@@ -0,0 +1,16 @@
+# x86-64-v2 (SSE2 through SSE4.2, no AVX) — the scalar realization, which LLVM
+# vectorizes to SSE.
+#
+# Usage:
+#   env -u RUSTFLAGS cargo --config .cargo/config-v2.toml <cmd>
+#   bash scripts/masking-parity.sh v2-qemu       (runs under qemu -cpu Nehalem)
+#
+# Selects the arm `all(target_arch = "x86_64", target_feature = "sse4.2",
+# not(target_feature = "avx"))`: Nehalem / Westmere, Silvermont-class Atom,
+# VMs that hide AVX. `simd.rs` routes it to `simd_scalar.rs`. Plan:
+# `.claude/plans/simd-sse-v2-tier-v1.md`. Overlay semantics as in
+# `config-v3.toml`: only the target-cpu is set here; the crypto cfgs come from
+# `.cargo/config.toml` through cargo's rustflags join. `env -u RUSTFLAGS` is
+# load-bearing for the same reason.
+[target.'cfg(target_arch = "x86_64")']
+rustflags = ["-Ctarget-cpu=x86-64-v2"]
@@ -1,3 +1,14 @@
+## 2026-10-10 — x86-64-v2 realization (SSE4.2 without AVX) via the scalar backend
+
+- **What:** an eighth realization, selected at compile time by `all(target_arch = "x86_64", target_feature = "sse4.2", not(target_feature = "avx"))`. You get it with `target-cpu=native` on a v2-only CPU (Nehalem/Westmere, Silvermont-class Atom, VMs that hide AVX) or with `.cargo/config-v2.toml`. `simd.rs` routes this arm to `simd_scalar.rs`: the AVX2/AVX-512 types wrap 256/512-bit registers and cannot be the v2 realization, so plain arrays that LLVM compiles to SSE are used instead. Baseline x86-64 builds (no SSE4.2) are unchanged: they keep the AVX2 arm and the `cpu_guard` AVX2 floor, which now exempts `sse4.2 && !avx2` (the AVX and v2 arms together).
+- **API gap closed in `simd_scalar.rs`:** `F32Mask16::to_bitmask` and `F32x16::gather` (unsafe, same contract as the AVX2 polyfill). sentinel-qa: `gather` CONDITIONAL, satisfied by the SAFETY comment added at the one call site in `hpc/cam_pq.rs`; `to_bitmask` PARITY-OK.
+- **Evidence:**
+  - `scripts/masking-parity.sh v2-qemu` passes all 15 groups under `qemu-x86_64-static -cpu Nehalem`. The objdump witness finds no `%ymm`, `%zmm`, mask register, or VEX op on xmm. Disable run: with the `simd.rs` routing reverted, the binary SIGILLs under Nehalem.
+  - Per-test sweep of all 2541 v2 lib tests, each as its own process under Nehalem: 28 SIGILLs, all in `simd_avx2`'s own test modules (`tests`, `u8x32_tests`, `f16_precision_tests`), which call AVX2 intrinsics directly. No library path faulted. Those modules are gated off v2 (36 tests). Full v2 suite afterwards: 2474 passed, 0 failed, 31 ignored.
+  - Clippy `-D warnings` clean on v2, v3, v4, avx and baseline. Codegen witness avx2/avx512 PASS. fmt clean.
+  - New CI row `realization/v2` runs the parity arm, the objdump witness and the SIMD unit tests under Nehalem.
+- **Open:** `crate::simd_avx2` stays public on v2 builds and nothing checks for AVX2 at startup there, so a direct caller SIGILLs. Documented in the `simd.rs` module doc; the safe path is the `crate::simd` re-exports.
+
 ## 2026-10-10 — `tests/splat3d_correctness.rs`: clippy 1.99 clean under `--features splat3d`
 
 - Five lints, all in the test file: an unneeded `mut` on a closure, an over-precise f32 literal (`0.28209479177387814` → `0.282_094_8`; same bits, `0x3e906ebb`), and three `chunks_exact(3)` → `as_chunks::<3>().0.iter()`.
 
@@ -30,16 +30,27 @@ goes through `crate::simd`, so the switch is confined to `simd.rs`.
 
