# Duplicate-implementation consolidation notes

Recorded during the 2026-07-09 wiring/dead-code sweep, updated 2026-07-09
during the deferred "rewrite call sites" pass. This pass audited every site
listed below; most turned out not to need (or not to be eligible for) a
call-site rewrite once actually read in full. Details per item below.

## Mixture of Experts

Canonical: `rtx-transformers/src/layers/mixture_of_experts/` (`MoEConfig`,
`Router`, `Expert`, `MixtureOfExperts`).

- `rtx-transformers/src/layers/metal_moe.rs` (Metal-specific) — **kept, no
  change needed.** Its types already mirror the canonical naming 1:1
  (`MetalMoEConfig`/`MetalExpert`/`MetalRouter`/`MetalMoE` vs.
  `MoEConfig`/`Expert`/`Router`/`MixtureOfExperts`), and its config fields
  (`num_experts`, `top_k`, `hidden_dim`, `expert_hidden_dim`,
  `capacity_factor`, `dropout`, `aux_loss_weight`, `activation`, `bias`) are
  identical to `MoEConfig`'s, plus Metal-only extras (`z_loss_weight`,
  `jitter_noise`, `use_metal`). This is a backend specialization done right;
  forcing a shared struct would just add an indirection layer for no benefit.
- `rtx-transformers/src/modular/router.rs` — **not a duplicate; left as is.**
  This is dynamic *module* selection (`ModuleRouter`, `TaskConditionedRouter`,
  `AttentionRouter`, `RLRouter`) over `Vec<Box<dyn Module>>` for the modular
  neural network system (`modular/modular_network.rs`), not token-to-FFN-expert
  gating. It's wired into `ModuleRouter` trait impls in `modular_network.rs`
  and has its own test suite (`modular_networks_tests.rs`). A genuinely
  different abstraction — MoE's `Router` routes tokens to FFN experts by
  learned gating logits; this routes whole inputs to heterogeneous modules
  (including RL-policy-based and task-conditioned selection). Not merging.
- `rtx-transformers/src/architectures/glam.rs` — **out of scope, dead code.**
  `mod glam` is commented out in `architectures/mod.rs` ("Temporarily
  disabled"), so this file is not part of the build at all. It also does not
  currently compile on its own: `GLaMBlock::forward` references an undefined
  `routing_info` binding (line ~229) — a pre-existing bug unrelated to
  consolidation. Migrating its expert layer to
  `layers::mixture_of_experts::{MixtureOfExperts, MoEConfig, Router}` is the
  right call *when* someone re-enables this module, but doing that migration
  on dead, non-compiling code with zero call sites and no test coverage today
  would be unverifiable busywork. Left as a note for whoever re-enables GLaM.

Deleted in the previous sweep (were orphaned, never declared by `mod`):
`layers/moe_layer.rs`, `layers/moe_integration.rs`.

## Flash Attention

Canonical: the `rtx-flash-attention` crate (v2+v3 kernels, CUDA + Metal,
public API: `flash_attention_forward`/`flash_attention_backward` and the
`FlashAttention`/`FlashAttentionBackend` types over `rtx_tensor::Tensor`).

Re-audited the three non-JEPA sites previously flagged as reimplementations.
None of them actually contained duplicate attention math to delegate. Two of
the three (`tensor_core_kernels.rs`, `orchestrator_core.rs`) have since been
given real execution paths (2026-07-10); see below.

