Add a generic version of trunc

The algorithm is identical for both types, so this is a straightforward
routine to port.

library/compiler-builtins/libm/src/math/generic/mod.rs

@@ -1,5 +1,7 @@
 mod copysign;
 mod fabs;
+mod trunc;
 
 pub use copysign::copysign;
 pub use fabs::fabs;
+pub use trunc::trunc;

library/compiler-builtins/libm/src/math/generic/trunc.rs

@@ -0,0 +1,54 @@
+use super::super::{Float, Int, IntTy, MinInt};
+
+pub fn trunc<F: Float>(x: F) -> F {
+    let mut xi: F::Int = x.to_bits();
+    let e: i32 = x.exp_unbiased();
+
+    // C1: The represented value has no fractional part, so no truncation is needed
+    if e >= F::SIG_BITS as i32 {
+        return x;
+    }
+
+    let mask = if e < 0 {
+        // C2: If the exponent is negative, the result will be zero so we mask out everything
+        // except the sign.
+        F::SIGN_MASK
+    } else {
+        // C3: Otherwise, we mask out the last `e` bits of the significand.
+        !(F::SIG_MASK >> e.unsigned())
+    };
+
+    // C4: If the to-be-masked-out portion is already zero, we have an exact result
+    if (xi & !mask) == IntTy::<F>::ZERO {
+        return x;
+    }
+
+    // C5: Otherwise the result is inexact and we will truncate. Raise `FE_INEXACT`, mask the
+    // result, and return.
+    force_eval!(x + F::MAX);
+    xi &= mask;
+    F::from_bits(xi)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn sanity_check() {
+        assert_biteq!(trunc(1.1f32), 1.0);
+        assert_biteq!(trunc(1.1f64), 1.0);
+
+        // C1
+        assert_biteq!(trunc(hf32!("0x1p23")), hf32!("0x1p23"));
+        assert_biteq!(trunc(hf64!("0x1p52")), hf64!("0x1p52"));
+        assert_biteq!(trunc(hf32!("-0x1p23")), hf32!("-0x1p23"));
+        assert_biteq!(trunc(hf64!("-0x1p52")), hf64!("-0x1p52"));
+
+        // C2
+        assert_biteq!(trunc(hf32!("0x1p-1")), 0.0);
+        assert_biteq!(trunc(hf64!("0x1p-1")), 0.0);
+        assert_biteq!(trunc(hf32!("-0x1p-1")), -0.0);
+        assert_biteq!(trunc(hf64!("-0x1p-1")), -0.0);
+    }
+}

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

@@ -121,7 +121,7 @@ use self::rem_pio2::rem_pio2;
 use self::rem_pio2_large::rem_pio2_large;
 use self::rem_pio2f::rem_pio2f;
 #[allow(unused_imports)]
-use self::support::{CastFrom, CastInto, DInt, Float, HInt, Int, MinInt};
+use self::support::{CastFrom, CastInto, DInt, Float, HInt, Int, IntTy, MinInt};
 
 // Public modules
 mod acos;

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

@@ -1,5 +1,3 @@
-use core::f64;
-
 /// Rounds the number toward 0 to the closest integral value (f64).
 ///
 /// This effectively removes the decimal part of the number, leaving the integral part.
@@ -11,31 +9,5 @@ pub fn trunc(x: f64) -> f64 {
         args: x,
     }
 
-    let x1p120 = f64::from_bits(0x4770000000000000); // 0x1p120f === 2 ^ 120
-
-    let mut i: u64 = x.to_bits();
-    let mut e: i64 = ((i >> 52) & 0x7ff) as i64 - 0x3ff + 12;
-    let m: u64;
-
-    if e >= 52 + 12 {
-        return x;
-    }
-    if e < 12 {
-        e = 1;
-    }
-    m = -1i64 as u64 >> e;
-    if (i & m) == 0 {
-        return x;
-    }
-    force_eval!(x + x1p120);
-    i &= !m;
-    f64::from_bits(i)
-}
-
-#[cfg(test)]
-mod tests {
-    #[test]
-    fn sanity_check() {
-        assert_eq!(super::trunc(1.1), 1.0);
-    }
+    super::generic::trunc(x)
 }

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

@@ -1,5 +1,3 @@
-use core::f32;
-
 /// Rounds the number toward 0 to the closest integral value (f32).
 ///
 /// This effectively removes the decimal part of the number, leaving the integral part.
@@ -11,25 +9,7 @@ pub fn truncf(x: f32) -> f32 {
         args: x,
     }
 
-    let x1p120 = f32::from_bits(0x7b800000); // 0x1p120f === 2 ^ 120
-
-    let mut i: u32 = x.to_bits();
-    let mut e: i32 = ((i >> 23) & 0xff) as i32 - 0x7f + 9;
-    let m: u32;
-
-    if e >= 23 + 9 {
-        return x;
-    }
-    if e < 9 {
-        e = 1;
-    }
-    m = -1i32 as u32 >> e;
-    if (i & m) == 0 {
-        return x;
-    }
-    force_eval!(x + x1p120);
-    i &= !m;
-    f32::from_bits(i)
+    super::generic::trunc(x)
 }
 
 // PowerPC tests are failing on LLVM 13: https://github.com/rust-lang/rust/issues/88520