diff --git a/build.rs b/build.rs
index bbae9e28..93e323e6 100644
--- a/build.rs
+++ b/build.rs
@@ -51,8 +51,9 @@ fn main() {
     println!("cargo:compiler-rt={}", cwd.join("compiler-rt").display());
 
     // Activate libm's unstable features to make full use of Nightly.
-    println!("cargo::rustc-check-cfg=cfg(feature, values(\"unstable\"))");
+    println!("cargo::rustc-check-cfg=cfg(feature, values(\"unstable\", \"force-soft-floats\"))");
     println!("cargo:rustc-cfg=feature=\"unstable\"");
+    println!("cargo:rustc-cfg=feature=\"force-soft-floats\"");
 
     // Emscripten's runtime includes all the builtins
     if target.os == "emscripten" {
diff --git a/libm b/libm
index 279e5f6a..300edb32 160000
--- a/libm
+++ b/libm
@@ -1 +1 @@
-Subproject commit 279e5f6abe0a2ca9066962d9ec894f0df1f417ac
+Subproject commit 300edb32520b1673e16d2411a0e2e6273959eb46