitive shaders would decrease bottlenecks on several fronts it doesn't mean we should expect gigantic performance upgrades on all games. But if it increases performance by 10% in general and 15% in some titles then it'll already be a significant difference in benchmarks against the competition, especially in the Vega 64 vs. GTX 1080 case.
The GTX 1070 Ti is also bound to bring a significant leap over the regular 1070 and therefore surpass the Vega 56, but put 10-15% more in the later and the competition could become fierce again.
The HBCC being enabled will likely only bring performance in cases when video memory is overcommitted.
Actually,
there are reports of HBCC giving a ~12% performance uplift in PUBG. Even though the game is most probably not using >8GB of VRAM, it seems HBCC might be scrambling memory in a way that increases effective bandwidth.
So I'm not so sure Vega will be able to catch up to the 1080ti with driver updates, except maybe in titles with high tesselation factors.
I don't think AMD is reasonably expecting Vega 64 to compete with GP102 cards within its lifetime on anything other than "gaming experience" when using Vega+FreeSync vs GP102+VSync. Maybe it will be close in a couple of cases like Wolfenstein 2 that uses instrinsic shaders, FP16 and is
extremely compute dependent: