Add a `Debug` impl and some basic functions to `f16` and `f128`

`compiler_builtins` uses some convenience functions like `is_nan` and
`is_sign_positive`. Add these, as well as a temporary implementation for
`Debug` that prints the bit representation.

3 files changed, 224 insertions, 0 deletions

diff --git a/library/core/src/fmt/float.rs b/library/core/src/fmt/float.rs
index 3bbf5d8770b..7f23d3c0956 100644
--- a/library/core/src/fmt/float.rs
+++ b/library/core/src/fmt/float.rs
@@ -228,3 +228,19 @@ macro_rules! floating {
 
 floating! { f32 }
 floating! { f64 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Debug for f16 {
+    #[inline]
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+        write!(f, "{:#06x}", self.to_bits())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Debug for f128 {
+    #[inline]
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+        write!(f, "{:#034x}", self.to_bits())
+    }
+}
diff --git a/library/core/src/num/f128.rs b/library/core/src/num/f128.rs
index 3bc3e2dbf12..8a94964c8c5 100644
--- a/library/core/src/num/f128.rs
+++ b/library/core/src/num/f128.rs
@@ -11,6 +11,110 @@
 
 #![unstable(feature = "f128", issue = "116909")]
 
+use crate::mem;
+
 /// Basic mathematical constants.
 #[unstable(feature = "f128", issue = "116909")]
 pub mod consts {}
+
+#[cfg(not(test))]
+impl f128 {
+    // FIXME(f16_f128): almost everything in this `impl` is missing examples and a const
+    // implementation. Add these once we can run code on all platforms and have f16/f128 in CTFE.
+
+    /// Returns `true` if this value is NaN.
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "f128", issue = "116909")]
+    #[allow(clippy::eq_op)] // > if you intended to check if the operand is NaN, use `.is_nan()` instead :)
+    pub const fn is_nan(self) -> bool {
+        self != self
+    }
+
+    /// 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
+    /// the bit pattern of NaNs are conserved over arithmetic operations, the result of
+    /// `is_sign_positive` on a NaN might produce an unexpected result in some cases.
+    /// See [explanation of NaN as a special value](f32) for more info.
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "f128", issue = "116909")]
+    pub fn is_sign_positive(self) -> bool {
+        !self.is_sign_negative()
+    }
+
+    /// Returns `true` if `self` has a negative sign, including `-0.0`, NaNs with
+    /// negative sign bit and negative 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
+    /// the bit pattern of NaNs are conserved over arithmetic operations, the result of
+    /// `is_sign_negative` on a NaN might produce an unexpected result in some cases.
+    /// See [explanation of NaN as a special value](f32) for more info.
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "f128", issue = "116909")]
+    pub fn is_sign_negative(self) -> bool {
+        // IEEE754 says: isSignMinus(x) is true if and only if x has negative sign. isSignMinus
+        // applies to zeros and NaNs as well.
+        // SAFETY: This is just transmuting to get the sign bit, it's fine.
+        (self.to_bits() & (1 << 127)) != 0
+    }
+
+    /// Raw transmutation to `u128`.
+    ///
+    /// This is currently identical to `transmute::<f128, u128>(self)` on all platforms.
+    ///
+    /// See [`from_bits`](#method.from_bits) for some discussion of the
+    /// portability of this operation (there are almost no issues).
+    ///
+    /// Note that this function is distinct from `as` casting, which attempts to
+    /// preserve the *numeric* value, and not the bitwise value.
+    #[inline]
+    #[unstable(feature = "f128", issue = "116909")]
+    #[must_use = "this returns the result of the operation, without modifying the original"]
+    pub fn to_bits(self) -> u128 {
+        // SAFETY: `u128` is a plain old datatype so we can always... uh...
+        // ...look, just pretend you forgot what you just read.
+        // Stability concerns.
+        unsafe { mem::transmute(self) }
+    }
+
+    /// Raw transmutation from `u128`.
+    ///
+    /// This is currently identical to `transmute::<u128, f128>(v)` on all platforms.
+    /// It turns out this is incredibly portable, for two reasons:
+    ///
+    /// * Floats and Ints have the same endianness on all supported platforms.
+    /// * IEEE 754 very precisely specifies the bit layout of floats.
+    ///
+    /// However there is one caveat: prior to the 2008 version of IEEE 754, how
+    /// to interpret the NaN signaling bit wasn't actually specified. Most platforms
+    /// (notably x86 and ARM) picked the interpretation that was ultimately
+    /// standardized in 2008, but some didn't (notably MIPS). As a result, all
+    /// signaling NaNs on MIPS are quiet NaNs on x86, and vice-versa.
+    ///
+    /// Rather than trying to preserve signaling-ness cross-platform, this
+    /// implementation favors preserving the exact bits. This means that
+    /// any payloads encoded in NaNs will be preserved even if the result of
+    /// this method is sent over the network from an x86 machine to a MIPS one.
+    ///
+    /// If the results of this method are only manipulated by the same
+    /// architecture that produced them, then there is no portability concern.
+    ///
+    /// If the input isn't NaN, then there is no portability concern.
+    ///
+    /// If you don't care about signalingness (very likely), then there is no
+    /// portability concern.
+    ///
+    /// Note that this function is distinct from `as` casting, which attempts to
+    /// preserve the *numeric* value, and not the bitwise value.
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "f128", issue = "116909")]
+    pub fn from_bits(v: u128) -> Self {
+        // SAFETY: `u128 is a plain old datatype so we can always... uh...
+        // ...look, just pretend you forgot what you just read.
+        // Stability concerns.
+        unsafe { mem::transmute(v) }
+    }
+}
diff --git a/library/core/src/num/f16.rs b/library/core/src/num/f16.rs
index 969ebdc5690..039f5188ba4 100644
--- a/library/core/src/num/f16.rs
+++ b/library/core/src/num/f16.rs
@@ -11,6 +11,110 @@
 
