[AdyaGMD]

What

A draw pass's relaxed end barrier ordered every later dispatch after the pass's vertex and fragment work. So each change from rendering to compute drained the GPU, even when the compute had nothing to do with the pass.

NVN only orders a dispatch after earlier rendering when the game calls nvnCommandBufferBarrier. With DM_NVN_COMPUTE_GAME_BAR (on by default on Macs without mesh-shading hardware, i.e. M1 and M2):

• A draw pass ends without the dispatch stage in its end barrier.
• The copy encoder opened for a dispatch right after it waits for earlier vertex / object / mesh / fragment work only if the game issued a barrier since dispatches were last ordered after rendering (g_cgb_need). Otherwise it waits for earlier dispatches and copies only.
• A copy, any other encoder in between, and the game barrier mode off all keep the full barrier.

Why

A counter-free Metal System Trace of Ocarina of Time's slow spot (Kokiri Forest) on an M1 showed the GPU idle about 25 % of each frame. About 3.5 ms of that was at render → compute changes, 134 µs each, roughly four times a pass → pass change.

Performance

Ocarina of Time (0400486024DEC000), Kokiri Forest, MacBook M1 (8-core GPU). Measured with the mesh compute and light-pass merge from #2413 / #2441 on, in a tree with other local changes. DM_NVN_COMPUTE_GAME_BAR=ab, DM_AB=600, one run, phases 2–9:

	GPU p50 per phase	Mean
On	34.97 / 35.08 / 35.02 / 34.69 ms	34.94 ms
Off	35.98 / 35.11 / 35.13 / 35.67 ms	35.47 ms

About −0.5 ms a frame, and on was lower in every adjacent pair. 68 % of the light opens still waited, because OoT issues a barrier before most of its post-pass compute.

Picture: screenshots at frames 1500 and 2100 (DM_NVN_SHOT) with the switch on, compared against two runs with it off. They differ no more than the two off runs differ from each other (animation), and look right.

Switch (in docs/env_switches.tsv)

• DM_NVN_COMPUTE_GAME_BAR=0|1|ab: on by default on M1 / M2 Macs, off elsewhere.

Not run

• This branch's host in game (the change was measured in the other tree).
• Any test suite.
• iPhone.
• M3 and later.
• Other games.

🤖 Generated with Claude Code