diff options
Diffstat (limited to 'library/core/src/num/f16.rs')
| -rw-r--r-- | library/core/src/num/f16.rs | 162 |
1 files changed, 161 insertions, 1 deletions
diff --git a/library/core/src/num/f16.rs b/library/core/src/num/f16.rs index 3c58b0af9c2..198a506f2b8 100644 --- a/library/core/src/num/f16.rs +++ b/library/core/src/num/f16.rs @@ -13,6 +13,7 @@ use crate::convert::FloatToInt; use crate::mem; +use crate::num::FpCategory; /// Basic mathematical constants. #[unstable(feature = "f16", issue = "116909")] @@ -244,7 +245,13 @@ impl f16 { /// Sign bit #[cfg(not(bootstrap))] - const SIGN_MASK: u16 = 0x8000; + pub(crate) const SIGN_MASK: u16 = 0x8000; + + /// Exponent mask + pub(crate) const EXP_MASK: u16 = 0x7c00; + + /// Mantissa mask + pub(crate) const MAN_MASK: u16 = 0x03ff; /// Minimum representable positive value (min subnormal) #[cfg(not(bootstrap))] @@ -344,6 +351,159 @@ impl f16 { self.abs_private() < Self::INFINITY } + /// Returns `true` if the number is [subnormal]. + /// + /// ``` + /// #![feature(f16)] + /// # #[cfg(target_arch = "aarch64")] { // FIXME(f16_F128): rust-lang/rust#123885 + /// + /// let min = f16::MIN_POSITIVE; // 6.1035e-5 + /// let max = f16::MAX; + /// let lower_than_min = 1.0e-7_f16; + /// let zero = 0.0_f16; + /// + /// assert!(!min.is_subnormal()); + /// assert!(!max.is_subnormal()); + /// + /// assert!(!zero.is_subnormal()); + /// assert!(!f16::NAN.is_subnormal()); + /// assert!(!f16::INFINITY.is_subnormal()); + /// // Values between `0` and `min` are Subnormal. + /// assert!(lower_than_min.is_subnormal()); + /// # } + /// ``` + /// [subnormal]: https://en.wikipedia.org/wiki/Denormal_number + #[inline] + #[must_use] + #[cfg(not(bootstrap))] + #[unstable(feature = "f16", issue = "116909")] + #[rustc_const_unstable(feature = "const_float_classify", issue = "72505")] + pub const fn is_subnormal(self) -> bool { + matches!(self.classify(), FpCategory::Subnormal) + } + + /// Returns `true` if the number is neither zero, infinite, [subnormal], or NaN. + /// + /// ``` + /// #![feature(f16)] + /// # #[cfg(target_arch = "aarch64")] { // FIXME(f16_F128): rust-lang/rust#123885 + /// + /// let min = f16::MIN_POSITIVE; // 6.1035e-5 + /// let max = f16::MAX; + /// let lower_than_min = 1.0e-7_f16; + /// let zero = 0.0_f16; + /// + /// assert!(min.is_normal()); + /// assert!(max.is_normal()); + /// + /// assert!(!zero.is_normal()); + /// assert!(!f16::NAN.is_normal()); + /// assert!(!f16::INFINITY.is_normal()); + /// // Values between `0` and `min` are Subnormal. + /// assert!(!lower_than_min.is_normal()); + /// # } + /// ``` + /// [subnormal]: https://en.wikipedia.org/wiki/Denormal_number + #[inline] + #[must_use] + #[cfg(not(bootstrap))] + #[unstable(feature = "f16", issue = "116909")] + #[rustc_const_unstable(feature = "const_float_classify", issue = "72505")] + pub const fn is_normal(self) -> bool { + matches!(self.classify(), FpCategory::Normal) + } + + /// Returns the floating point category of the number. If only one property + /// is going to be tested, it is generally faster to use the specific + /// predicate instead. + /// + /// ``` + /// #![feature(f16)] + /// # #[cfg(target_arch = "aarch64")] { // FIXME(f16_F128): rust-lang/rust#123885 + /// + /// use std::num::FpCategory; + /// + /// let num = 12.4_f16; + /// let inf = f16::INFINITY; + /// + /// assert_eq!(num.classify(), FpCategory::Normal); + /// assert_eq!