Work with explicit SIMD lanes

Prefer ordinary pipelines for repeated column scans: the columnar-list lessons show how Plenty can accelerate them automatically. Use std.simd when the algorithm itself is clearer as a fixed set of lanes.

This example compares four measurements with one threshold, then selects the threshold for lanes below it. The mask is a vector of logical booleans; count counts its true lanes, and select chooses between already-computed values.

import std.simd as simd

def main() -> Result[(), IoError]:
    readings = simd.f32x4.Lanes(18.0f32, 21.0f32, 16.0f32, 24.0f32)
    floor = simd.splat_f32x4(20.0f32)
    below = readings.simd_lt(floor)
    adjusted = below.select(floor, readings)
    print(below.count(), adjusted.to_tuple())?
    print(readings.to_tuple())?
    Ok(())

2 (20.0, 21.0, 20.0, 24.0)
(18.0, 21.0, 16.0, 24.0)

Vectors are copyable and allocate nothing. Operations borrow their inputs and produce new values, so the original readings remain unchanged. i32x4, u32x4 and f64x2 provide the same pattern with other lane types. Mask shapes must match the vectors they select.

Use add, sub and mul for ordinary arithmetic. Integer versions return a Result with a lane-typed overflow error; propagate it with ? or handle it with match. Integer wrapping_add, wrapping_sub and wrapping_mul explicitly choose modular arithmetic. Floating sum_ordered(initial) preserves scalar addition order. Selection is eager: compute and handle both inputs before selecting between them.

The module also provides checked list loads and stores; they require a complete vector to fit and never allocate. Handle any leftover elements with a scalar loop. Ordinary lists and these initial vector values use tagged scalar storage, so explicit lanes do not promise packed storage or faster code. The default optimized profile enables supported native SIMD operations on x86-64. AArch64 currently uses the scalar implementation, which preserves the same results. See the reference for the exact operations, bounds contracts and current limits.