 ## Work items
 
-- [ ] P0 TESTS FIRST: `v2-qemu` parity arm (`--config .cargo/config-v2.toml`,
+- [x] P0 TESTS FIRST: `v2-qemu` parity arm (`--config .cargo/config-v2.toml`,
       run under `qemu-x86_64-static -cpu Nehalem`) in `scripts/masking-parity.sh`
       and a `realization/v2` row in `simd-matrix.yaml`, with an objdump witness
       (no AVX instruction at all: no `%ymm`, no VEX `v`-prefixed op). Expect it
       to fail before the arm exists.
-- [ ] P1 `.cargo/config-v2.toml`; `simd.rs` routing; `cpu_guard` floor gated off
+- [x] P1 `.cargo/config-v2.toml`; `simd.rs` routing; `cpu_guard` floor gated off
       the v2 arm.
-- [ ] P2 close the API gap: compile v2, list every missing item, add each to
+- [x] P2 close the API gap: compile v2, list every missing item, add each to
       `simd_scalar.rs` (types `U8x32`, `F32Mask16`, `F64Mask8` and the x86-only
       methods). Safe Rust only.
-- [ ] P3 gates: lib tests on v2 under qemu Nehalem; parity on every arm; clippy
+- [x] P3 gates: lib tests on v2 under qemu Nehalem; parity on every arm; clippy
       v2/v3/v4/avx/baseline; codegen witness avx2/avx512 unchanged.
-- [ ] P4 docs: CLAUDE.md realization list, blackboard, `simd.rs` dispatch comment.
+- [x] P4 docs: CLAUDE.md realization list, blackboard, `simd.rs` dispatch comment.
+
+## Outcome (2026-10-10)
+
+- Per-test sweep: all 2541 v2 lib tests, each run as its own process under
+  `qemu-x86_64-static -cpu Nehalem`. 28 faulted with SIGILL, every one in
+  `simd_avx2`'s own test modules (`tests`, `u8x32_tests`,
+  `f16_precision_tests`), which call AVX2 intrinsics directly. No library
+  path faulted. Those three modules are now gated off the v2 arm.
+- Open: `crate::simd_avx2` stays public on v2 builds and has no startup
+  check there, so a direct caller faults with SIGILL. Documented in the
+  `simd.rs` module doc; the safe path is the `crate::simd` re-exports.
@@ -266,6 +266,48 @@ jobs:
           # the multithreaded harness while a test was panicking.
           qemu-x86_64-static -cpu SandyBridge "$bin" simd --test-threads=1
 
+  v2:
+    # x86-64-v2 realization (SSE4.2, no AVX): `simd.rs` routes it to the
+    # scalar backend, which LLVM vectorizes to SSE. Built for `x86-64-v2`
+    # (`.cargo/config-v2.toml`) and RUN under `qemu-x86_64-static -cpu
+    # Nehalem`, so any AVX instruction on the executed path faults. Three
+    # rungs, as for `avx`: parity, an assembly witness that the parity binary
+    # has no AVX encoding at all, and the SIMD unit tests under emulation.
+    runs-on: ubuntu-latest
+    name: realization/v2 × x86_64
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
+        with:
+          key: v2
+      - name: install qemu-user
+        run: sudo apt-get update && sudo apt-get install -y qemu-user-static
+      - name: masking parity (x86-64-v2 build, qemu -cpu Nehalem)
+        run: bash scripts/masking-parity.sh v2-qemu
+      - name: no AVX instruction in the parity binary
+        run: |
+          bin=crates/simd-masking-parity/target/release/simd-masking-parity
+          # Nehalem has no AVX: no ymm/zmm or mask register, and no VEX-encoded
+          # (v-prefixed) op on xmm either. The parity binary has no
+          # runtime-gated paths, so any hit came from an ungated intrinsic.
+          bad=$(objdump -d --no-show-raw-insn "$bin" | grep -E '%ymm|%zmm|%k[0-7]|\sv[a-z0-9]+\s.*%xmm' || true)
+          if [ -n "$bad" ]; then echo "$bad" | head -20; echo "::error::AVX instruction in the v2 parity binary"; exit 1; fi
+          echo "no AVX instruction found"
+      - name: SIMD unit tests under qemu -cpu Nehalem
+        env:
+          CARGO_PROFILE_DEV_DEBUG: "0"
+          CARGO_PROFILE_TEST_DEBUG: "0"
+          CARGO_PROFILE_DEV_OPT_LEVEL: "1"
+          CARGO_PROFILE_TEST_OPT_LEVEL: "1"
+        run: |
+          bin=$(env -u RUSTFLAGS cargo --config .cargo/config-v2.toml test --lib --no-run --message-format=json \
+            | jq -r 'select(.executable != null and .target.kind == ["lib"] and .target.name == "ndarray") | .executable')
+          # One thread: see the `avx` job.
+          qemu-x86_64-static -cpu Nehalem "$bin" simd --test-threads=1
+
   neon:
     # aarch64 realization: cross-build on the x86 runner, run under qemu-user.
     # Three rungs: parity under qemu (bits), the codegen witness (opt-3 NEON
 