 #![unstable(feature = "f16", issue = "116909")]
 
+use crate::mem;
+
 /// Basic mathematical constants.
 #[unstable(feature = "f16", issue = "116909")]
 pub mod consts {}
+
+#[cfg(not(test))]
+impl f16 {
+    // FIXME(f16_f128): almost everything in this `impl` is missing examples and a const
+    // implementation. Add these once we can run code on all platforms and have f16/f128 in CTFE.
+
+    /// Returns `true` if this value is NaN.
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "f16", issue = "116909")]
+    #[allow(clippy::eq_op)] // > if you intended to check if the operand is NaN, use `.is_nan()` instead :)
+    pub const fn is_nan(self) -> bool {
+        self != self
+    }
+
+    /// 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
+    /// the bit pattern of NaNs are conserved over arithmetic operations, the result of
+    /// `is_sign_positive` on a NaN might produce an unexpected result in some cases.
+    /// See [explanation of NaN as a special value](f32) for more info.
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "f128", issue = "116909")]
+    pub fn is_sign_positive(self) -> bool {
+        !self.is_sign_negative()
+    }
+
+    /// Returns `true` if `self` has a negative sign, including `-0.0`, NaNs with
+    /// negative sign bit and negative 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
+    /// the bit pattern of NaNs are conserved over arithmetic operations, the result of
+    /// `is_sign_negative` on a NaN might produce an unexpected result in some cases.
+    /// See [explanation of NaN as a special value](f32) for more info.
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "f128", issue = "116909")]
+    pub fn is_sign_negative(self) -> bool {
+        // IEEE754 says: isSignMinus(x) is true if and only if x has negative sign. isSignMinus
+        // applies to zeros and NaNs as well.
+        // SAFETY: This is just transmuting to get the sign bit, it's fine.
+        (self.to_bits() & (1 << 15)) != 0
+    }
+
+    /// Raw transmutation to `u16`.
+    ///
+    /// This is currently identical to `transmute::<f16, u16>(self)` on all platforms.
+    ///
+    /// See [`from_bits`](#method.from_bits) for some discussion of the
+    /// portability of this operation (there are almost no issues).
+    ///
+    /// Note that this function is distinct from `as` casting, which attempts to
+    /// preserve the *numeric* value, and not the bitwise value.
+    #[inline]
+    #[unstable(feature = "f16", issue = "116909")]
+    #[must_use = "this returns the result of the operation, without modifying the original"]
+    pub fn to_bits(self) -> u16 {
+        // SAFETY: `u16` is a plain old datatype so we can always... uh...
+        // ...look, just pretend you forgot what you just read.
+        // Stability concerns.
+        unsafe { mem::transmute(self) }
+    }
+
+    /// Raw transmutation from `u16`.
+    ///
+    /// This is currently identical to `transmute::<u16, f16>(v)` on all platforms.
+    /// It turns out this is incredibly portable, for two reasons:
+    ///
+    /// * Floats and Ints have the same endianness on all supported platforms.
+    /// * IEEE 754 very precisely specifies the bit layout of floats.
+    ///
+    /// However there is one caveat: prior to the 2008 version of IEEE 754, how
+    /// to interpret the NaN signaling bit wasn't actually specified. Most platforms
+    /// (notably x86 and ARM) picked the interpretation that was ultimately
+    /// standardized in 2008, but some didn't (notably MIPS). As a result, all
+    /// signaling NaNs on MIPS are quiet NaNs on x86, and vice-versa.
+    ///
+    /// Rather than trying to preserve signaling-ness cross-platform, this
+    /// implementation favors preserving the exact bits. This means that
+    /// any payloads encoded in NaNs will be preserved even if the result of
+    /// this method is sent over the network from an x86 machine to a MIPS one.
+    ///
+    /// If the results of this method are only manipulated by the same
+    /// architecture that produced them, then there is no portability concern.
+    ///
+    /// If the input isn't NaN, then there is no portability concern.
+    ///
+    /// If you don't care about signalingness (very likely), then there is no
+    /// portability concern.
+    ///
+    /// Note that this function is distinct from `as` casting, which attempts to
+    /// preserve the *numeric* value, and not the bitwise value.
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "f16", issue = "116909")]
+    pub fn from_bits(v: u16) -> Self {
+        // SAFETY: `u16` is a plain old datatype so we can always... uh...
+        // ...look, just pretend you forgot what you just read.
+        // Stability concerns.
+        unsafe { mem::transmute(v) }
+    }
+}