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Implement total_cmp for f32, f64 #72568

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merged 7 commits into from
May 29, 2020
Merged

Implement total_cmp for f32, f64 #72568

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68bab3e
Add total_cmp to f32 and f64, plus tests
golddranks May 22, 2020
b6eec22
Fix typo in src/libcore/num/f32.rs
golddranks May 25, 2020
d6650e0
Fix typo in src/libcore/num/f32.rs
golddranks May 25, 2020
bd68de8
remove unneeded and unidiomatic must_use
golddranks May 25, 2020
6973fd7
Add bit twiddling
golddranks May 25, 2020
8bc31ff
Fix the same typos again orz
golddranks May 25, 2020
66da735
Add tracing issue for total_cmp
golddranks May 26, 2020
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74 changes: 74 additions & 0 deletions src/libcore/num/f32.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -810,4 +810,78 @@ impl f32 {
pub fn from_ne_bytes(bytes: [u8; 4]) -> Self {
Self::from_bits(u32::from_ne_bytes(bytes))
}

/// Returns an ordering between self and other values.
/// Unlike the standard partial comparison between floating point numbers,
/// this comparison always produces an ordering in accordance to
/// the totalOrder predicate as defined in IEEE 754 (2008 revision)
/// floating point standard. The values are ordered in following order:
/// - Negative quiet NaN
/// - Negative signaling NaN
/// - Negative infinity
/// - Negative numbers
/// - Negative subnormal numbers
/// - Negative zero
/// - Positive zero
/// - Positive subnormal numbers
/// - Positive numbers
/// - Positive infinity
/// - Positive signaling NaN
/// - Positive quiet NaN
///
/// # Example
/// ```
/// #![feature(total_cmp)]
/// struct GoodBoy {
/// name: String,
/// weight: f32,
/// }
///
/// let mut bois = vec![
/// GoodBoy { name: "Pucci".to_owned(), weight: 0.1 },
/// GoodBoy { name: "Woofer".to_owned(), weight: 99.0 },
/// GoodBoy { name: "Yapper".to_owned(), weight: 10.0 },
/// GoodBoy { name: "Chonk".to_owned(), weight: f32::INFINITY },
/// GoodBoy { name: "Abs. Unit".to_owned(), weight: f32::NAN },
/// GoodBoy { name: "Floaty".to_owned(), weight: -5.0 },
/// ];
///
/// bois.sort_by(|a, b| a.weight.total_cmp(&b.weight));
/// # assert!(bois.into_iter().map(|b| b.weight)
/// # .zip([-5.0, 0.1, 10.0, 99.0, f32::INFINITY, f32::NAN].iter())
/// # .all(|(a, b)| a.to_bits() == b.to_bits()))
/// ```
#[unstable(feature = "total_cmp", issue = "72599")]
#[inline]
pub fn total_cmp(&self, other: &Self) -> crate::cmp::Ordering {
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Is this implementation sourced from somewhere else? Might be nice to link to that for easy reference.

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The code is from scratch but the idea was adapted from an earlier thread here: rust-lang/rfcs#1249 (comment)

let mut left = self.to_bits() as i32;
let mut right = other.to_bits() as i32;

// In case of negatives, flip all the bits except the sign
// to achieve a similar layout as two's complement integers
//
// Why does this work? IEEE 754 floats consist of three fields:
// Sign bit, exponent and mantissa. The set of exponent and mantissa
// fields as a whole have the property that their bitwise order is
// equal to the numeric magnitude where the magnitude is defined.
// The magnitude is not normally defined on NaN values, but
// IEEE 754 totalOrder defines the NaN values also to follow the
// bitwise order. This leads to order explained in the doc comment.
// However, the representation of magnitude is the same for negative
// and positive numbers – only the sign bit is different.
// To easily compare the floats as signed integers, we need to
// flip the exponent and mantissa bits in case of negative numbers.
// We effectively convert the numbers to "two's complement" form.
//
// To do the flipping, we construct a mask and XOR against it.
// We branchlessly calculate an "all-ones except for the sign bit"
// mask from negative-signed values: right shifting sign-extends
// the integer, so we "fill" the mask with sign bits, and then
// convert to unsigned to push one more zero bit.
// On positive values, the mask is all zeros, so it's a no-op.
left ^= (((left >> 31) as u32) >> 1) as i32;
right ^= (((right >> 31) as u32) >> 1) as i32;

left.cmp(&right)
}
}
74 changes: 74 additions & 0 deletions src/libcore/num/f64.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -824,4 +824,78 @@ impl f64 {
pub fn from_ne_bytes(bytes: [u8; 8]) -> Self {
Self::from_bits(u64::from_ne_bytes(bytes))
}

/// Returns an ordering between self and other values.
/// Unlike the standard partial comparison between floating point numbers,
/// this comparison always produces an ordering in accordance to
/// the totalOrder predicate as defined in IEEE 754 (2008 revision)
/// floating point standard. The values are ordered in following order:
/// - Negative quiet NaN
/// - Negative signaling NaN
/// - Negative infinity
/// - Negative numbers
/// - Negative subnormal numbers
/// - Negative zero
/// - Positive zero
/// - Positive subnormal numbers
/// - Positive numbers
/// - Positive infinity
/// - Positive signaling NaN
/// - Positive quiet NaN
///
/// # Example
/// ```
/// #![feature(total_cmp)]
/// struct GoodBoy {
/// name: String,
/// weight: f64,
/// }
///
/// let mut bois = vec![
/// GoodBoy { name: "Pucci".to_owned(), weight: 0.1 },
/// GoodBoy { name: "Woofer".to_owned(), weight: 99.0 },
/// GoodBoy { name: "Yapper".to_owned(), weight: 10.0 },
/// GoodBoy { name: "Chonk".to_owned(), weight: f64::INFINITY },
/// GoodBoy { name: "Abs. Unit".to_owned(), weight: f64::NAN },
/// GoodBoy { name: "Floaty".to_owned(), weight: -5.0 },
/// ];
///
/// bois.sort_by(|a, b| a.weight.total_cmp(&b.weight));
/// # assert!(bois.into_iter().map(|b| b.weight)
/// # .zip([-5.0, 0.1, 10.0, 99.0, f64::INFINITY, f64::NAN].iter())
/// # .all(|(a, b)| a.to_bits() == b.to_bits()))
/// ```
#[unstable(feature = "total_cmp", issue = "72599")]
#[inline]
pub fn total_cmp(&self, other: &Self) -> crate::cmp::Ordering {
let mut left = self.to_bits() as i64;
let mut right = other.to_bits() as i64;

// In case of negatives, flip all the bits except the sign
// to achieve a similar layout as two's complement integers
//
// Why does this work? IEEE 754 floats consist of three fields:
// Sign bit, exponent and mantissa. The set of exponent and mantissa
// fields as a whole have the property that their bitwise order is
// equal to the numeric magnitude where the magnitude is defined.
// The magnitude is not normally defined on NaN values, but
// IEEE 754 totalOrder defines the NaN values also to follow the
// bitwise order. This leads to order explained in the doc comment.
// However, the representation of magnitude is the same for negative
// and positive numbers – only the sign bit is different.
// To easily compare the floats as signed integers, we need to
// flip the exponent and mantissa bits in case of negative numbers.
// We effectively convert the numbers to "two's complement" form.
//
// To do the flipping, we construct a mask and XOR against it.
// We branchlessly calculate an "all-ones except for the sign bit"
// mask from negative-signed values: right shifting sign-extends
// the integer, so we "fill" the mask with sign bits, and then
// convert to unsigned to push one more zero bit.
// On positive values, the mask is all zeros, so it's a no-op.
left ^= (((left >> 63) as u64) >> 1) as i64;
right ^= (((right >> 63) as u64) >> 1) as i64;

left.cmp(&right)
}
}
143 changes: 143 additions & 0 deletions src/libstd/f32.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1531,4 +1531,147 @@ mod tests {
fn test_clamp_max_is_nan() {
let _ = 1.0f32.clamp(3.0, NAN);
}

