agent/algorithms.md

# Restructure the maths before optimizing the code

The biggest wins are algorithmic. A well-tiled implementation of the wrong algorithm loses to a
restructured one. Work through this before touching tile sizes.

## Questions to ask

1. **Can I avoid materializing the large intermediate?** (attention's N×N score matrix)
2. **Can a sequential dependence become an associative reduction?** (scan, recurrence)
3. **Can I trade compute for memory, or recompute instead of store?** (checkpointing, backward recompute)
4. **Can I change what stays resident?** (block the loop so the working set fits smem/registers)
5. **Does the output have structure the algorithm ignores?** (sparsity, low rank, symmetry, banding)

## Catalog

| transform | what it buys | seen in |
|---|---|---|
| **online softmax** (running max + rescaled sum) | O(N²)→O(N) memory, single pass | FlashAttention |
| **chunked/blocked recurrence** | turns a sequential scan into a sequence of GEMMs | Mamba-2 SSD, Gated DeltaNet |
| **WY representation** | expresses a product of Householder-like updates as one matmul | DeltaNet family |
| **associative / Blelloch scan** | parallelizes a prefix dependence | cumsum, linear attention |
| **radix partitioning** | avoids a full sort for top-k / grouping | radix top-k, MoE token permute |
| **counting sort + exclusive scan** | stable grouping in two passes, no comparison sort | MoE dispatch |
| **low-rank factorization** | shrinks a GEMM's K | LoRA, MLA's compressed KV, PowerSGD |
| **absorb / fold weights** | removes an entire GEMM by folding it into a neighbour | MLA V-absorb |
| **split-K / stream-K** | fills the machine when M·N is too small for the SM count | skinny GEMMs |
| **recompute in backward** | trades FLOPs for HBM traffic | activation checkpointing |
| **two-pass → one-pass** | fuse statistics with the transform | fused LayerNorm/RMSNorm |
| **persistent kernel** | removes launch and pipeline-drain cost between stages | megakernels |
| **symmetry exploitation** | halves the work | `X @ X.T` is symmetric (Muon/Newton-Schulz) |

## Sanity check before implementing

Compute the arithmetic intensity of the *restructured* algorithm and re-run the roofline. If the
restructure does not move you toward the compute-bound side (or cut absolute traffic), it is not
actually a win — write it down in the ledger and move on.