library/portable-simd/crates/core_simd/examples/nbody.rs

   1 #![feature(portable_simd)]
   2 extern crate std_float;
   3
   4 /// Benchmarks game nbody code
   5 /// Taken from the `packed_simd` crate
   6 /// Run this benchmark with `cargo test --example nbody`
   7 mod nbody {
   8     use core_simd::simd::*;
   9     #[allow(unused)] // False positive?
  10     use std_float::StdFloat;
  11
  12     use std::f64::consts::PI;
  13     const SOLAR_MASS: f64 = 4.0 * PI * PI;
  14     const DAYS_PER_YEAR: f64 = 365.24;
  15
  16     #[derive(Debug, Clone, Copy)]
  17     struct Body {
  18         pub x: f64x4,
  19         pub v: f64x4,
  20         pub mass: f64,
  21     }
  22
  23     const N_BODIES: usize = 5;
  24     const BODIES: [Body; N_BODIES] = [
  25         // sun:
  26         Body {
  27             x: f64x4::from_array([0., 0., 0., 0.]),
  28             v: f64x4::from_array([0., 0., 0., 0.]),
  29             mass: SOLAR_MASS,
  30         },
  31         // jupiter:
  32         Body {
  33             x: f64x4::from_array([
  34                 4.84143144246472090e+00,
  35                 -1.16032004402742839e+00,
  36                 -1.03622044471123109e-01,
  37                 0.,
  38             ]),
  39             v: f64x4::from_array([
  40                 1.66007664274403694e-03 * DAYS_PER_YEAR,
  41                 7.69901118419740425e-03 * DAYS_PER_YEAR,
  42                 -6.90460016972063023e-05 * DAYS_PER_YEAR,
  43                 0.,
  44             ]),
  45             mass: 9.54791938424326609e-04 * SOLAR_MASS,
  46         },
  47         // saturn:
  48         Body {
  49             x: f64x4::from_array([
  50                 8.34336671824457987e+00,
  51                 4.12479856412430479e+00,
  52                 -4.03523417114321381e-01,
  53                 0.,
  54             ]),
  55             v: f64x4::from_array([
  56                 -2.76742510726862411e-03 * DAYS_PER_YEAR,
  57                 4.99852801234917238e-03 * DAYS_PER_YEAR,
  58                 2.30417297573763929e-05 * DAYS_PER_YEAR,
  59                 0.,
  60             ]),
  61             mass: 2.85885980666130812e-04 * SOLAR_MASS,
  62         },
  63         // uranus:
  64         Body {
  65             x: f64x4::from_array([
  66                 1.28943695621391310e+01,
  67                 -1.51111514016986312e+01,
  68                 -2.23307578892655734e-01,
  69                 0.,
  70             ]),
  71             v: f64x4::from_array([
  72                 2.96460137564761618e-03 * DAYS_PER_YEAR,
  73                 2.37847173959480950e-03 * DAYS_PER_YEAR,
  74                 -2.96589568540237556e-05 * DAYS_PER_YEAR,
  75                 0.,
  76             ]),
  77             mass: 4.36624404335156298e-05 * SOLAR_MASS,
  78         },
  79         // neptune:
  80         Body {
  81             x: f64x4::from_array([
  82                 1.53796971148509165e+01,
  83                 -2.59193146099879641e+01,
  84                 1.79258772950371181e-01,
  85                 0.,
  86             ]),
  87             v: f64x4::from_array([
  88                 2.68067772490389322e-03 * DAYS_PER_YEAR,
  89                 1.62824170038242295e-03 * DAYS_PER_YEAR,
  90                 -9.51592254519715870e-05 * DAYS_PER_YEAR,
  91                 0.,
  92             ]),
  93             mass: 5.15138902046611451e-05 * SOLAR_MASS,
  94         },
  95     ];
  96
  97     fn offset_momentum(bodies: &mut [Body; N_BODIES]) {
  98         let (sun, rest) = bodies.split_at_mut(1);
  99         let sun = &mut sun[0];
 100         for body in rest {
 101             let m_ratio = body.mass / SOLAR_MASS;
 102             sun.v -= body.v * Simd::splat(m_ratio);
 103         }
 104     }
 105
 106     fn energy(bodies: &[Body; N_BODIES]) -> f64 {
 107         let mut e = 0.;
 108         for i in 0..N_BODIES {
 109             let bi = &bodies[i];
 110             e += bi.mass * (bi.v * bi.v).reduce_sum() * 0.5;
 111             for bj in bodies.iter().take(N_BODIES).skip(i + 1) {
 112                 let dx = bi.x - bj.x;
 113                 e -= bi.mass * bj.mass / (dx * dx).reduce_sum().sqrt()
 114             }
 115         }
 116         e
 117     }
 118
 119     fn advance(bodies: &mut [Body; N_BODIES], dt: f64) {
 120         const N: usize = N_BODIES * (N_BODIES - 1) / 2;
 121
 122         // compute distance between bodies:
 123         let mut r = [f64x4::splat(0.); N];
 124         {
 125             let mut i = 0;
 126             for j in 0..N_BODIES {
 127                 for k in j + 1..N_BODIES {
 128                     r[i] = bodies[j].x - bodies[k].x;
 129                     i += 1;
 130                 }
 131             }
 132         }
 133
 134         let mut mag = [0.0; N];
 135         for i in (0..N).step_by(2) {
 136             let d2s = f64x2::from_array([
 137                 (r[i] * r[i]).reduce_sum(),
 138                 (r[i + 1] * r[i + 1]).reduce_sum(),
 139             ]);
 140             let dmags = f64x2::splat(dt) / (d2s * d2s.sqrt());
 141             mag[i] = dmags[0];
 142             mag[i + 1] = dmags[1];
 143         }
 144
 145         let mut i = 0;
 146         for j in 0..N_BODIES {
 147             for k in j + 1..N_BODIES {
 148                 let f = r[i] * Simd::splat(mag[i]);
 149                 bodies[j].v -= f * Simd::splat(bodies[k].mass);
 150                 bodies[k].v += f * Simd::splat(bodies[j].mass);
 151                 i += 1
 152             }
 153         }
 154         for body in bodies {
 155             body.x += Simd::splat(dt) * body.v
 156         }
 157     }
 158
 159     pub fn run(n: usize) -> (f64, f64) {
 160         let mut bodies = BODIES;
 161         offset_momentum(&mut bodies);
 162         let energy_before = energy(&bodies);
 163         for _ in 0..n {
 164             advance(&mut bodies, 0.01);
 165         }
 166         let energy_after = energy(&bodies);
 167
 168         (energy_before, energy_after)
 169     }
 170 }
 171
 172 #[cfg(test)]
 173 mod tests {
 174     // Good enough for demonstration purposes, not going for strictness here.
 175     fn approx_eq_f64(a: f64, b: f64) -> bool {
 176         (a - b).abs() < 0.00001
 177     }
 178     #[test]
 179     fn test() {
 180         const OUTPUT: [f64; 2] = [-0.169075164, -0.169087605];
 181         let (energy_before, energy_after) = super::nbody::run(1000);
 182         assert!(approx_eq_f64(energy_before, OUTPUT[0]));
 183         assert!(approx_eq_f64(energy_after, OUTPUT[1]));
 184     }
 185 }
 186
 187 fn main() {
 188     {
 189         let (energy_before, energy_after) = nbody::run(1000);
 190         println!("Energy before: {energy_before}");
 191         println!("Energy after:  {energy_after}");
 192     }
 193 }