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//! `sub_f64x4` — packed `f64x4` subtraction.
//!
//! * `x86_64` + AVX2: single `vsubpd ymm` (256-bit, 4 lanes at once).
//! * `AArch64`: two `fsub v.2d` NEON ops (128-bit each; NEON is mandatory).
//! * riscv64 + RVV: `vfsub.vv` with `vsetvli` for 4×f64.
//! * fallback: scalar loop.
#![allow(unsafe_code)]
/// Subtracts two 4-lane `f64` vectors element-wise (`lhs - rhs`), writing
/// the result to `out`. All three slices must have length 4.
#[allow(clippy::inline_always)]
#[inline(always)]
pub fn apply(lhs: &[f64], rhs: &[f64], out: &mut [f64]) {
debug_assert!(
lhs.len() == 4 && rhs.len() == 4 && out.len() == 4,
"sub_f64x4::apply requires exactly 4-element slices \
(got lhs={}, rhs={}, out={})",
lhs.len(),
rhs.len(),
out.len()
);
if lhs.len() != 4 || rhs.len() != 4 || out.len() != 4 {
return;
}
#[cfg(target_arch = "x86_64")]
{
if std::is_x86_feature_detected!("avx2") {
// SAFETY: lengths checked above; AVX2 detected.
unsafe {
use core::arch::asm;
asm!(
"vmovupd ymm0, ymmword ptr [{lhs}]",
"vmovupd ymm1, ymmword ptr [{rhs}]",
"vsubpd ymm0, ymm0, ymm1",
"vmovupd ymmword ptr [{out}], ymm0",
lhs = in(reg) lhs.as_ptr(),
rhs = in(reg) rhs.as_ptr(),
out = in(reg) out.as_mut_ptr(),
out("ymm0") _,
out("ymm1") _,
options(nostack, preserves_flags),
);
}
return;
}
}
#[cfg(target_arch = "aarch64")]
{
// SAFETY: lengths checked above; NEON is mandatory on AArch64.
unsafe {
use core::arch::asm;
asm!(
"ld1 {{v0.2d}}, [{lhs}]",
"ld1 {{v1.2d}}, [{rhs}]",
"fsub v0.2d, v0.2d, v1.2d",
"st1 {{v0.2d}}, [{out}]",
"ld1 {{v0.2d}}, [{lhs}, #16]",
"ld1 {{v1.2d}}, [{rhs}, #16]",
"fsub v0.2d, v0.2d, v1.2d",
"st1 {{v0.2d}}, [{out}, #16]",
lhs = in(reg) lhs.as_ptr(),
rhs = in(reg) rhs.as_ptr(),
out = in(reg) out.as_mut_ptr(),
out("v0") _,
out("v1") _,
options(nostack, preserves_flags),
);
}
return;
}
#[cfg(all(target_arch = "riscv64", target_feature = "v"))]
{
// SAFETY: lengths checked above; RVV activated via target_feature = "v".
unsafe {
use core::arch::asm;
asm!(
"li t0, 4",
"vsetvli t0, t0, e64, m1, ta, ma",
"vle64.v v0, ({lhs})",
"vle64.v v1, ({rhs})",
"vfsub.vv v0, v0, v1",
"vse64.v v0, ({out})",
lhs = in(reg) lhs.as_ptr(),
rhs = in(reg) rhs.as_ptr(),
out = in(reg) out.as_mut_ptr(),
out("t0") _,
out("v0") _,
out("v1") _,
options(nostack),
);
}
return;
}
// Scalar fallback.
for i in 0..4 {
out[i] = lhs[i] - rhs[i];
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn vector_sub_matches_scalar() {
let a = [10.0_f64, 20.0, 30.0, 40.0];
let b = [1.0_f64, 2.0, 3.0, 4.0];
let mut out = [0.0_f64; 4];
apply(&a, &b, &mut out);
assert_eq!(out, [9.0, 18.0, 27.0, 36.0]);
}
#[test]
fn sub_negative_result() {
let a = [1.0_f64, 2.0, 3.0, 4.0];
let b = [10.0_f64; 4];
let mut out = [0.0_f64; 4];
apply(&a, &b, &mut out);
assert_eq!(out, [-9.0, -8.0, -7.0, -6.0]);
}
}