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#![feature(portable_simd)]
#![allow(clippy::excessive_precision)]
extern crate std_float;
/// Benchmarks game nbody code
/// Taken from the `packed_simd` crate
/// Run this benchmark with `cargo test --example nbody`
mod nbody {
use core_simd::simd::prelude::*;
#[allow(unused)] // False positive?
use std_float::StdFloat;
use std::f64::consts::PI;
const SOLAR_MASS: f64 = 4.0 * PI * PI;
const DAYS_PER_YEAR: f64 = 365.24;
#[derive(Debug, Clone, Copy)]
struct Body {
pub x: f64x4,
pub v: f64x4,
pub mass: f64,
}
const N_BODIES: usize = 5;
const BODIES: [Body; N_BODIES] = [
// sun:
Body {
x: f64x4::from_array([0., 0., 0., 0.]),
v: f64x4::from_array([0., 0., 0., 0.]),
mass: SOLAR_MASS,
},
// jupiter:
Body {
x: f64x4::from_array([
4.84143144246472090e+00,
-1.16032004402742839e+00,
-1.03622044471123109e-01,
0.,
]),
v: f64x4::from_array([
1.66007664274403694e-03 * DAYS_PER_YEAR,
7.69901118419740425e-03 * DAYS_PER_YEAR,
-6.90460016972063023e-05 * DAYS_PER_YEAR,
0.,
]),
mass: 9.54791938424326609e-04 * SOLAR_MASS,
},
// saturn:
Body {
x: f64x4::from_array([
8.34336671824457987e+00,
4.12479856412430479e+00,
-4.03523417114321381e-01,
0.,
]),
v: f64x4::from_array([
-2.76742510726862411e-03 * DAYS_PER_YEAR,
4.99852801234917238e-03 * DAYS_PER_YEAR,
2.30417297573763929e-05 * DAYS_PER_YEAR,
0.,
]),
mass: 2.85885980666130812e-04 * SOLAR_MASS,
},
// uranus:
Body {
x: f64x4::from_array([
1.28943695621391310e+01,
-1.51111514016986312e+01,
-2.23307578892655734e-01,
0.,
]),
v: f64x4::from_array([
2.96460137564761618e-03 * DAYS_PER_YEAR,
2.37847173959480950e-03 * DAYS_PER_YEAR,
-2.96589568540237556e-05 * DAYS_PER_YEAR,
0.,
]),
mass: 4.36624404335156298e-05 * SOLAR_MASS,
},
// neptune:
Body {
x: f64x4::from_array([
1.53796971148509165e+01,
-2.59193146099879641e+01,
1.79258772950371181e-01,
0.,
]),
v: f64x4::from_array([
2.68067772490389322e-03 * DAYS_PER_YEAR,
1.62824170038242295e-03 * DAYS_PER_YEAR,
-9.51592254519715870e-05 * DAYS_PER_YEAR,
0.,
]),
mass: 5.15138902046611451e-05 * SOLAR_MASS,
},
];
fn offset_momentum(bodies: &mut [Body; N_BODIES]) {
let (sun, rest) = bodies.split_at_mut(1);
let sun = &mut sun[0];
for body in rest {
let m_ratio = body.mass / SOLAR_MASS;
sun.v -= body.v * Simd::splat(m_ratio);
}
}
fn energy(bodies: &[Body; N_BODIES]) -> f64 {
let mut e = 0.;
for i in 0..N_BODIES {
let bi = &bodies[i];
e += bi.mass * (bi.v * bi.v).reduce_sum() * 0.5;
for bj in bodies.iter().take(N_BODIES).skip(i + 1) {
let dx = bi.x - bj.x;
e -= bi.mass * bj.mass / (dx * dx).reduce_sum().sqrt()
}
}
e
}
fn advance(bodies: &mut [Body; N_BODIES], dt: f64) {
const N: usize = N_BODIES * (N_BODIES - 1) / 2;
// compute distance between bodies:
let mut r = [f64x4::splat(0.); N];
{
let mut i = 0;
for j in 0..N_BODIES {
for k in j + 1..N_BODIES {
r[i] = bodies[j].x - bodies[k].x;
i += 1;
}
}
}
let mut mag = [0.0; N];
for i in (0..N).step_by(2) {
let d2s = f64x2::from_array([
(r[i] * r[i]).reduce_sum(),
(r[i + 1] * r[i + 1]).reduce_sum(),
]);
let dmags = f64x2::splat(dt) / (d2s * d2s.sqrt());
mag[i] = dmags[0];
mag[i + 1] = dmags[1];
}
let mut i = 0;
for j in 0..N_BODIES {
for k in j + 1..N_BODIES {
let f = r[i] * Simd::splat(mag[i]);
bodies[j].v -= f * Simd::splat(bodies[k].mass);
bodies[k].v += f * Simd::splat(bodies[j].mass);
i += 1
}
}
for body in bodies {
body.x += Simd::splat(dt) * body.v
}
}
pub fn run(n: usize) -> (f64, f64) {
let mut bodies = BODIES;
offset_momentum(&mut bodies);
let energy_before = energy(&bodies);
for _ in 0..n {
advance(&mut bodies, 0.01);
}
let energy_after = energy(&bodies);
(energy_before, energy_after)
}
}
#[cfg(test)]
mod tests {
// Good enough for demonstration purposes, not going for strictness here.
fn approx_eq_f64(a: f64, b: f64) -> bool {
(a - b).abs() < 0.00001
}
#[test]
fn test() {
const OUTPUT: [f64; 2] = [-0.169075164, -0.169087605];
let (energy_before, energy_after) = super::nbody::run(1000);
assert!(approx_eq_f64(energy_before, OUTPUT[0]));
assert!(approx_eq_f64(energy_after, OUTPUT[1]));
}
}
fn main() {
{
let (energy_before, energy_after) = nbody::run(1000);
println!("Energy before: {energy_before}");
println!("Energy after: {energy_after}");
}
}
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