@@ -196,7 +196,7 @@ src/
 ### The parity arms are ALL reachable here — never report one as blocked without apt
 
 `scripts/masking-parity.sh` takes `native | nightly | wasm | wasm-scalar |
-neon-qemu | avx-qemu`, and the cross arms are an `apt-get` away, not an environment
+neon-qemu | avx-qemu | v2-qemu`, and the cross arms are an `apt-get` away, not an environment
 limit. Measured 2026-09-16: `neon-qemu` failed with a bare
 `No such file or directory (os error 2)` — which reads as "this target is not
 available here" and is in fact a **missing linker**, then a missing
@@ -239,12 +239,14 @@ env -u RUSTFLAGS cargo --config ../../.cargo/config-v4.toml run --release
 
 Read the program's own header line to confirm which arm you actually got
 (`avx512f=true`, `neon=true`, …) — that line exists precisely because the
-config can silently not apply. Six of the seven realizations are reachable
-without nightly (AVX2, AVX-512, AVX-without-AVX2, NEON, wasm-simd128,
-wasm-scalar); the AVX arm (`src/simd_avx.rs`, `.cargo/config-avx.toml`) runs
-under `qemu-x86_64-static -cpu SandyBridge` (`apt-get install -y
-qemu-user-static`); only
-`nightly-simd` needs a toolchain this repo does not pin. The same lesson the
+config can silently not apply. Seven of the eight realizations are reachable
+without nightly (AVX2, AVX-512, AVX-without-AVX2, x86-64-v2, NEON,
+wasm-simd128, wasm-scalar). The AVX arm (`src/simd_avx.rs`,
+`.cargo/config-avx.toml`) runs under `qemu-x86_64-static -cpu SandyBridge`;
+the v2 arm (SSE4.2 without AVX, routed to `simd_scalar.rs`,
+`.cargo/config-v2.toml`) runs under `-cpu Nehalem` (both need `apt-get install
+-y qemu-user-static`). Only `nightly-simd` needs a toolchain this repo does
+not pin. The same lesson the
 sibling `lance-graph-java` records for the JDK: **a stale index or a missing
 helper binary reporting absence is not evidence of absence.**
 
 
@@ -9,12 +9,13 @@
 #   scripts/masking-parity.sh wasm-scalar   wasm32 WITHOUT simd128 under node = the scalar arm
 #   scripts/masking-parity.sh neon-qemu     aarch64 cross-build run under qemu-aarch64-static
 #   scripts/masking-parity.sh avx-qemu      AVX-without-AVX2 build (config-avx) under qemu -cpu SandyBridge
+#   scripts/masking-parity.sh v2-qemu       x86-64-v2 build (config-v2, scalar realization) under qemu -cpu Nehalem
 #
 # Extra cargo arguments (e.g. `--config .cargo/config-v4.toml` for the v4
 # realization on the native row) pass through CARGO_ARGS, see
 # scripts/codegen-witness.sh for why that form and not the env var.
 set -euo pipefail
-ARM="${1:?native|nightly|wasm|wasm-scalar|neon-qemu|avx-qemu}"
+ARM="${1:?native|nightly|wasm|wasm-scalar|neon-qemu|avx-qemu|v2-qemu}"
 ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 MANIFEST="$ROOT/crates/simd-masking-parity/Cargo.toml"
 TD="${CARGO_TARGET_DIR:-$ROOT/crates/simd-masking-parity/target}"
@@ -70,6 +71,14 @@ case "$ARM" in
     env -u RUSTFLAGS cargo ${CARGO_ARGS:-} --config "$ROOT/.cargo/config-avx.toml" build --release --manifest-path "$MANIFEST" --bin simd-masking-parity
     "$QEMU" -cpu SandyBridge "$TD/release/simd-masking-parity"
     ;;
+  v2-qemu)
+    # The x86-64-v2 arm: SSE4.2, no AVX, routed to the scalar realization.
+    # Built for x86-64-v2 and run on an emulated Nehalem, so an AVX
+    # instruction anywhere in the binary faults instead of passing silently.
+    QEMU="${QEMU_X86_64:-qemu-x86_64-static}"
+    env -u RUSTFLAGS cargo ${CARGO_ARGS:-} --config "$ROOT/.cargo/config-v2.toml" build --release --manifest-path "$MANIFEST" --bin simd-masking-parity
+    "$QEMU" -cpu Nehalem "$TD/release/simd-masking-parity"
+    ;;
   *) echo "unknown arm: $ARM"; exit 2 ;;
 esac
 echo "masking parity ($ARM): PASS"
@@ -98,7 +98,8 @@ impl fmt::Display for BuildCpuMismatch {
                 f,
                 " This build uses ndarray's AVX2 SIMD backend. For a CPU with AVX but \
                  not AVX2, rebuild with `target-cpu=native` on it or with \
-                 `--config .cargo/config-avx.toml`; a CPU without AVX is not supported."
+                 `--config .cargo/config-avx.toml`; for one with SSE4.2 but not AVX, \
+                 `--config .cargo/config-v2.toml`. A CPU without SSE4.2 is not supported."
             )
         } else {
             write!(
@@ -274,12 +275,14 @@ const FEATURES: &[FeatureRow] = {
 