#[test]
fn test_total_cmp() {
use core::cmp::Ordering;

fn quiet_bit_mask() -> u32 {
1 << (f32::MANTISSA_DIGITS - 2)
}

fn min_subnorm() -> f32 {
f32::MIN_POSITIVE / f32::powf(2.0, f32::MANTISSA_DIGITS as f32 - 1.0)
}

fn max_subnorm() -> f32 {
f32::MIN_POSITIVE - min_subnorm()
}

fn q_nan() -> f32 {
f32::from_bits(f32::NAN.to_bits() | quiet_bit_mask())
}

fn s_nan() -> f32 {
f32::from_bits((f32::NAN.to_bits() & !quiet_bit_mask()) + 42)
}

assert_eq!(Ordering::Equal, (-q_nan()).total_cmp(&-q_nan()));
assert_eq!(Ordering::Equal, (-s_nan()).total_cmp(&-s_nan()));
assert_eq!(Ordering::Equal, (-f32::INFINITY).total_cmp(&-f32::INFINITY));
assert_eq!(Ordering::Equal, (-f32::MAX).total_cmp(&-f32::MAX));
assert_eq!(Ordering::Equal, (-2.5_f32).total_cmp(&-2.5));
assert_eq!(Ordering::Equal, (-1.0_f32).total_cmp(&-1.0));
assert_eq!(Ordering::Equal, (-1.5_f32).total_cmp(&-1.5));
assert_eq!(Ordering::Equal, (-0.5_f32).total_cmp(&-0.5));
assert_eq!(Ordering::Equal, (-f32::MIN_POSITIVE).total_cmp(&-f32::MIN_POSITIVE));
assert_eq!(Ordering::Equal, (-max_subnorm()).total_cmp(&-max_subnorm()));
assert_eq!(Ordering::Equal, (-min_subnorm()).total_cmp(&-min_subnorm()));
assert_eq!(Ordering::Equal, (-0.0_f32).total_cmp(&-0.0));
assert_eq!(Ordering::Equal, 0.0_f32.total_cmp(&0.0));
assert_eq!(Ordering::Equal, min_subnorm().total_cmp(&min_subnorm()));
assert_eq!(Ordering::Equal, max_subnorm().total_cmp(&max_subnorm()));
assert_eq!(Ordering::Equal, f32::MIN_POSITIVE.total_cmp(&f32::MIN_POSITIVE));
assert_eq!(Ordering::Equal, 0.5_f32.total_cmp(&0.5));
assert_eq!(Ordering::Equal, 1.0_f32.total_cmp(&1.0));
assert_eq!(Ordering::Equal, 1.5_f32.total_cmp(&1.5));
assert_eq!(Ordering::Equal, 2.5_f32.total_cmp(&2.5));
assert_eq!(Ordering::Equal, f32::MAX.total_cmp(&f32::MAX));
assert_eq!(Ordering::Equal, f32::INFINITY.total_cmp(&f32::INFINITY));
assert_eq!(Ordering::Equal, s_nan().total_cmp(&s_nan()));
assert_eq!(Ordering::Equal, q_nan().total_cmp(&q_nan()));

assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-s_nan()));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&-f32::INFINITY));
assert_eq!(Ordering::Less, (-f32::INFINITY).total_cmp(&-f32::MAX));
assert_eq!(Ordering::Less, (-f32::MAX).total_cmp(&-2.5));
assert_eq!(Ordering::Less, (-2.5_f32).total_cmp(&-1.5));
assert_eq!(Ordering::Less, (-1.5_f32).total_cmp(&-1.0));
assert_eq!(Ordering::Less, (-1.0_f32).total_cmp(&-0.5));
assert_eq!(Ordering::Less, (-0.5_f32).total_cmp(&-f32::MIN_POSITIVE));
assert_eq!(Ordering::Less, (-f32::MIN_POSITIVE).total_cmp(&-max_subnorm()));
assert_eq!(Ordering::Less, (-max_subnorm()).total_cmp(&-min_subnorm()));
assert_eq!(Ordering::Less, (-min_subnorm()).total_cmp(&-0.0));
assert_eq!(Ordering::Less, (-0.0_f32).total_cmp(&0.0));
assert_eq!(Ordering::Less, 0.0_f32.total_cmp(&min_subnorm()));
assert_eq!(Ordering::Less, min_subnorm().total_cmp(&max_subnorm()));
assert_eq!(Ordering::Less, max_subnorm().total_cmp(&f32::MIN_POSITIVE));
assert_eq!(Ordering::Less, f32::MIN_POSITIVE.total_cmp(&0.5));
assert_eq!(Ordering::Less, 0.5_f32.total_cmp(&1.0));
assert_eq!(Ordering::Less, 1.0_f32.total_cmp(&1.5));
assert_eq!(Ordering::Less, 1.5_f32.total_cmp(&2.5));
assert_eq!(Ordering::Less, 2.5_f32.total_cmp(&f32::MAX));
assert_eq!(Ordering::Less, f32::MAX.total_cmp(&f32::INFINITY));
assert_eq!(Ordering::Less, f32::INFINITY.total_cmp(&s_nan()));
assert_eq!(Ordering::Less, s_nan().total_cmp(&q_nan()));