(inf.classify(), FpCategory::Infinite); + /// # } + /// ``` + #[inline] + #[cfg(not(bootstrap))] + #[unstable(feature = "f16", issue = "116909")] + #[rustc_const_unstable(feature = "const_float_classify", issue = "72505")] + pub const fn classify(self) -> FpCategory { + // A previous implementation for f32/f64 tried to only use bitmask-based checks, + // using `to_bits` to transmute the float to its bit repr and match on that. + // Unfortunately, floating point numbers can be much worse than that. + // This also needs to not result in recursive evaluations of `to_bits`. + // + + // Platforms without native support generally convert to `f32` to perform operations, + // and most of these platforms correctly round back to `f16` after each operation. + // However, some platforms have bugs where they keep the excess `f32` precision (e.g. + // WASM, see llvm/llvm-project#96437). This implementation makes a best-effort attempt + // to account for that excess precision. + if self.is_infinite() { + // Thus, a value may compare unequal to infinity, despite having a "full" exponent mask. + FpCategory::Infinite + } else if self.is_nan() { + // And it may not be NaN, as it can simply be an "overextended" finite value. + FpCategory::Nan + } else { + // However, std can't simply compare to zero to check for zero, either, + // as correctness requires avoiding equality tests that may be Subnormal == -0.0 + // because it may be wrong under "denormals are zero" and "flush to zero" modes. + // Most of std's targets don't use those, but they are used for thumbv7neon. + // So, this does use bitpattern matching for the rest. + + // SAFETY: f16 to u16 is fine. Usually. + // If classify has gotten this far, the value is definitely in one of these categories. + unsafe { f16::partial_classify(self) } + } + } + + /// This doesn't actually return a right answer for NaN on purpose, + /// seeing as how it cannot correctly discern between a floating point NaN, + /// and some normal floating point numbers truncated from an x87 FPU. + /// + /// # Safety + /// + /// This requires making sure you call this function for values it answers correctly on, + /// otherwise it returns a wrong answer. This is not important for memory safety per se, + /// but getting floats correct is important for not accidentally leaking const eval + /// runtime-deviating logic which may or may not be acceptable. + #[inline] + #[cfg(not(bootstrap))] + #[rustc_const_unstable(feature = "const_float_classify", issue = "72505")] + const unsafe fn partial_classify(self) -> FpCategory { + // SAFETY: The caller is not asking questions for which this will tell lies. + let b = unsafe { mem::transmute::<f16, u16>(self) }; + match (b & Self::MAN_MASK, b & Self::EXP_MASK) { + (0, Self::EXP_MASK) => FpCategory::Infinite, + (0, 0) => FpCategory::Zero, + (_, 0) => FpCategory::Subnormal, + _ => FpCategory::Normal, + } + } + + /// This operates on bits, and only bits, so it can ignore concerns about weird FPUs. + /// FIXME(jubilee): In a just world, this would be the entire impl for classify, + /// plus a transmute. We do not live in a just world, but we can make it more so. + #[inline] + #[rustc_const_unstable(feature = "const_float_classify", issue = "72505")] + const fn classify_bits(b: u16) -> FpCategory { + match (b & Self::MAN_MASK, b & Self::EXP_MASK) { + (0, Self::EXP_MASK) => FpCategory::Infinite, + (_, Self::EXP_MASK) => FpCategory::Nan, + (0, 0) => FpCategory::Zero, + (_, 0) => FpCategory::Subnormal, + _ => FpCategory::Normal, + } + } + /// Returns `true` if `self` has a positive sign, including `+0.0`, NaNs with /// positive sign bit and positive infinity. Note that IEEE 754 doesn't assign any /// meaning to the sign bit in case of a NaN, and as Rust doesn't guarantee that |