Add an enum representation of rounding mode

We only round using nearest, but some incoming code has more handling of
rounding modes that would be nice to `match` on. Rather than checking
integer values, add an enum representation.

library/compiler-builtins/libm/src/math/fenv.rs

@@ -5,6 +5,9 @@ pub(crate) const FE_UNDERFLOW: i32 = 0;
 pub(crate) const FE_INEXACT: i32 = 0;
 
 pub(crate) const FE_TONEAREST: i32 = 0;
+pub(crate) const FE_DOWNWARD: i32 = 1;
+pub(crate) const FE_UPWARD: i32 = 2;
+pub(crate) const FE_TOWARDZERO: i32 = 3;
 
 #[inline]
 pub(crate) fn feclearexcept(_mask: i32) -> i32 {
@@ -25,3 +28,22 @@ pub(crate) fn fetestexcept(_mask: i32) -> i32 {
 pub(crate) fn fegetround() -> i32 {
     FE_TONEAREST
 }
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub(crate) enum Rounding {
+    Nearest = FE_TONEAREST as isize,
+    Downward = FE_DOWNWARD as isize,
+    Upward = FE_UPWARD as isize,
+    ToZero = FE_TOWARDZERO as isize,
+}
+
+impl Rounding {
+    pub(crate) fn get() -> Self {
+        match fegetround() {
+            x if x == FE_DOWNWARD => Self::Downward,
+            x if x == FE_UPWARD => Self::Upward,
+            x if x == FE_TOWARDZERO => Self::ToZero,
+            _ => Self::Nearest,
+        }
+    }
+}