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+# KNOWLEDGE: Taking `ndarray::simd` to the Browser
+
+## READ BY:
+- Any contributor wiring a consumer crate to `wasm32-unknown-unknown` and expecting `ndarray::simd` to vectorise there
+- `savant-architect` agent — when a SIMD addition must also hold on the wasm32 backend
+- `sentinel-qa` agent — when auditing a claim of the form "this is SIMD-accelerated in the browser"
+- Any contributor about to *measure* whether a wasm artifact carries SIMD
+
+## P0 TRIGGERS:
+- About to write "the browser gets SIMD" in a PR body → read §"Verifying it, without fooling yourself"
+- A wasm build compiles but the vector path seems absent → read §"Two ways this silently does nothing"
+- About to count SIMD instructions with a text window around a function name → read §"The measurement trap"
+
+---
+
+## Why this doc exists
+
+The polyfill's whole promise is that one source line vectorises on every
+target. On x86 and ARM that promise is nearly free: even without the
+polyfill, an autovectoriser has registers to aim at, so a scalar loop
+often *happens* to become SIMD and nobody notices the difference.
+
+**wasm32 is the target where the promise stops being free.** Without
+`+simd128` there are no vector registers at all, so the autovectoriser
+cannot find anything, no matter how well-shaped the loop is. The browser
+is therefore the one place where "we rely on LLVM" degrades all the way
+to scalar — silently, with no build error and no warning.
+
+That makes wasm both the strongest argument for routing consumer math
+through `ndarray::simd` **and** the easiest place to believe you have
+done so when you have not. This doc is the recipe plus the two ways it
+silently fails, both hit in practice on 2026-08-14 while wiring
+`AdaWorldAPI/a2ui-rs`'s GPU field renderer.
+
+---
+
+## The recipe
+
+### 1. Depend on the fork, with `std`
+
+```toml
+# `std` is the MINIMUM that exposes the lane types: `src/lib.rs` gates the
+# module as `#[cfg(feature = "std")] pub mod simd`. `default-features =
+# false` then keeps `hpc-extras` out, which matters for a wasm payload.
+ndarray = { git = "https://github.com/AdaWorldAPI/ndarray", branch = "master",
+            default-features = false, features = ["std"] }
+```
+
+Two details that cost a build each if guessed:
+
+- **The default branch is `master`, not `main`.** `branch = "main"` fails
+  with `cannot locate remote-tracking branch 'origin/main'`.
+- **`default-features = false` alone removes `simd`.** The module lives
+  behind `std`; without it the import is `unresolved import ndarray::simd`
+  and the cause is not obvious from the error.
+
+For local work against a sibling checkout, redirect the git coordinate in
+`.cargo/config.toml` rather than editing the manifest — a path in the
+manifest ships to CI and container builds, which have no sibling:
+
+```toml
+[patch."https://github.com/AdaWorldAPI/ndarray"]
+ndarray = { path = "../ndarray" }
+```
+
+### 2. Build with `+simd128`
+
+```bash
+RUSTFLAGS='-C target-feature=+simd128' \
+  cargo build --target wasm32-unknown-unknown --release
+```
+
+or use the checked-in profile, which exists for exactly this:
+
+```bash
+cargo build --target wasm32-unknown-unknown --config .cargo/config-wasm.toml
+```
+
+Without the flag the dispatch in `src/simd.rs` falls through to the scalar
+arm — correct results, no vectors. That is a deliberate design (the code
+must still *work* on a browser without SIMD support), which is precisely
+why nothing complains.
+
+### 3. Write against the lane types, not intrinsics
+
+```rust
+use ndarray::simd::F32x16;
+const LANES: usize = 16;
+
+let mut i = 0;
+while i + LANES <= n {
+    let x = F32x16::from_slice(&xs[i..]);
+    let v = (F32x16::from_slice(&vs[i..]) + f / m) * damping;
+    v.copy_to_slice(&mut vs[i..]);
+    i += LANES;
+}
+while i < n {
+    scalar_step(i);   // the tail — and the parity reference, see below
+    i += 1;
+}
+```
+
+The consumer crate can stay `#![forbid(unsafe_code)]`: every intrinsic
+lives behind the polyfill boundary.
+
+### What the wasm backend actually provides
+
+`src/simd.rs` re-exports the v128-backed types from `simd_wasm::wasm32_simd`
+under `all(target_arch = "wasm32", target_feature = "simd128")` —
+`F32x16`, `F64x8`, `I8x16`, `U32x16` and their masks. The long-tail
+integer and 256-bit-shaped types come from the scalar fallback. That is
+the same split `simd_neon` uses on aarch64: native float kernels, scalar
+for the rest. Reaching for `I16x32` on wasm compiles and runs, but it is
+scalar; only the four above are vectors.
