float: Add f16 parsing and printing

Use the existing Lemire (decimal -> float) and Dragon / Grisu algorithms
(float -> decimal) to add support for `f16`. This allows updating the
implementation for `Display` to the expected behavior for `Display`
(currently it prints the a hex bitwise representation), matching other
floats, and adds a `FromStr` implementation.

In order to avoid crashes when compiling with Cranelift or on targets
where f16 is not well supported, a fallback is used if
`cfg(target_has_reliable_f16)` is not true.

Author: tgross35

library/core/Cargo.toml

@@ -35,4 +35,6 @@ check-cfg = [
     # and to stdarch `core_arch` crate which messes-up with Cargo list
     # of declared features, we therefor expect any feature cfg
     'cfg(feature, values(any()))',
+    # Internal features aren't marked known config by default
+    'cfg(target_has_reliable_f16)',
 ]

library/core/src/fmt/float.rs

@@ -20,6 +20,8 @@ macro_rules! impl_general_format {
     }
 }
 
+#[cfg(target_has_reliable_f16)]
+impl_general_format! { f16 }
 impl_general_format! { f32 f64 }
 
 // Don't inline this so callers don't use the stack space this function
@@ -231,6 +233,13 @@ macro_rules! floating {
 
 floating! { f32 f64 }
 
+#[cfg(target_has_reliable_f16)]
+floating! { f16 }
+
+// FIXME(f16_f128): A fallback is used when the backend+target does not support f16 well, in order
+// to avoid ICEs.
+
+#[cfg(not(target_has_reliable_f16))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Debug for f16 {
     #[inline]
@@ -239,6 +248,33 @@ impl Debug for f16 {
     }
 }
 
+#[cfg(not(target_has_reliable_f16))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Display for f16 {
+    #[inline]
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
+        Debug::fmt(self, fmt)
+    }
+}
+
+#[cfg(not(target_has_reliable_f16))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl LowerExp for f16 {
+    #[inline]
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
+        Debug::fmt(self, fmt)
+    }
+}
+
+#[cfg(not(target_has_reliable_f16))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl UpperExp for f16 {
+    #[inline]
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
+        Debug::fmt(self, fmt)
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Debug for f128 {
     #[inline]

library/core/src/lib.rs

@@ -101,6 +101,7 @@
 #![feature(bstr)]
 #![feature(bstr_internals)]
 #![feature(cfg_match)]
+#![feature(cfg_target_has_reliable_f16_f128)]
 #![feature(const_carrying_mul_add)]
 #![feature(const_eval_select)]
 #![feature(core_intrinsics)]

library/core/src/num/dec2flt/float.rs

@@ -45,7 +45,7 @@ macro_rules! int {
     }
 }
 
-int!(u32, u64);
+int!(u16, u32, u64);
 
 /// A helper trait to avoid duplicating basically all the conversion code for IEEE floats.
 ///
@@ -189,9 +189,14 @@ pub trait RawFloat:
 
     /// Returns the mantissa, exponent and sign as integers.
     ///
-    /// That is, this returns `(m, p, s)` such that `s * m * 2^p` represents the original float.
-    /// For 0, the exponent will be `-(EXP_BIAS + SIG_BITS`, which is the
-    /// minimum subnormal power.
+    /// This returns `(m, p, s)` such that `s * m * 2^p` represents the original float. For 0, the
+    /// exponent will be `-(EXP_BIAS + SIG_BITS)`, which is the minimum subnormal power. For
+    /// infinity or NaN, the exponent will be `EXP_SAT - EXP_BIAS - SIG_BITS`.
+    ///
+    /// If subnormal, the mantissa will be shifted one bit to the left. Otherwise, it is returned
+    /// with the explicit bit set but otherwise unshifted
+    ///
+    /// `s` is only ever +/-1.
     fn integer_decode(self) -> (u64, i16, i8) {
         let bits = self.to_bits();
         let sign: i8 = if bits >> (Self::BITS - 1) == Self::Int::ZERO { 1 } else { -1 };
@@ -213,6 +218,50 @@ const fn pow2_to_pow10(a: i64) -> i64 {
     res as i64
 }
 
+#[cfg(target_has_reliable_f16)]
+impl RawFloat for f16 {
+    type Int = u16;
+
+    const INFINITY: Self = Self::INFINITY;
+    const NEG_INFINITY: Self = Self::NEG_INFINITY;
+    const NAN: Self = Self::NAN;
+    const NEG_NAN: Self = -Self::NAN;
+
+    const BITS: u32 = 16;
+    const SIG_TOTAL_BITS: u32 = Self::MANTISSA_DIGITS;
+    const EXP_MASK: Self::Int = Self::EXP_MASK;
+    const SIG_MASK: Self::Int = Self::MAN_MASK;
+
+    const MIN_EXPONENT_ROUND_TO_EVEN: i32 = -22;
+    const MAX_EXPONENT_ROUND_TO_EVEN: i32 = 5;
+    const SMALLEST_POWER_OF_TEN: i32 = -27;
+
+    #[inline]
+    fn from_u64(v: u64) -> Self {
+        debug_assert!(v <= Self::MAX_MANTISSA_FAST_PATH);
+        v as _
+    }
+
+    #[inline]
+    fn from_u64_bits(v: u64) -> Self {
+        Self::from_bits((v & 0xFFFF) as u16)
+    }
+
+    fn pow10_fast_path(exponent: usize) -> Self {
+        #[allow(clippy::use_self)]
+        const TABLE: [f16; 8] = [1e0, 1e1, 1e2, 1e3, 1e4, 0.0, 0.0, 0.];
+        TABLE[exponent & 7]
+    }
+
+    fn to_bits(self) -> Self::Int {
+        self.to_bits()
+    }
+
+    fn classify(self) -> FpCategory {
+        self.classify()
+    }
+}
+
 impl RawFloat for f32 {
     type Int = u32;
 

library/core/src/num/dec2flt/mod.rs

@@ -171,9 +171,26 @@ macro_rules! from_str_float_impl {
         }
     };
 }
+
+#[cfg(target_has_reliable_f16)]
+from_str_float_impl!(f16);
 from_str_float_impl!(f32);
 from_str_float_impl!(f64);
 
+// FIXME(f16_f128): A fallback is used when the backend+target does not support f16 well, in order
+// to avoid ICEs.
+
+// After the bootstrap bump this should be: `#[cfg(not(target_has_reliable_f16))`
+#[cfg(not(target_has_reliable_f16))]
+impl FromStr for f16 {
+    type Err = ParseFloatError;
+
+    #[inline]
+    fn from_str(_src: &str) -> Result<Self, ParseFloatError> {
+        unimplemented!("requires target_has_reliable_f16")
+    }
+}
+
 /// An error which can be returned when parsing a float.
 ///
 /// This error is used as the error type for the [`FromStr`] implementation

library/core/src/num/flt2dec/decoder.rs

@@ -45,6 +45,13 @@ pub trait DecodableFloat: RawFloat + Copy {
     fn min_pos_norm_value() -> Self;
 }
 
+#[cfg(target_has_reliable_f16)]
+impl DecodableFloat for f16 {
+    fn min_pos_norm_value() -> Self {
+        f16::MIN_POSITIVE
+    }
+}
+
 impl DecodableFloat for f32 {
     fn min_pos_norm_value() -> Self {
         f32::MIN_POSITIVE