diff --git a/library/compiler-builtins/libm/src/lib.rs b/library/compiler-builtins/libm/src/lib.rs
index f112aaaca6b1..56ab82eace0f 100644
--- a/library/compiler-builtins/libm/src/lib.rs
+++ b/library/compiler-builtins/libm/src/lib.rs
@@ -42,6 +42,8 @@ pub trait F32Ext: private::Sealed {
 
     fn trunc(self) -> Self;
 
+    fn fdim(self, rhs: Self) -> Self;
+
     #[cfg(todo)]
     fn fract(self) -> Self;
 
@@ -155,6 +157,11 @@ impl F32Ext for f32 {
         truncf(self)
     }
 
+    #[inline]
+    fn fdim(self, rhs: Self) -> Self {
+        fdimf(self, rhs)
+    }
+
     #[cfg(todo)]
     #[inline]
     fn fract(self) -> Self {
@@ -353,6 +360,8 @@ pub trait F64Ext: private::Sealed {
 
     fn trunc(self) -> Self;
 
+    fn fdim(self, rhs: Self) -> Self;
+
     #[cfg(todo)]
     fn fract(self) -> Self;
 
@@ -468,6 +477,10 @@ impl F64Ext for f64 {
         trunc(self)
     }
 
+    #[inline]
+    fn fdim(self, rhs: Self) -> Self {
+        fdim(self, rhs)
+    }
     #[cfg(todo)]
     #[inline]
     fn fract(self) -> Self {
diff --git a/library/compiler-builtins/libm/src/math/fdim.rs b/library/compiler-builtins/libm/src/math/fdim.rs
new file mode 100644
index 000000000000..2b277eab03e1
--- /dev/null
+++ b/library/compiler-builtins/libm/src/math/fdim.rs
@@ -0,0 +1,15 @@
+use core::f64;
+
+pub fn fdim(x: f64, y: f64) -> f64 {
+    if x.is_nan() {
+        x
+    } else if y.is_nan() {
+        y
+    } else {
+        if x > y {
+            x - y
+        } else {
+            0.0
+        }
+    }
+}
diff --git a/library/compiler-builtins/libm/src/math/fdimf.rs b/library/compiler-builtins/libm/src/math/fdimf.rs
new file mode 100644
index 000000000000..44bf2d6806ef
--- /dev/null
+++ b/library/compiler-builtins/libm/src/math/fdimf.rs
@@ -0,0 +1,15 @@
+use core::f32;
+
+pub fn fdimf(x: f32, y: f32) -> f32 {
+    if x.is_nan() {
+        x
+    } else if y.is_nan() {
+        y
+    } else {
+        if x > y {
+            x - y
+        } else {
+            0.0
+        }
+    }
+}
diff --git a/library/compiler-builtins/libm/src/math/fmodf.rs b/library/compiler-builtins/libm/src/math/fmodf.rs
index 909775249085..d84cfeb01b83 100644
--- a/library/compiler-builtins/libm/src/math/fmodf.rs
+++ b/library/compiler-builtins/libm/src/math/fmodf.rs
@@ -1,7 +1,6 @@
+use core::f32;
 use core::u32;
 
-use super::isnanf;
-
 #[inline]
 pub fn fmodf(x: f32, y: f32) -> f32 {
     let mut uxi = x.to_bits();
@@ -11,7 +10,7 @@ pub fn fmodf(x: f32, y: f32) -> f32 {
     let sx = uxi & 0x80000000;
     let mut i;
 
-    if uyi << 1 == 0 || isnanf(y) || ex == 0xff {
+    if uyi << 1 == 0 || y.is_nan() || ex == 0xff {
         return (x * y) / (x * y);
     }
 
diff --git a/library/compiler-builtins/libm/src/math/mod.rs b/library/compiler-builtins/libm/src/math/mod.rs
index 2a84b463d9fd..0b52e3fa74c9 100644
--- a/library/compiler-builtins/libm/src/math/mod.rs
+++ b/library/compiler-builtins/libm/src/math/mod.rs
@@ -13,6 +13,8 @@ mod exp;
 mod expf;
 mod fabs;
 mod fabsf;
+mod fdim;
+mod fdimf;
 mod floor;
 mod floorf;
 mod fmodf;
@@ -44,6 +46,8 @@ pub use self::exp::exp;
 pub use self::expf::expf;
 pub use self::fabs::fabs;
 pub use self::fabsf::fabsf;
+pub use self::fdim::fdim;
+pub use self::fdimf::fdimf;
 pub use self::floor::floor;
 pub use self::floorf::floorf;
 pub use self::fmodf::fmodf;
@@ -73,7 +77,3 @@ mod rem_pio2_large;
 mod rem_pio2f;
 
 use self::{k_cosf::k_cosf, k_sinf::k_sinf, rem_pio2_large::rem_pio2_large, rem_pio2f::rem_pio2f};
-
-fn isnanf(x: f32) -> bool {
-    x.to_bits() & 0x7fffffff > 0x7f800000
-}
diff --git a/library/compiler-builtins/libm/test-generator/src/main.rs b/library/compiler-builtins/libm/test-generator/src/main.rs
index a54d8b2710f5..afc56cdcf974 100644
--- a/library/compiler-builtins/libm/test-generator/src/main.rs
+++ b/library/compiler-builtins/libm/test-generator/src/main.rs
@@ -662,7 +662,6 @@ f32_f32! {
     // coshf,
     // exp2f,
     expf,
-    // fdimf,
     log10f,
     log1pf,
     log2f,
@@ -679,6 +678,7 @@ f32_f32! {
 // With signature `fn(f32, f32) -> f32`
 f32f32_f32! {
     // atan2f,
+    fdimf,
     hypotf,
     fmodf,
     powf,
@@ -724,7 +724,7 @@ f64_f64! {
 // With signature `fn(f64, f64) -> f64`
 f64f64_f64! {
     // atan2,
-    // fdim,
+    fdim,
     // fmod,
     hypot,
     // pow,