video_core: GPU thread split on by default for Switch 2 titles; cheaper front paths

Clean A/B in title 1 (3 rounds each): 1.46x the draws a second of one
thread, frames 17% shorter, no crashes, front register copies equal to
the back's in 57,089 checks. EDEN_NX2_SPLIT=0 runs everything on one
thread, =1 keeps macros on the back.

The front's per-write paths no longer check guarded statics
(GpuSplit::Get, Nx2Profile::Enabled, the skip-same switches) and the
ring's method writes are inline. Notes row 145.

1 files changed, 2 insertions, 1 deletions

diff --git a/NX2_NOTES.md b/NX2_NOTES.md
index e24a87f774..620708a59e 100644
--- a/NX2_NOTES.md
+++ b/NX2_NOTES.md
@@ -206,6 +206,7 @@ With the patched decoder, games 1/3/4 only leave ~60-110 rare SIMD encodings in
 | 142 | GPU thread ~100% busy, host GPU 20-40%: one thread parses, runs macros, sets state and records draws | the work is sequential but only a few points need a result from what came before (semaphore acquires, MME DMA reads) | GPU thread split (babd395, `gpu_split.h`, `EDEN_NX2_SPLIT=1`): engine methods run in order on a second thread fed by a lock-free ring; releases, syncpoints, wait-for-idle, flushes and channel binds are deferred into the ring, only acquires wait (~2,400 drains a second). ~47,400 draws a second against ~44,000 (3 A/B rounds), same per-call costs |
 | 143 | crashes with the split: CommandChunk::Record (null chunk), vkCmdEndRenderPass on the Vulkan worker | two threads recording into the scheduler: (a) the DMA pusher's split-method debug log read guest memory with the cache-flushing ReadBlock on the front; (b) with async presentation, Swapchain::AcquireNextImage -> Scheduler::Wait flushed the main scheduler from the present thread whenever its tick was not submitted yet (pre-existing race, the split made it frequent) | (a) ReadBlockUnsafe; (b) e4523ab: the present thread waits up to 20 ms for the GPU thread to submit, never flushes. Found with `EDEN_NX2_SPLIT_CHECK=1` (stack of any other thread using the scheduler while the back runs) |
 | 144 | macros on the split's front (`EDEN_NX2_SPLIT=2`) need the registers they read before the back has them | the front keeps copies: every write it sends plus what the back changes on its own (constant buffer offset, shadow_scratch[0], macro bind pointer, shadow RAM). `EDEN_NX2_SPLIT_VERIFY=1` found three differences: writes still in the back's method sink at the handover; a multi-word run that changes the shadow RAM mode mid-run; and shadow_state.shadow_ram_control being the shadow copy of register 0x49 itself (a Replay write reads the mode back) | all three modelled: 0 differences in 40,193 checks (~165 million macro calls). Front: macros 128 ns a call (180 on one thread, cache-hot), drops ~55% of 3D state writes as same-value. Back: 50,000-66,500 draws a second against 41,500-47,000 without the split; frames 20.5 / 25.5 ms on two courses where the unsplit runs measured 25.6-40.7 ms. Default stays off until a clean A/B |
+| 145 | is the split worth having on by default | clean A/B, nothing else running, 3 rounds each (profiler on in both): one thread 41,100 / 48,600 / 37,000 draws a second, frames 30.5 / 35.0 / 27.1 ms; `EDEN_NX2_SPLIT=2` 59,200 / 62,400 / 62,700 draws a second, frames 28.6 / 23.3 / 25.1 ms. Throughput 1.46x, frames 17% shorter (the back is now ~92% busy, ~53% of it draw setup); 0 crashes, 0 register differences in 57,089 checks after the front's fast paths (no guarded statics, inline ring writes) | on by default for Switch 2 titles (`EDEN_NX2_SPLIT=0` one thread, `=1` macros on the back). Default run without the profiler: 16.9 ms frames (~59 fps) on a light course |
 
 Run settings used (in `build/bin/user/config/sdl2-config.ini`): `memory_layout_mode=4`
 (12 GB). Debug knobs: `EDEN_NX2_TC_EXPECTED_MIB` / `EDEN_NX2_TC_CRITICAL_MIB` (collector thresholds),
@@ -220,7 +221,7 @@ faulting stack as module+offset, then a minidump), `EDEN_NX2_AUTO_STUB`, `EDEN_N
 `EDEN_NX2_BUFFER_DOWNLOADS=0/1`, `EDEN_NX2_INLINE_DEFER=0`, `EDEN_NX2_TC_MIN_AGE=<frames>` (rows 131, 134, 133),
 `EDEN_NX2_SKIP_SAME=0` (row 135),
 `EDEN_NX2_MME_JIT=0` (row 140),
-`EDEN_NX2_SPLIT=1/2`, `EDEN_NX2_SPLIT_VERIFY=1`, `EDEN_NX2_SPLIT_CHECK=1`, `EDEN_NX2_SPLIT_SKIP=0` (rows 142-144),
+`EDEN_NX2_SPLIT=0/1` (default 2), `EDEN_NX2_SPLIT_VERIFY=1`, `EDEN_NX2_SPLIT_CHECK=1`, `EDEN_NX2_SPLIT_SKIP=0` (rows 142-145),
 `EDEN_NX2_FRAME_CAPTURE=<dir>` (every 120th presented frame as TGA), `EDEN_NX2_PB_CHECK`
 (pushbuffer entries changed between submit and execution), `EDEN_NX2_QUERY_TRACE` (every GPU
 report/semaphore write). `C:\dev\crashrun.sh` runs eden-cli under cdb and prints the crash stack;