HOT site: `train_step_gpu` was uploading the 4-byte Adam step counter via
`stream.memcpy_htod(&[step], &mut scratch.adam_t)` on every backward pass.
Promoted `DtScratch.adam_t` from `CudaSlice<i32>[1]` to `MappedI32Buffer(1)`
allocated once in `alloc_dt_scratch`. Per-step CPU writes go through
`host_ptr`; the Adam kernel reads via `dev_ptr` u64. Trivial fix, massive
frequency — runs once per gradient step.
Other sites:
- L478 ctor params init: `params` is mutated by Adam (must stay CudaSlice).
Upload via mapped-pinned staging + `memcpy_dtod_async` through new
helper `upload_host_to_cuda_f32`.
- L501 ctor wd_mask init: wd_mask is read-only by Adam. Promoted to
`MappedF32Buffer`; CPU writes 1.0s through host_ptr, kernel reads dev_ptr.
Caller switched from `wd_mask.raw_ptr()` to `wd_mask.dev_ptr`.
- L1105 trajectory build episode_starts: one-time, replaced `clone_htod`
with `MappedI32Buffer` (direct host write, kernel read via dev_ptr).
- L1147 trajectory build ep_rewards: one-time, replaced `htod_f32` with
`MappedF32Buffer` (direct host write, return_to_go reads via dev_ptr).
Also added `#[allow(missing_debug_implementations)]` to MappedI32Buffer
and MappedF32Buffer in `mapped_pinned.rs` — raw pointers + CUdeviceptr
have no useful Debug impl, and the workspace `missing_debug_implementations`
lint now reaches them through the public DT type that holds them.
Audit doc: docs/dqn-gpu-hot-path-audit.md updated.
cargo check -p ml --lib: 11 warnings (down from 13 baseline; net-zero
regression, 2 minor unrelated warnings vanished as a side-effect).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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