fix: remove avx2 runtime detection (#53)

Author: DaniPopes

src/nibbles.rs

@@ -1283,30 +1283,24 @@ fn longest_prefix_bit(a: &U256, b: &U256) -> usize {
 #[inline]
 fn longest_prefix<const EXACT: bool>(a: &U256, b: &U256) -> usize {
     cfg_if! {
-        if #[cfg(target_arch = "x86_64")] {
-            #[cfg(feature = "std")]
-            let enabled = std::is_x86_feature_detected!("avx2");
-            #[cfg(not(feature = "std"))]
-            let enabled = cfg!(target_feature = "avx2");
-            if enabled {
-                return unsafe {
-                    use core::arch::x86_64::*;
-                    let x = _mm256_loadu_si256(a.as_limbs().as_ptr().cast());
-                    let y = _mm256_loadu_si256(b.as_limbs().as_ptr().cast());
-                    let diff = _mm256_cmpeq_epi8(x, y);
-                    let mask = _mm256_movemask_epi8(diff);
-                    let bytes = mask.leading_ones() as usize;
-                    if !EXACT || bytes == 32 {
-                        return bytes * 8;
-                    }
-                    let le_idx = 31 - bytes;
-                    let a = *a.as_le_slice().get_unchecked(le_idx);
-                    let b = *b.as_le_slice().get_unchecked(le_idx);
-                    let diff = a ^ b;
-                    let bits = diff.leading_zeros() as usize;
-                    bytes * 8 + bits
-                };
-            }
+        if #[cfg(all(target_arch = "x86_64", target_feature = "avx2"))] {
+            return unsafe {
+                use core::arch::x86_64::*;
+                let x = _mm256_loadu_si256(a.as_limbs().as_ptr().cast());
+                let y = _mm256_loadu_si256(b.as_limbs().as_ptr().cast());
+                let diff = _mm256_cmpeq_epi8(x, y);
+                let mask = _mm256_movemask_epi8(diff);
+                let bytes = mask.leading_ones() as usize;
+                if !EXACT || bytes == 32 {
+                    return bytes * 8;
+                }
+                let le_idx = 31 - bytes;
+                let a = *a.as_le_slice().get_unchecked(le_idx);
+                let b = *b.as_le_slice().get_unchecked(le_idx);
+                let diff = a ^ b;
+                let bits = diff.leading_zeros() as usize;
+                bytes * 8 + bits
+            };
         }
     }