diff --git a/src/libcore/num/f32.rs b/src/libcore/num/f32.rs
index d1bd97552024d..68da79135d3a3 100644
--- a/src/libcore/num/f32.rs
+++ b/src/libcore/num/f32.rs
@@ -161,6 +161,14 @@ impl f32 {
         self != self
     }
 
+    // FIXME(#50145): `abs` is publicly unavailable in libcore due to
+    // concerns about portability, so this implementation is for
+    // private use internally.
+    #[inline]
+    fn abs_private(self) -> f32 {
+        f32::from_bits(self.to_bits() & 0x7fff_ffff)
+    }
+
     /// Returns `true` if this value is positive infinity or negative infinity and
     /// false otherwise.
     ///
@@ -181,7 +189,7 @@ impl f32 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn is_infinite(self) -> bool {
-        self == INFINITY || self == NEG_INFINITY
+        self.abs_private() == INFINITY
     }
 
     /// Returns `true` if this number is neither infinite nor `NaN`.
@@ -203,7 +211,9 @@ impl f32 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn is_finite(self) -> bool {
-        !(self.is_nan() || self.is_infinite())
+        // There's no need to handle NaN separately: if self is NaN,
+        // the comparison is not true, exactly as desired.
+        self.abs_private() < INFINITY
     }
 
     /// Returns `true` if the number is neither zero, infinite,
diff --git a/src/libcore/num/f64.rs b/src/libcore/num/f64.rs
index 8ada5b6756c38..b677391548146 100644
--- a/src/libcore/num/f64.rs
+++ b/src/libcore/num/f64.rs
@@ -161,6 +161,14 @@ impl f64 {
         self != self
     }
 
+    // FIXME(#50145): `abs` is publicly unavailable in libcore due to
+    // concerns about portability, so this implementation is for
+    // private use internally.
+    #[inline]
+    fn abs_private(self) -> f64 {
+        f64::from_bits(self.to_bits() & 0x7fff_ffff_ffff_ffff)
+    }
+
     /// Returns `true` if this value is positive infinity or negative infinity and
     /// false otherwise.
     ///
@@ -181,7 +189,7 @@ impl f64 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn is_infinite(self) -> bool {
-        self == INFINITY || self == NEG_INFINITY
+        self.abs_private() == INFINITY
     }
 
     /// Returns `true` if this number is neither infinite nor `NaN`.
@@ -203,7 +211,9 @@ impl f64 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn is_finite(self) -> bool {
-        !(self.is_nan() || self.is_infinite())
+        // There's no need to handle NaN separately: if self is NaN,
+        // the comparison is not true, exactly as desired.
+        self.abs_private() < INFINITY
     }
 
     /// Returns `true` if the number is neither zero, infinite,