diff --git a/src/simd.rs b/src/simd.rs
index fb97af3..8a87386 100644
--- a/src/simd.rs
+++ b/src/simd.rs
@@ -5,8 +5,12 @@ use crate::util;
 
 use util::cast;
 
+// these may be unused depending on target features.
+#[allow(unused)]
 const W_128: usize = 128 / 8;
+#[allow(unused)]
 const W_256: usize = 256 / 8;
+#[allow(unused)]
 const W_512: usize = 512 / 8;
 
 /// The value which cause `vpshufb` to write 0 instead of indexing.