+
+---
+
+## Two ways this silently does nothing
+
+Both were hit on the same afternoon, and neither produces a warning.
+
+### (a) The flag is missing
+
+Covered above: scalar fallback, correct results, zero vectors. The tell is
+a SIMD instruction count of exactly **0** in the artifact.
+
+### (b) The code is not in the artifact at all
+
+This one is nastier and is specific to `cdylib` (i.e. every browser build).
+**Only exported items survive the link.** A crate can compile, link, and
+emit a `.wasm` in which the entire subsystem you care about is absent,
+because nothing reachable from a `#[wasm_bindgen]` export calls it.
+
+Measured instance: a field renderer whose client struct carried no
+`#[wasm_bindgen]` attribute at all. The `.wasm` was 1.27 MB, contained
+**2** SIMD instructions and **zero** symbols from the layout module — while
+the same code compiled as an rlib carried **800** in one function. Adding
+the exports took the module to 4.88 MB and **13 375** SIMD instructions.
+
+So `crate-type = ["cdylib", "rlib"]` is necessary and **not sufficient**.
+If a wasm SIMD measurement comes out near zero, check whether the code is
+in the module *before* concluding the flag failed:
+
+```bash
+wasm-objdump -x module.wasm | grep -ci 'your_crate.*your_module'
+```
+
+---
+
+## Verifying it, without fooling yourself
+
+The dispatch is a `cfg`, so it is checkable rather than assumable. Build
+both ways and count:
+
+```bash
+# wabt provides wasm-objdump; llvm-objdump is unusably slow on .wasm
+apt-get install -y wabt
+
+RUSTFLAGS='-C target-feature=+simd128' cargo build --target wasm32-unknown-unknown --release
+wasm-objdump -d target/wasm32-unknown-unknown/release/mod.wasm \
+  | grep -cE 'f32x4|i32x4|v128\.'
+# a large number
+
+cargo build --target wasm32-unknown-unknown --release      # no flag
+wasm-objdump -d ... | grep -cE 'f32x4|i32x4|v128\.'
+# 0
+```
+
+The **zero** in the second run is what makes the first meaningful. A count
+without its contrast proves nothing: 0xFD is also an ordinary data byte, and
+any large module contains vectorised code from dependencies.
+
+### The measurement trap
+
+Do **not** isolate a function by grepping a text window around its name:
+
+```bash
+#  WRONG — /integrate/ matches a string, not a function boundary, and an
+#  rlib is full of ndarray's own vectorised functions. The number can be
+#  almost entirely somebody else's code.
+llvm-objdump -d lib.rlib | awk '/integrate/{f=1} f&&/^$/{f=0} f' | grep -c v128
+```
+
+Ask the symbol table instead:
+
+```bash
+SYM=$(llvm-nm lib.rlib | grep 'Layout9integrate17' | awk '{print $3}')
+llvm-objdump -d --disassemble-symbols="$SYM" lib.rlib | grep -cE 'f32x4|v128\.'
+```
+
+The text-window form was used once here and came out **801** where the
+symbol-scoped answer is **800**. It was right by luck — and a measurement
+that is right by luck is not a measurement. What made the error visible was
+an unrelated contradiction (2 instructions in the shipped module vs 801 in
+the rlib), not the number itself.
+
+---
+
+## Cross-backend semantics — write the test with tolerance
+
+`src/simd_wasm.rs` documents where wasm32 diverges from the other backends.
+Any parity test that must pass on more than one target has to allow for it:
+
+| | wasm32 SIMD128 | scalar fallback | AVX / NEON |
+|---|---|---|---|
+| `mul_add` | `mul`+`add`, two roundings (unless `relaxed-simd`) | fused | fused |
+| `reduce_sum` | balanced tree | sequential fold | tree |
+| `round` | round-half-to-even | half-away-from-zero | half-to-even |
+| `min`/`max` with NaN | IEEE, NaN-propagating | returns the non-NaN operand | varies |
+
+So the honest shape of a vector/scalar parity test is a **relative
+tolerance**, not equality:
+
+```rust
+assert!((a - b).abs() <= 1e-4 * b.abs().max(1.0));
+```
+
+and the scalar reference should be **the same function the tail uses** —
+one definition, so the two cannot drift apart.
+
+Enabling `relaxed-simd` restores a fused `mul_add` via `f32x4_relaxed_madd`,
+at the cost of a target feature not every runtime has. Prefer the tolerance.
+
+---
+
+## Worked consumer
+
+`AdaWorldAPI/a2ui-rs` `crates/a2ui-graph` — a GPU field renderer whose
+per-frame layout integration runs through `F32x16`. Its
+`docs/WASM-INTEGRATION.md` carries the end-to-end recipe including the
+`wasm-bindgen` surface; this doc is the polyfill half.