diff --git a/library/compiler-builtins/libm/src/lib.rs b/library/compiler-builtins/libm/src/lib.rs
index 7d8ae903a12f..df96aadc725b 100644
--- a/library/compiler-builtins/libm/src/lib.rs
+++ b/library/compiler-builtins/libm/src/lib.rs
@@ -363,6 +363,8 @@ pub trait F64Ext: private::Sealed {
 
     fn trunc(self) -> Self;
 
+    fn fdim(self, rhs: Self) -> Self;
+
     #[cfg(todo)]
     fn fract(self) -> Self;
 
@@ -480,6 +482,10 @@ 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
index 3cf65d9beeef..44bf2d6806ef 100644
--- a/library/compiler-builtins/libm/src/math/fdimf.rs
+++ b/library/compiler-builtins/libm/src/math/fdimf.rs
@@ -1,9 +1,9 @@
-use super::isnanf;
+use core::f32;
 
 pub fn fdimf(x: f32, y: f32) -> f32 {
-    if isnanf(x) {
+    if x.is_nan() {
         x
-    } else if isnanf(y) {
+    } else if y.is_nan() {
         y
     } else {
         if x > y {
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 a9ba87aa2ce4..e526314a6673 100644
--- a/library/compiler-builtins/libm/src/math/mod.rs
+++ b/library/compiler-builtins/libm/src/math/mod.rs
@@ -10,6 +10,7 @@ mod ceilf;
 mod expf;
 mod fabs;
 mod fabsf;
+mod fdim;
 mod fdimf;
 mod floor;
 mod floorf;
@@ -37,13 +38,9 @@ mod truncf;
 //mod service;
 
 pub use self::{
-    ceilf::ceilf, expf::expf, fabs::fabs, fabsf::fabsf, fdimf::fdimf, floor::floor, floorf::floorf,
-    fmodf::fmodf, hypot::hypot, hypotf::hypotf, log::log, log10::log10, log10f::log10f,
-    log1p::log1p, log1pf::log1pf, log2::log2, log2f::log2f, logf::logf, powf::powf, round::round,
-    roundf::roundf, scalbn::scalbn, scalbnf::scalbnf, sqrt::sqrt, sqrtf::sqrtf, trunc::trunc,
-    truncf::truncf,
+    ceilf::ceilf, expf::expf, fabs::fabs, fabsf::fabsf, fdim::fdim, fdimf::fdimf, floor::floor,
+    floorf::floorf, fmodf::fmodf, hypot::hypot, hypotf::hypotf, log::log, log10::log10,
+    log10f::log10f, log1p::log1p, log1pf::log1pf, log2::log2, log2f::log2f, logf::logf, powf::powf,
+    round::round, roundf::roundf, scalbn::scalbn, scalbnf::scalbnf, sqrt::sqrt, sqrtf::sqrtf,
+    trunc::trunc, truncf::truncf,
 };
-
-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 29529356247c..bf679ca64191 100644
--- a/library/compiler-builtins/libm/test-generator/src/main.rs
+++ b/library/compiler-builtins/libm/test-generator/src/main.rs
@@ -724,7 +724,7 @@ f64_f64! {
 // With signature `fn(f64, f64) -> f64`
 f64f64_f64! {
     // atan2,
-    // fdim,
+    fdim,
     // fmod,
     hypot,
     // pow,