Benchmark different nearest functions for Rust

Cargo.toml

[package]
name = "bench"
version = "0.1.0"
authors = ["MaxGraey <[email protected]>"]

[profile.bench]
codegen-units = 1
opt-level = 3
lto = true
debug = false
rpath = false
debug-assertions = false
panic = 'unwind'

[profile.release]
codegen-units = 1
opt-level = 3
lto = true
debug = true
rpath = false
debug-assertions = false
panic = 'unwind'

nearest_bench.rs

#![feature(stdsimd)]
#![feature(test)]
extern crate test;

#[cfg(target_arch = "x86")]
use std::arch::x86::*;
#[cfg(target_arch = "x86_64")]
use std::arch::x86_64::*;

const TOINT_64: f64 = 1.0 / f64::EPSILON;

pub extern "C" fn f64_nearest_original(x: f64) -> f64 {
  // Rust doesn't have a nearest function, so do it manually.
  if x == 0.0 {
    // Preserve the sign of zero.
    x
  } else {
    // Nearest is either ceil or floor depending on which is nearest or even.
    let u = x.ceil();
    let d = x.floor();
    let um = (x - u).abs();
    let dm = (x - d).abs();
    if um < dm
      || (um == dm && {
        let h = u / 2.;
        h.floor() == h
      })
    {
      u
    } else {
      d
    }
  }
}

pub extern "C" fn f64_nearest_branch(x: f64) -> f64 {
  if x == 0.0 {
    x
  } else {
    let i = x.to_bits();
    let e = i >> 52 & 0x7ff;
    if e < 0x3ff + 52 {
      if i >> 63 != 0 {
        x - TOINT_64 + TOINT_64
      } else {
        x + TOINT_64 - TOINT_64
      }
    } else {
      x
    }
  }
}

pub extern "C" fn f64_nearest_copysign(x: f64) -> f64 {
  let i = x.to_bits();
  let e = i >> 52 & 0x7ff;
  if e < 0x3ff + 52 {
    if i >> 63 != 0 {
      x - TOINT_64 + TOINT_64
    } else {
      x + TOINT_64 - TOINT_64
    }.copysign(x)
  } else {
    x
  }
}

pub extern "C" fn f64_nearest_abs_copysign(x: f64) -> f64 {
  let i = x.to_bits();
  let e = i >> 52 & 0x7ff;
  if e < 0x3ff + 52 {
    (x.abs() + TOINT_64 - TOINT_64).copysign(x)
  } else {
    x
  }
}

pub extern "C" fn f64_nearest_abs_copysign_without_bits(x: f64) -> f64 {
  let y = x.abs();
  let m = f64::from_bits(0x3ff_u64 + 52 << 52_u64);
  if y < m {
    (y + TOINT_64 - TOINT_64).copysign(x)
  } else {
    x
  }
}

#[repr(C)]
#[repr(align(16))]
struct F64x2(f64, f64);

pub extern "C" fn f64_nearest_sse41(x: f64) -> f64 {
  // woraround due to stdsimd missed _mm_extract_pd
  let mut inout = F64x2(x, 0_f64);
  unsafe {
    _mm_store_pd(
      &mut inout.0,
      _mm_round_pd(
        _mm_load_pd(&inout.0),
        _MM_FROUND_RINT
      )
    )
  };
  inout.0
}


use test::{Bencher, black_box};

#[bench]
fn nearest_original(b: &mut Bencher) {
  let min = black_box(-10_000);
  let max = black_box( 10_000);
  b.iter(|| {
    let mut sum = black_box(0_f64);
    for x in min..=max {
      sum += f64_nearest_original(x as f64 * 0.5);
    }
    black_box(sum)
  });
}

#[bench]
fn nearest_branch(b: &mut Bencher) {
  let min = black_box(-10_000);
  let max = black_box( 10_000);
  b.iter(|| {
    let mut sum = black_box(0_f64);
    for x in min..=max {
      sum += f64_nearest_branch(x as f64 * 0.5);
    }
    black_box(sum)
  });
}

#[bench]
fn nearest_copysign(b: &mut Bencher) {
  let min = black_box(-10_000);
  let max = black_box( 10_000);
  b.iter(|| {
    let mut sum = black_box(0_f64);
    for x in min..=max {
      sum += f64_nearest_copysign(x as f64 * 0.5);
    }
    black_box(sum)
  });
}

#[bench]
fn nearest_abs_copysign(b: &mut Bencher) {
  let min = black_box(-10_000);
  let max = black_box( 10_000);
  b.iter(|| {
    let mut sum = black_box(0_f64);
    for x in min..=max {
      sum += f64_nearest_abs_copysign(x as f64 * 0.5);
    }
    black_box(sum)
  });
}

#[bench]
fn nearest_abs_copysign_without_bits(b: &mut Bencher) {
  let min = black_box(-10_000);
  let max = black_box( 10_000);
  b.iter(|| {
    let mut sum = black_box(0_f64);
    for x in min..=max {
      sum += f64_nearest_abs_copysign_without_bits(x as f64 * 0.5);
    }
    black_box(sum)
  });
}

#[bench]
fn nearest_sse41(b: &mut Bencher) {
  let min = black_box(-10_000);
  let max = black_box( 10_000);
  b.iter(|| {
    let mut sum = black_box(0_f64);
    for x in min..=max {
      sum += f64_nearest_sse41(x as f64 * 0.5);
    }
    black_box(sum)
  });
}

launch with cargo bench:

test nearest_abs_copysign              ... bench:      35,993 ns/iter (+/- 7,475)
test nearest_abs_copysign_without_bits ... bench:      37,380 ns/iter (+/- 16,714)
test nearest_branch                    ... bench:      37,300 ns/iter (+/- 7,593)
test nearest_copysign                  ... bench:      32,348 ns/iter (+/- 4,869)
test nearest_original                  ... bench:      99,693 ns/iter (+/- 16,491)
test nearest_sse41                     ... bench:      40,587 ns/iter (+/- 3,854)

launch with RUSTFLAGS='-C target-cpu=native' cargo bench:

test nearest_abs_copysign              ... bench:      29,055 ns/iter (+/- 5,596)
test nearest_abs_copysign_without_bits ... bench:      28,080 ns/iter (+/- 9,036)
test nearest_branch                    ... bench:      45,084 ns/iter (+/- 3,673)
test nearest_copysign                  ... bench:      32,565 ns/iter (+/- 4,966)
test nearest_original                  ... bench:      53,212 ns/iter (+/- 9,682)
test nearest_sse41                     ... bench:      20,099 ns/iter (+/- 2,394)