diff --git a/util/Cargo.toml b/util/Cargo.toml
index 6a18f80de..b7eafd905 100644
--- a/util/Cargo.toml
+++ b/util/Cargo.toml
@@ -39,7 +39,7 @@ target_info = "0.1"
 [features]
 default = []
 dev = ["clippy"]
-x64asm = []
 
 [build-dependencies]
 vergen = "*"
+rustc_version = "0.1"
diff --git a/util/build.rs b/util/build.rs
index 32ee30472..2b581642b 100644
--- a/util/build.rs
+++ b/util/build.rs
@@ -1,6 +1,13 @@
 extern crate vergen;
+extern crate rustc_version;
+
 use vergen::*;
+use rustc_version::{version_meta, Channel};
 
 fn main() {
 	vergen(OutputFns::all()).unwrap();
-}
\ No newline at end of file
+
+	if let Channel::Nightly = version_meta().channel {
+		println!("cargo:rustc-cfg=x64asm");
+	}
+}
diff --git a/util/src/lib.rs b/util/src/lib.rs
index be1e788c9..2a47eb438 100644
--- a/util/src/lib.rs
+++ b/util/src/lib.rs
@@ -16,7 +16,7 @@
 
 #![warn(missing_docs)]
 #![cfg_attr(feature="dev", feature(plugin))]
-#![cfg_attr(feature="x64asm", feature(asm))]
+#![cfg_attr(x64asm, feature(asm))]
 #![cfg_attr(feature="dev", plugin(clippy))]
 
 // Clippy settings
diff --git a/util/src/uint.rs b/util/src/uint.rs
index 67a6f2ff4..517b7a29f 100644
--- a/util/src/uint.rs
+++ b/util/src/uint.rs
@@ -51,7 +51,7 @@ macro_rules! impl_map_from {
 	}
 }
 
-#[cfg(not(all(feature="x64asm", target_arch="x86_64")))]
+#[cfg(not(all(x64asm, target_arch="x86_64")))]
 macro_rules! uint_overflowing_add {
 	($name:ident, $n_words:expr, $self_expr: expr, $other: expr) => ({
 		uint_overflowing_add_reg!($name, $n_words, $self_expr, $other)
@@ -89,7 +89,7 @@ macro_rules! uint_overflowing_add_reg {
 }
 
 
-#[cfg(all(feature="x64asm", target_arch="x86_64"))]
+#[cfg(all(x64asm, target_arch="x86_64"))]
 macro_rules! uint_overflowing_add {
 	(U256, $n_words: expr, $self_expr: expr, $other: expr) => ({
 		let mut result: [u64; 4] = unsafe { mem::uninitialized() };
@@ -165,7 +165,7 @@ macro_rules! uint_overflowing_add {
 	)
 }
 
-#[cfg(not(all(feature="x64asm", target_arch="x86_64")))]
+#[cfg(not(all(x64asm, target_arch="x86_64")))]
 macro_rules! uint_overflowing_sub {
 	($name:ident, $n_words: expr, $self_expr: expr, $other: expr) => ({
 		let res = overflowing!((!$other).overflowing_add(From::from(1u64)));
@@ -174,7 +174,7 @@ macro_rules! uint_overflowing_sub {
 	})
 }
 
-#[cfg(all(feature="x64asm", target_arch="x86_64"))]
+#[cfg(all(x64asm, target_arch="x86_64"))]
 macro_rules! uint_overflowing_sub {
 	(U256, $n_words: expr, $self_expr: expr, $other: expr) => ({
 		let mut result: [u64; 4] = unsafe { mem::uninitialized() };
@@ -250,7 +250,7 @@ macro_rules! uint_overflowing_sub {
 	})
 }
 
-#[cfg(all(feature="x64asm", target_arch="x86_64"))]
+#[cfg(all(x64asm, target_arch="x86_64"))]
 macro_rules! uint_overflowing_mul {
 	(U256, $n_words: expr, $self_expr: expr, $other: expr) => ({
 		let mut result: [u64; 4] = unsafe { mem::uninitialized() };
@@ -370,7 +370,7 @@ macro_rules! uint_overflowing_mul {
 	)
 }
 
-#[cfg(not(all(feature="x64asm", target_arch="x86_64")))]
+#[cfg(not(all(x64asm, target_arch="x86_64")))]
 macro_rules! uint_overflowing_mul {
 	($name:ident, $n_words: expr, $self_expr: expr, $other: expr) => ({
 		uint_overflowing_mul_reg!($name, $n_words, $self_expr, $other)