 /// The floor of `crate::simd`'s AVX2 realization (see the module docs):
 /// required on every x86_64 build that compiles that realization in, whatever
-/// its target features. The one x86_64 build that does not is the
-/// AVX-without-AVX2 arm (`simd_avx.rs`, `.cargo/config-avx.toml`), whose
-/// `avx` requirement the compiled-feature table already enforces.
+/// its target features. Two x86_64 builds do not, and the compiled-feature
+/// table already enforces what each needs: the AVX-without-AVX2 arm
+/// (`simd_avx.rs`, `.cargo/config-avx.toml`) and the x86-64-v2 arm
+/// (SSE4.2 without AVX, the scalar realization, `.cargo/config-v2.toml`).
+/// Together they are exactly `sse4.2` without `avx2`.
 #[cfg(target_arch = "x86_64")]
 const BACKEND_FLOOR: &[FeatureRow] =
-    &[("avx2", cfg!(not(all(target_feature = "avx", not(target_feature = "avx2")))), || {
+    &[("avx2", cfg!(not(all(target_feature = "sse4.2", not(target_feature = "avx2")))), || {
         cpuid::bit(7, 0, 'b', 5) && cpuid::os_avx()
     })];
 
@@ -448,21 +451,23 @@ mod tests {
 
     #[test]
     #[cfg(target_arch = "x86_64")]
-    fn avx2_is_required_unless_the_avx_arm_is_compiled_in() {
-        // A baseline build (no `avx2` compiled in) still reaches the AVX2 SIMD
-        // backend, so the floor applies to it; only the AVX-without-AVX2 arm
-        // is exempt, because it never selects that backend.
+    fn avx2_is_required_unless_the_avx_or_v2_arm_is_compiled_in() {
+        // A baseline build (no `sse4.2` compiled in) still reaches the AVX2
+        // SIMD backend, so the floor applies to it; the AVX-without-AVX2 arm
+        // and the x86-64-v2 arm are exempt, because neither selects it.
         let avx_arm = cfg!(all(target_feature = "avx", not(target_feature = "avx2")));
+        let v2_arm = cfg!(all(target_feature = "sse4.2", not(target_feature = "avx")));
         assert!(BACKEND_FLOOR
             .iter()
-            .any(|&(n, compiled, _)| n == "avx2" && compiled == !avx_arm));
+            .any(|&(n, compiled, _)| n == "avx2" && compiled == !(avx_arm || v2_arm)));
     }
 
     #[test]
     fn the_message_explains_the_avx2_floor_only_when_avx2_is_missing() {
         let floor = BuildCpuMismatch { missing: vec!["avx2"] }.to_string();
         assert!(floor.contains("AVX2 SIMD backend"), "{floor}");
         assert!(floor.contains("config-avx.toml"), "{floor}");
+        assert!(floor.contains("config-v2.toml"), "{floor}");
         let other = BuildCpuMismatch {
             missing: vec!["avx512f"],
         }
 
@@ -248,6 +248,11 @@ impl DistanceTables {
 
                 // Gather distances from precomputed table
                 let base_ptr = self.tables[s].as_ptr();
+                // SAFETY: every index is a `u8` code (`CamFingerprint = [u8; 6]`),
+                // so 0..=255 < NUM_CENTROIDS, and `base_ptr` starts the
+                // `[f32; NUM_CENTROIDS]` table `self.tables[s]` (`s <
+                // NUM_SUBSPACES`): each offset lands on an initialised, aligned
+                // `f32` in that one array.
                 let distances = F32x16::gather(indices, base_ptr);
 
                 acc = acc + distances;