diff --git a/README.md b/README.md
index 97ef340..30abd92 100644
--- a/README.md
+++ b/README.md
@@ -146,7 +146,7 @@ features = ["c"]
 - [x] divsf3.c
 - [x] divsi3.c
 - [ ] extendhfsf2.c
-- [ ] extendsfdf2.c
+- [x] extendsfdf2.c
 - [x] fixdfdi.c
 - [x] fixdfsi.c
 - [x] fixsfdi.c
diff --git a/build.rs b/build.rs
index 89a4a83..0d32e97 100644
--- a/build.rs
+++ b/build.rs
@@ -171,7 +171,6 @@ mod c {
                 "divdc3.c",
                 "divsc3.c",
                 "divxc3.c",
-                "extendsfdf2.c",
                 "extendhfsf2.c",
                 "floatdisf.c",
                 "floatundisf.c",
diff --git a/src/float/extend.rs b/src/float/extend.rs
new file mode 100644
index 0000000..1abb23c
--- /dev/null
+++ b/src/float/extend.rs
@@ -0,0 +1,77 @@
+use int::{CastInto, Int};
+use float::Float;
+
+/// Generic conversion from a narrower to a wider IEEE-754 floating-point type
+fn extend<F: Float, R: Float>(a: F) -> R where
+    F::Int: CastInto<u64>,
+    u64: CastInto<F::Int>,
+    u32: CastInto<R::Int>,
+    R::Int: CastInto<u32>,
+    R::Int: CastInto<u64>,
+    u64: CastInto<R::Int>,
+    F::Int: CastInto<R::Int>,
+{
+    let src_zero = F::Int::ZERO;
+    let src_one = F::Int::ONE;
+    let src_bits = F::BITS;
+    let src_sign_bits = F::SIGNIFICAND_BITS;
+    let src_exp_bias = F::EXPONENT_BIAS;
+    let src_min_normal = F::IMPLICIT_BIT;
+    let src_infinity = F::EXPONENT_MASK;
+    let src_sign_mask = F::SIGN_MASK as F::Int;
+    let src_abs_mask = src_sign_mask - src_one;
+    let src_qnan = F::SIGNIFICAND_MASK;
+    let src_nan_code = src_qnan - src_one;
+
+    let dst_bits = R::BITS;
+    let dst_sign_bits = R::SIGNIFICAND_BITS;
+    let dst_inf_exp = R::EXPONENT_MAX;
+    let dst_exp_bias = R::EXPONENT_BIAS;
+    let dst_min_normal = R::IMPLICIT_BIT;
+
+    let sign_bits_delta = dst_sign_bits - src_sign_bits;
+    let exp_bias_delta = dst_exp_bias - src_exp_bias;
+    let a_abs = a.repr() & src_abs_mask;
+    let mut abs_result = R::Int::ZERO;
+
+    if a_abs.wrapping_sub(src_min_normal) < src_infinity.wrapping_sub(src_min_normal) {
+        // a is a normal number.
+        // Extend to the destination type by shifting the significand and
+        // exponent into the proper position and rebiasing the exponent.
+        let abs_dst: R::Int = a_abs.cast();
+        let bias_dst: R::Int = exp_bias_delta.cast();
+        abs_result = abs_dst.wrapping_shl(sign_bits_delta);
+        abs_result |= bias_dst.wrapping_shl(dst_sign_bits);
+    } else if a_abs >= src_infinity {
+        // a is NaN or infinity.
+        // Conjure the result by beginning with infinity, then setting the qNaN
+        // bit (if needed) and right-aligning the rest of the trailing NaN
+        // payload field.
+        let qnan_dst: R::Int = (a_abs & src_qnan).cast();
+        let nan_code_dst: R::Int = (a_abs & src_nan_code).cast();
+        let inf_exp_dst: R::Int = dst_inf_exp.cast();
+        abs_result = inf_exp_dst.wrapping_shl(dst_sign_bits);
+        abs_result |= qnan_dst.wrapping_shl(sign_bits_delta);
+        abs_result |= nan_code_dst.wrapping_shl(sign_bits_delta);
+    } else if a_abs != src_zero {
+        // a is denormal.
+        // Renormalize the significand and clear the leading bit, then insert
+        // the correct adjusted exponent in the destination type.
+        let scale = a_abs.leading_zeros() - src_min_normal.leading_zeros();
+        let abs_dst: R::Int = a_abs.cast();
+        let bias_dst: R::Int = (exp_bias_delta - scale + 1).cast();
+        abs_result = abs_dst.wrapping_shl(sign_bits_delta + scale);
+        abs_result = (abs_result ^ dst_min_normal) | (bias_dst.wrapping_shl(dst_sign_bits));
+    }
+
+    let sign_result: R::Int = (a.repr() & src_sign_mask).cast();
+    R::from_repr(abs_result | (sign_result.wrapping_shl(dst_bits - src_bits)))
+}
+
+intrinsics! {
+    #[aapcs_on_arm]
+    #[arm_aeabi_alias = __aeabi_f2d]
+    pub extern "C" fn  __extendsfdf2(a: f32) -> f64 {
+        extend(a)
+    }
+}
\ No newline at end of file
diff --git a/src/float/mod.rs b/src/float/mod.rs
index 51ed667..3bb13ab 100644
--- a/src/float/mod.rs
+++ b/src/float/mod.rs
@@ -10,6 +10,7 @@ pub mod pow;
 pub mod sub;
 pub mod mul;
 pub mod div;
+pub mod extend;
 
 /// Trait for some basic operations on floats
 pub trait Float:
diff --git a/testcrate/build.rs b/testcrate/build.rs
index a300e28..9269983 100644
--- a/testcrate/build.rs
+++ b/testcrate/build.rs
@@ -235,6 +235,15 @@ fn main() {
             "compiler_builtins::float::cmp::__aeabi_dcmpgt(a, b)");
     }
 
+    // float/extend.rs
+    gen(|a: MyF32| {
+            if a.0.is_nan() {
+                return None;
+            }
+            Some(f64(a.0))
+        },
+        "compiler_builtins::float::extend::__extendsfdf2(a)");
+
     // float/conv.rs
     gen(|a: MyF64| i64(a.0).ok(),
         "compiler_builtins::float::conv::__fixdfdi(a)");