impl std::simd::StdFloat

While consulting with Simulacrum on how to make available the float
functions that currently require runtime support for `Simd<f32, N>` and
`Simd<f64, N>`, we realized breaking coherence with the classic approach
of lang items was, since `{core,std}::simd::Simd` is a `ty::Adt`, likely
to be quite a bit nasty. The project group has a long-term plan for how
to get around this kind of issue and move the associated functions into
libcore, but that will likely take time as well. Since all routes
forward are temporally costly, we probably will skip the lang item
approach entirely and go the "proper" route, but in the interests of
having something this year for people to play around with, this
extension trait was whipped up.

For now, while it involves a lot of fairly internal details most users
shouldn't have to care about, I went ahead and fully documented the
situation for any passerby to read on the trait, as the situation is
quite unusual and puzzling to begin with.

Cargo.toml

@@ -2,5 +2,6 @@
 
 members = [
     "crates/core_simd",
+    "crates/std_float",
     "crates/test_helpers",
 ]

crates/std_float/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "std_float"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+core_simd = { path = "../core_simd" }
+
+[features]
+default = ["as_crate"]
+as_crate = []

crates/std_float/src/lib.rs

@@ -0,0 +1,165 @@
+#![cfg_attr(feature = "as_crate", no_std)] // We are std!
+#![cfg_attr(
+    feature = "as_crate",
+    feature(platform_intrinsics),
+    feature(portable_simd)
+)]
+#[cfg(not(feature = "as_crate"))]
+use core::simd;
+#[cfg(feature = "as_crate")]
+use core_simd::simd;
+
+use simd::{LaneCount, Simd, SupportedLaneCount};
+
+#[cfg(feature = "as_crate")]
+mod experimental {
+    pub trait Sealed {}
+}
+
+#[cfg(feature = "as_crate")]
+use experimental as sealed;
+
+use crate::sealed::Sealed;
+
+// "platform intrinsics" are essentially "codegen intrinsics"
+// each of these may be scalarized and lowered to a libm call
+extern "platform-intrinsic" {
+    // ceil
+    fn simd_ceil<T>(x: T) -> T;
+
+    // floor
+    fn simd_floor<T>(x: T) -> T;
+
+    // round
+    fn simd_round<T>(x: T) -> T;
+
+    // trunc
+    fn simd_trunc<T>(x: T) -> T;
+
+    // fsqrt
+    fn simd_fsqrt<T>(x: T) -> T;
+
+    // fma
+    fn simd_fma<T>(x: T, y: T, z: T) -> T;
+}
+
+/// This trait provides a possibly-temporary implementation of float functions
+/// that may, in the absence of hardware support, canonicalize to calling an
+/// operating system's `math.h` dynamically-loaded library (also known as a
+/// shared object). As these conditionally require runtime support, they
+/// should only appear in binaries built assuming OS support: `std`.
+///
+/// However, there is no reason SIMD types, in general, need OS support,
+/// as for many architectures an embedded binary may simply configure that
+/// support itself. This means these types must be visible in `core`
+/// but have these functions available in `std`.
+///
+/// [`f32`] and [`f64`] achieve a similar trick by using "lang items", but
+/// due to compiler limitations, it is harder to implement this approach for
+/// abstract data types like [`Simd`]. From that need, this trait is born.
+///
+/// It is possible this trait will be replaced in some manner in the future,
+/// when either the compiler or its supporting runtime functions are improved.
+/// For now this trait is available to permit experimentation with SIMD float
+/// operations that may lack hardware support, such as `mul_add`.
+pub trait StdFloat: Sealed + Sized {
+    /// Fused multiply-add.  Computes `(self * a) + b` with only one rounding error,
+    /// yielding a more accurate result than an unfused multiply-add.
+    ///
+    /// Using `mul_add` *may* be more performant than an unfused multiply-add if the target
+    /// architecture has a dedicated `fma` CPU instruction.  However, this is not always
+    /// true, and will be heavily dependent on designing algorithms with specific target
+    /// hardware in mind.
+    #[inline]
+    #[must_use = "method returns a new vector and does not mutate the original value"]
+    fn mul_add(self, a: Self, b: Self) -> Self {
+        unsafe { simd_fma(self, a, b) }
+    }
+
+    /// Produces a vector where every lane has the square root value
+    /// of the equivalently-indexed lane in `self`
+    #[inline]
+    #[must_use = "method returns a new vector and does not mutate the original value"]
+    fn sqrt(self) -> Self {
+        unsafe { simd_fsqrt(self) }
+    }
+
+    /// Returns the smallest integer greater than or equal to each lane.
+    #[must_use = "method returns a new vector and does not mutate the original value"]
+    #[inline]
+    fn ceil(self) -> Self {
+        unsafe { simd_ceil(self) }
+    }
+
+    /// Returns the largest integer value less than or equal to each lane.
+    #[must_use = "method returns a new vector and does not mutate the original value"]
+    #[inline]
+    fn floor(self) -> Self {
+        unsafe { simd_floor(self) }
+    }
+
+    /// Rounds to the nearest integer value. Ties round toward zero.
+    #[must_use = "method returns a new vector and does not mutate the original value"]
+    #[inline]
+    fn round(self) -> Self {
+        unsafe { simd_round(self) }
+    }
+
+    /// Returns the floating point's integer value, with its fractional part removed.
+    #[must_use = "method returns a new vector and does not mutate the original value"]
+    #[inline]
+    fn trunc(self) -> Self {
+        unsafe { simd_trunc(self) }
+    }
+
+    /// Returns the floating point's fractional value, with its integer part removed.
+    #[must_use = "method returns a new vector and does not mutate the original value"]
+    fn fract(self) -> Self;
+}
+
+impl<const N: usize> Sealed for Simd<f32, N> where LaneCount<N>: SupportedLaneCount {}
+impl<const N: usize> Sealed for Simd<f64, N> where LaneCount<N>: SupportedLaneCount {}
+
+// We can safely just use all the defaults.
+impl<const N: usize> StdFloat for Simd<f32, N>
+where
+    LaneCount<N>: SupportedLaneCount,
+{
+    /// Returns the floating point's fractional value, with its integer part removed.
+    #[must_use = "method returns a new vector and does not mutate the original value"]
+    #[inline]
+    fn fract(self) -> Self {
+        self - self.trunc()
+    }
+}
+
+impl<const N: usize> StdFloat for Simd<f64, N>
+where
+    LaneCount<N>: SupportedLaneCount,
+{
+    /// Returns the floating point's fractional value, with its integer part removed.
+    #[must_use = "method returns a new vector and does not mutate the original value"]
+    #[inline]
+    fn fract(self) -> Self {
+        self - self.trunc()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use simd::*;
+
+    #[test]
+    fn everything_works() {
+        let x = f32x4::from_array([0.1, 0.5, 0.6, -1.5]);
+        let x2 = x + x;
+        let _xc = x.ceil();
+        let _xf = x.floor();
+        let _xr = x.round();
+        let _xt = x.trunc();
+        let _xfma = x.mul_add(x, x);
+        let _xsqrt = x.sqrt();
+        let _ = x2.abs() * x2;
+    }
+}