diff --git a/src/operators/upwind4.rs b/src/operators/upwind4.rs
index 14136d2..a8330b0 100644
--- a/src/operators/upwind4.rs
+++ b/src/operators/upwind4.rs
@@ -1,6 +1,9 @@
 use super::SbpOperator;
 use ndarray::{arr1, arr2, s, ArrayView1, ArrayView2, ArrayViewMut1, ArrayViewMut2};
 
+/// Simdtype used in diffy_simd
+type SimdT = packed_simd::f32x8;
+
 pub struct Upwind4 {}
 
 impl Upwind4 {
@@ -57,7 +60,7 @@ impl Upwind4 {
     fn diff_simd(prev: &[f32], fut: &mut [f32]) {
         use packed_simd::{f32x8, u32x8};
         assert_eq!(prev.len(), fut.len());
-        assert_eq!(prev.len() % 8, 0);
+        assert!(prev.len() >= 2 * Self::BLOCK.len());
         let nx = prev.len();
         let dx = 1.0 / (nx - 1) as f32;
         let idx = 1.0 / dx;
@@ -147,25 +150,24 @@ impl Upwind4 {
 
     #[inline(never)]
     fn diffy_simd(prev: &[f32], fut: &mut [f32], nx: usize, ny: usize) {
-        use packed_simd::f32x4;
-        assert!(ny >= 8);
-        assert!(nx > 4);
-        assert!(nx % 4 == 0);
+        assert!(ny >= 2 * Self::BLOCK.len());
+        assert!(nx >= SimdT::lanes());
+        assert!(nx % SimdT::lanes() == 0);
         assert_eq!(prev.len(), fut.len());
         assert_eq!(prev.len(), nx * ny);
 
         let dy = 1.0 / (ny - 1) as f32;
         let idy = 1.0 / dy;
 
-        for j in (0..nx).step_by(4) {
+        for j in (0..nx).step_by(SimdT::lanes()) {
             let a = [
-                f32x4::from_slice_unaligned(&prev[0 * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[1 * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[2 * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[3 * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[4 * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[5 * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[6 * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[0 * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[1 * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[2 * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[3 * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[4 * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[5 * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[6 * nx + j..]),
             ];
 
             for (i, bl) in Self::BLOCK.iter().enumerate() {
@@ -190,7 +192,7 @@ impl Upwind4 {
                     a[4],
                     a[5],
                     a[6],
-                    f32x4::from_slice_unaligned(&prev[nx * (i + 3) + j..]),
+                    SimdT::from_slice_unaligned(&prev[nx * (i + 3) + j..]),
                 ];
                 let b = idy
                     * (a[0] * Self::DIAG[0]
@@ -204,13 +206,13 @@ impl Upwind4 {
             }
 
             let a = [
-                f32x4::from_slice_unaligned(&prev[(ny - 1) * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[(ny - 2) * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[(ny - 3) * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[(ny - 4) * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[(ny - 5) * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[(ny - 6) * nx + j..]),
-                f32x4::from_slice_unaligned(&prev[(ny - 7) * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[(ny - 1) * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[(ny - 2) * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[(ny - 3) * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[(ny - 4) * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[(ny - 5) * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[(ny - 6) * nx + j..]),
+                SimdT::from_slice_unaligned(&prev[(ny - 7) * nx + j..]),
             ];
 
             for (i, bl) in Self::BLOCK.iter().enumerate() {
@@ -230,13 +232,11 @@ impl Upwind4 {
     fn diff(prev: ArrayView1<f32>, mut fut: ArrayViewMut1<f32>) {
         assert_eq!(prev.shape(), fut.shape());
         let nx = prev.shape()[0];
-        assert!(nx >= 8);
+        assert!(nx >= 2 * Self::BLOCK.len());
 
-        if nx % 8 == 0 {
-            if let (Some(p), Some(f)) = (prev.as_slice(), fut.as_slice_mut()) {
-                Self::diff_simd(p, f);
-                return;
-            }
+        if let (Some(p), Some(f)) = (prev.as_slice(), fut.as_slice_mut()) {
+            Self::diff_simd(p, f);
+            return;
         }
 
         let dx = 1.0 / (nx - 1) as f32;
@@ -273,7 +273,7 @@ impl Upwind4 {
 impl SbpOperator for Upwind4 {
     fn diffx(prev: ArrayView2<f32>, mut fut: ArrayViewMut2<f32>) {
         assert_eq!(prev.shape(), fut.shape());
-        assert!(prev.shape()[1] >= 8);
+        assert!(prev.shape()[1] >= 2 * Self::BLOCK.len());
         for (r0, r1) in prev.outer_iter().zip(fut.outer_iter_mut()) {
             Self::diff(r0, r1)
         }
@@ -281,10 +281,10 @@ impl SbpOperator for Upwind4 {
 
     fn diffy(prev: ArrayView2<f32>, mut fut: ArrayViewMut2<f32>) {
         assert_eq!(prev.shape(), fut.shape());
-        assert!(prev.shape()[0] >= 8);
+        assert!(prev.shape()[0] >= 2 * Self::BLOCK.len());
         let nx = prev.shape()[1];
         let ny = prev.shape()[0];
-        if nx >= 4 && nx % 4 == 0 {
+        if nx >= SimdT::lanes() && nx % SimdT::lanes() == 0 {
             if let (Some(p), Some(f)) = (prev.as_slice(), fut.as_slice_mut()) {
                 Self::diffy_simd(p, f, nx, ny);
                 return;