assert_eq!(Ordering::Greater, (-s_nan()).total_cmp(&-q_nan()));
assert_eq!(Ordering::Greater, (-f32::INFINITY).total_cmp(&-s_nan()));
assert_eq!(Ordering::Greater, (-f32::MAX).total_cmp(&-f32::INFINITY));
assert_eq!(Ordering::Greater, (-2.5_f32).total_cmp(&-f32::MAX));
assert_eq!(Ordering::Greater, (-1.5_f32).total_cmp(&-2.5));
assert_eq!(Ordering::Greater, (-1.0_f32).total_cmp(&-1.5));
assert_eq!(Ordering::Greater, (-0.5_f32).total_cmp(&-1.0));
assert_eq!(Ordering::Greater, (-f32::MIN_POSITIVE).total_cmp(&-0.5));
assert_eq!(Ordering::Greater, (-max_subnorm()).total_cmp(&-f32::MIN_POSITIVE));
assert_eq!(Ordering::Greater, (-min_subnorm()).total_cmp(&-max_subnorm()));
assert_eq!(Ordering::Greater, (-0.0_f32).total_cmp(&-min_subnorm()));
assert_eq!(Ordering::Greater, 0.0_f32.total_cmp(&-0.0));
assert_eq!(Ordering::Greater, min_subnorm().total_cmp(&0.0));
assert_eq!(Ordering::Greater, max_subnorm().total_cmp(&min_subnorm()));
assert_eq!(Ordering::Greater, f32::MIN_POSITIVE.total_cmp(&max_subnorm()));
assert_eq!(Ordering::Greater, 0.5_f32.total_cmp(&f32::MIN_POSITIVE));
assert_eq!(Ordering::Greater, 1.0_f32.total_cmp(&0.5));
assert_eq!(Ordering::Greater, 1.5_f32.total_cmp(&1.0));
assert_eq!(Ordering::Greater, 2.5_f32.total_cmp(&1.5));
assert_eq!(Ordering::Greater, f32::MAX.total_cmp(&2.5));
assert_eq!(Ordering::Greater, f32::INFINITY.total_cmp(&f32::MAX));
assert_eq!(Ordering::Greater, s_nan().total_cmp(&f32::INFINITY));
assert_eq!(Ordering::Greater, q_nan().total_cmp(&s_nan()));

assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-s_nan()));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-f32::INFINITY));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-f32::MAX));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-2.5));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-1.5));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-1.0));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-0.5));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-f32::MIN_POSITIVE));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-max_subnorm()));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-min_subnorm()));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&-0.0));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&0.0));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&min_subnorm()));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&max_subnorm()));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&f32::MIN_POSITIVE));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&0.5));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&1.0));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&1.5));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&2.5));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&f32::MAX));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&f32::INFINITY));
assert_eq!(Ordering::Less, (-q_nan()).total_cmp(&s_nan()));

assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&-f32::INFINITY));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&-f32::MAX));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&-2.5));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&-1.5));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&-1.0));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&-0.5));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&-f32::MIN_POSITIVE));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&-max_subnorm()));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&-min_subnorm()));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&-0.0));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&0.0));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&min_subnorm()));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&max_subnorm()));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&f32::MIN_POSITIVE));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&0.5));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&1.0));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&1.5));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&2.5));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&f32::MAX));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&f32::INFINITY));
assert_eq!(Ordering::Less, (-s_nan()).total_cmp(&s_nan()));
}
}
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