- `src/tensor_core_kernels.rs` — was config/strategy scaffolding only
  (`AttentionConfig`, `SoftmaxOptimizationStrategy`,
  `AttentionComputationOptimizer`) with no `compute`/`forward` method
  anywhere in the file. **Now real:**
  `AttentionComputationOptimizer::execute(&self, q, k, v: &Tensor) -> Result<Tensor>`
  dispatches on `self.softmax_strategy`:
  - `Standard` / `Online` → `Tensor::scaled_dot_product_attention`
    (`rtx_tensor` matmul + softmax + matmul). Online softmax is
    mathematically equivalent to standard softmax (same result, different
    accumulation order) and no separate online-softmax kernel exists here,
    so it intentionally shares the standard path rather than faking a
    distinct computation.
  - `FlashAttention` → delegates to `rtx_flash_attention::flash_attention_forward`
    (the crate's own CUDA/Metal-or-naive-CPU-fallback logic applies
    unchanged).
  - `Approximated` → explicit `Err`: no approximation kernel is implemented,
    so this strategy refuses to silently compute exact attention under an
    "approximated" label.
  - Accepts 3D `[batch, seq, head_dim]` or 4D `[batch, heads, seq, head_dim]`
    q/k/v; other ranks are a clear `Err`.
  - Tests (`tensor_core_kernels::real_execution_tests`, always-on, not
    feature-gated): standard path is finite and input-dependent; flash vs.
    standard agree within 1e-3 on a small `[1,2,4,8]` shape; `Approximated`
    errors; wrong-rank input errors.
- `src/revolutionary/orchestrator_core.rs` — `execute_flash_attention` and
  its siblings `execute_classical`/`execute_hybrid`/`execute_edge`/
  `execute_distributed` were all identical no-op stubs (`Ok(input.clone())`)
  while the scoring/decision machinery around them was real. **Now real:**
  - `execute_classical`: genuine QKV self-attention — deterministic
    fixed-seed weight matrices (`Tensor::randn_seeded`) project the input,
    then `scaled_dot_product_attention`, then an output projection.
    Input-dependent, reproducible, not an identity.
  - `execute_flash_attention`: reshapes `[batch, seq, hidden]` into flash's
    `[batch, heads, seq, head_dim]` convention (8 heads; `Err` if `hidden`
    doesn't divide evenly) and calls
    `rtx_flash_attention::flash_attention_forward` directly.
  - `execute_hybrid`: real composition — `execute_classical`'s projection
    feeds into `execute_flash_attention`'s attention step, not either path
    run alone.
  - `execute_edge` / `execute_distributed`: explicit
    `Err("modality not implemented: ...")` — these need a target-device
    runtime handle / cluster communicator that this orchestrator has no way
    to obtain, so they no longer fake success. This also makes the
    orchestrator's `fallback_modalities` machinery real: `execute_task`
    already retried the next modality on `Err`, but previously nothing ever
    errored, so the fallback path was dead code; now Edge/Distributed
    failures genuinely exercise it.
  - Tests (`orchestrator_core::real_modality_execution_tests`, always-on):
    classical/flash/hybrid produce finite, input-dependent output (not equal
    to the input, different for different inputs, deterministic for the same
    input); flash rejects an indivisible hidden dim; edge/distributed error;
    a decision with `primary_modality: Edge` and
    `fallback_modalities: [Distributed, Classical]` ends up executed via
    Classical with `result.success == true`.
- `src/training/training_loop.rs` — the only mention of `FlashAttention` is a
  doc comment listing it as an example LLM op; no implementation present.

`src/ssl/jepa_gpu.rs` is intentionally **not** in this list: its attention is
part of the fused GPU-resident ViT block written for the JEPA platform, not
a standalone duplicate of `rtx-flash-attention`'s API — the fusion is the
point (avoids materializing intermediate tensors across the ViT block same
as flash attention's inner loop does, but tied to ViT-specific tensor
layouts). Revisit only if `rtx-flash-attention` grows a fused-block API that
covers this shape.

## Speculative decoding

Layering is intentional (documented in `rtx-inference/src/speculative/mod.rs`):
`speculative/` is the orchestration layer (traits/configs/trees);
`rtx-inference/src/medusa.rs` and `src/lookahead.rs` are the concrete
implementations. Not a duplication to remove, but keep the two `MedusaConfig`
types (orchestration vs implementation, re-exported as `MedusaHeadsConfig`)
from drifting.