Just for clarification, by whole thing do you mean the start of the batch of n dispatches, or start of that particular test mode?
Code generation for GCN has in other cases appeared to be more conservative.
Perhaps the heuristics favor smaller footprint for better occupancy and cache presence, since just switching to another wavefront for periodic stalls is what the architecture leans on.
The branch overhead is modest for GCN, and the upshot of the 4-cycle execution loop is that from the software point of view there is generally no forwarding latency.
Other architectures like Maxwell would have different trade-offs, since their time window for resolving their instruction latencies and overheads is significantly shorter, and they have not cut down single-threaded performance quite as much.
Naively, it's utilizing GCN twice as badly as Maxwell, at 1/64 versus 1/32. That seems like something minor enough for the test in question.
If there are other bottlenecks besides it that are being hit, I would like to find them.
I would want to find more cases where things break. The vendors describe things with varying degrees of opacity, and sometimes it's more illuminating to break the things they say work just great when you're not looking.
Unfortunately, we could have a taste test between GPUs and the data would be twisted into a debate on which vendor is more salty.
In games, sure.
There's other software that GCN had ambitions for once upon a time.
It also may have implications as to the hardware and what directions it can take in the future, but the importance of that is a matter of personal preference.
That makes GPUs more honest than most marketing departments.
