simd: I8x16::saturating_abs no longer needs SSSE3 at run time

.claude/plans/simd-sse-v2-tier-v1.md

@@ -0,0 +1,45 @@
+# Plan: x86-64-v2 SSE tier (v1)
+
+Operator decision 2026-10-10: a compile-time SIMD tier for **x86-64-v2**
+builds (SSE2 through SSE4.2, no AVX). Baseline x86-64 builds (no SSE4.2) are
+unchanged: they keep the AVX2 arm and the `cpu_guard` AVX2 startup floor.
+No runtime feature checks in the SIMD code.
+
+## Selection
+
+* Arm predicate: `all(target_arch = "x86_64", target_feature = "sse4.2",
+  not(target_feature = "avx"))`, written out at each gate.
+* Reached by `target-cpu=native` on a v2-only CPU (Nehalem/Westmere, Atom
+  Silvermont-class, VMs that hide AVX), or the new `.cargo/config-v2.toml`
+  (`-Ctarget-cpu=x86-64-v2`).
+* Unchanged: AVX-512, AVX2, AVX-without-AVX2 (`simd_avx.rs`), and baseline.
+
+## Shape: route to the scalar realization, not a new intrinsic file
+
+Every type in `simd_avx2.rs` / `simd_avx512.rs` wraps a 256- or 512-bit
+register (`U8x32(pub __m256i)`, …), so none of them can be the v2 realization:
+unlike the AVX arm, there is no impl-block split that works. `simd.rs` instead
+routes the v2 arm to `simd_scalar.rs` — plain `[T; N]` arrays that LLVM
+vectorizes to SSE. Measured earlier (existing `simd_scalar.rs` compiled at
+x86-64 / v2 / sandybridge / v3): element-wise ops ~2× the AVX2 instruction
+count, SSE2/SSSE3/SSE4.2 produce identical code, reductions stay scalar, and
+`mul_add` becomes a software `fmaf` call on any tier without FMA.
+
+Only the `simd_*` backends import the x86 modules directly; every consumer
+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`,
+      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
+      the v2 arm.
+- [ ] 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
+      v2/v3/v4/avx/baseline; codegen witness avx2/avx512 unchanged.
+- [ ] P4 docs: CLAUDE.md realization list, blackboard, `simd.rs` dispatch comment.

src/simd.rs

@@ -1083,6 +1083,26 @@ mod tests {
     /// transpose that drops, duplicates or misroutes any word fails, and the
     /// fixture is asserted not to be symmetric, so an identity "transpose"
     /// fails too. Transposing twice must return the input.
+    /// Every `i8` value through `I8x16`/`I8x32::saturating_abs`, on whichever
+    /// path this build compiled: the SSSE3 intrinsic (`cfg(target_feature =
+    /// "ssse3")`: v3, v4, native) or the scalar fallback (a baseline build,
+    /// e.g. CI's `tests/*` jobs, whose RUSTFLAGS drop `target-cpu`). Lives
+    /// here, not in `simd_avx512.rs`, whose test modules are `avx512f`-only.
+    #[cfg(all(feature = "std", target_arch = "x86_64"))]
+    #[test]
+    fn x86_saturating_abs_is_exact_for_all_256_i8_values() {
+        use crate::simd_avx512::{I8x16, I8x32};
+        let all: [i8; 256] = core::array::from_fn(|i| i as u8 as i8);
+        for c in all.chunks_exact(16) {
+            let v: [i8; 16] = c.try_into().unwrap();
+            assert_eq!(I8x16::from_array(v).saturating_abs().to_array(), v.map(i8::saturating_abs));
+        }
+        for c in all.chunks_exact(32) {
+            let v: [i8; 32] = c.try_into().unwrap();
+            assert_eq!(I8x32::from_array(v).saturating_abs().to_array(), v.map(i8::saturating_abs));
+        }
+    }
+
     /// The x86 256-bit integer methods that had no test on any stable arm,
     /// each checked lane-by-lane against a scalar reference on asymmetric
     /// data. They run on the AVX2, AVX-512 and AVX-without-AVX2 realizations;

src/simd_avx512.rs

@@ -2943,24 +2943,33 @@ impl I8x16 {
     /// ```
     #[inline(always)]
     pub fn saturating_abs(self) -> Self {
-        // SAFETY: `_mm_abs_epi8` needs SSSE3 and `_mm_min_epu8` needs SSE2. SSE2
-        // is in the x86_64 baseline; SSSE3 is NOT, and this file compiles for
-        // every x86_64 build, so SSSE3 at run time is the caller's obligation.
-        // `target-cpu=native` (default) and `config-v3`/`config-v4` include it;
-        // a baseline build (e.g. a RUSTFLAGS env replacing the config) compiles
-        // this and would SIGILL. The unaligned load/store match the
-        // `[i8; 16]` storage. VPABSB returns 0x80 for `i8::MIN` (the bit pattern
-        // of +128, which does not fit in i8); VPMINUB then clamps 0x80 (= 128
-        // unsigned) down to 0x7f (= 127 = `i8::MAX`), producing the saturating
-        // result bare VPABSB cannot — per the consumer contract's VPABSB
-        // correction. All 16 lanes are saturated branchlessly.
-        use core::arch::x86_64::*;
-        unsafe {
-            let v = _mm_loadu_si128(self.0.as_ptr() as *const __m128i);
-            let clamped = _mm_min_epu8(_mm_abs_epi8(v), _mm_set1_epi8(0x7f_u8 as i8));
-            let mut o = [0i8; 16];
-            _mm_storeu_si128(o.as_mut_ptr() as *mut __m128i, clamped);
-            Self(o)
+        // SSSE3 (`_mm_abs_epi8`) is not in the x86_64 baseline, and this file
+        // compiles for every x86_64 build. With SSSE3 compiled in (the default
+        // `target-cpu=native` on any host since 2006, `config-v3`, `config-v4`)
+        // the intrinsic path runs; otherwise the scalar loop does, so no build
+        // of this method can SIGILL. Both give identical results.
+        #[cfg(target_feature = "ssse3")]
+        {
+            // SAFETY: SSSE3 is enabled at compile time (`cfg` above), so the
+            // whole build already assumes it; `_mm_min_epu8` is SSE2, which is
+            // baseline. The unaligned load/store match the `[i8; 16]` storage.
+            // VPABSB returns 0x80 for `i8::MIN` (the bit pattern of +128,
+            // which does not fit in i8); VPMINUB then clamps 0x80 (= 128
+            // unsigned) down to 0x7f (= 127 = `i8::MAX`), producing the
+            // saturating result bare VPABSB cannot — per the consumer
+            // contract's VPABSB correction.
+            use core::arch::x86_64::*;
+            unsafe {
+                let v = _mm_loadu_si128(self.0.as_ptr() as *const __m128i);
+                let clamped = _mm_min_epu8(_mm_abs_epi8(v), _mm_set1_epi8(0x7f_u8 as i8));
+                let mut o = [0i8; 16];
+                _mm_storeu_si128(o.as_mut_ptr() as *mut __m128i, clamped);
+                Self(o)
+            }
+        }
+        #[cfg(not(target_feature = "ssse3"))]
+        {
+            Self(core::array::from_fn(|i| self.0[i].saturating_abs()))
         }
     }
 }