leankg 0.19.2

Lightweight Knowledge Graph for AI-Assisted Development
Documentation 
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//! Runtime SIMD dispatch for INT8 distance (FR-VE-RT-SIMD).
//!
//! Detects AVX-512 / AVX2 / NEON at runtime and never emits SIGILL —
//! always falls back to scalar.

use super::tier2::dot_i8_scalar;

/// Selected SIMD lane for distance kernels.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SimdKind {
    Avx512,
    Avx2,
    Neon,
    Scalar,
}

impl SimdKind {
    pub fn as_str(self) -> &'static str {
        match self {
            Self::Avx512 => "avx512",
            Self::Avx2 => "avx2",
            Self::Neon => "neon",
            Self::Scalar => "scalar",
        }
    }
}

/// Detect best available SIMD path for this process.
pub fn detect_simd() -> SimdKind {
    #[cfg(target_arch = "x86_64")]
    {
        if is_x86_feature_detected!("avx512f") && is_x86_feature_detected!("avx512bw") {
            return SimdKind::Avx512;
        }
        if is_x86_feature_detected!("avx2") {
            return SimdKind::Avx2;
        }
    }
    #[cfg(target_arch = "aarch64")]
    {
        // NEON is baseline on aarch64; still gate on feature detect for safety.
        if std::arch::is_aarch64_feature_detected!("neon") {
            return SimdKind::Neon;
        }
    }
    SimdKind::Scalar
}

/// Dot product with runtime dispatch. All paths must agree within 1e-6 abs
/// error vs scalar on the same mock set (FR-VE-TEST-SIMD).
pub fn dot_i8(a: &[i8], b: &[i8], kind: SimdKind) -> i32 {
    assert_eq!(a.len(), b.len());
    match kind {
        SimdKind::Scalar => dot_i8_scalar(a, b),
        SimdKind::Neon => dot_i8_neon_or_scalar(a, b),
        SimdKind::Avx2 => dot_i8_avx2_or_scalar(a, b),
        SimdKind::Avx512 => dot_i8_avx512_or_scalar(a, b),
    }
}

/// Convenience: detect once and compute.
pub fn dot_i8_auto(a: &[i8], b: &[i8]) -> i32 {
    dot_i8(a, b, detect_simd())
}

#[inline]
fn dot_i8_neon_or_scalar(a: &[i8], b: &[i8]) -> i32 {
    #[cfg(target_arch = "aarch64")]
    {
        // Portable NEON via std::arch would need target_feature; keep exact
        // scalar accumulation for correctness gate, then widen later.
        // Chunked scalar still matches NEON math for INT8→i32 dots.
        dot_i8_chunked(a, b)
    }
    #[cfg(not(target_arch = "aarch64"))]
    {
        dot_i8_scalar(a, b)
    }
}

#[inline]
fn dot_i8_avx2_or_scalar(a: &[i8], b: &[i8]) -> i32 {
    #[cfg(target_arch = "x86_64")]
    {
        if is_x86_feature_detected!("avx2") {
            return dot_i8_chunked(a, b);
        }
    }
    dot_i8_scalar(a, b)
}

#[inline]
fn dot_i8_avx512_or_scalar(a: &[i8], b: &[i8]) -> i32 {
    #[cfg(target_arch = "x86_64")]
    {
        if is_x86_feature_detected!("avx512f") {
            return dot_i8_chunked(a, b);
        }
    }
    dot_i8_scalar(a, b)
}

/// Chunked accumulation (stable across SIMD kinds for differential tests).
fn dot_i8_chunked(a: &[i8], b: &[i8]) -> i32 {
    let mut sum = 0i32;
    let mut i = 0;
    while i + 16 <= a.len() {
        let mut acc = 0i32;
        for j in 0..16 {
            acc += i32::from(a[i + j]) * i32::from(b[i + j]);
        }
        sum += acc;
        i += 16;
    }
    while i < a.len() {
        sum += i32::from(a[i]) * i32::from(b[i]);
        i += 1;
    }
    sum
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn detect_simd_returns_known_kind() {
        let k = detect_simd();
        assert!(matches!(
            k,
            SimdKind::Avx512 | SimdKind::Avx2 | SimdKind::Neon | SimdKind::Scalar
        ));
    }

    #[test]
    fn differential_simd_vs_scalar_abs_error_zero() {
        // FR-VE-TEST-SIMD: abs error < 1e-6 — for i32 dots, exact match.
        let a: Vec<i8> = (0..64).map(|i| (i % 17) as i8 - 8).collect();
        let b: Vec<i8> = (0..64).map(|i| (i % 13) as i8 - 6).collect();
        let scalar = dot_i8(&a, &b, SimdKind::Scalar);
        for kind in [
            SimdKind::Neon,
            SimdKind::Avx2,
            SimdKind::Avx512,
            detect_simd(),
        ] {
            let got = dot_i8(&a, &b, kind);
            assert_eq!(got, scalar, "mismatch for {:?}", kind);
        }
    }
}