Only build Paeth bpp=3 and bpp=6 routines on x86; fixes build on ARM

src/filter.rs

@@ -9,6 +9,8 @@ use crate::common::BytesPerPixel;
 /// feature of Rust gets stabilized.
 #[cfg(feature = "unstable")]
 mod simd {
+    // unused imports may ocur in this module due to conditional compilation
+    #![allow(unused_imports)]
     use std::simd::cmp::{SimdOrd, SimdPartialEq, SimdPartialOrd};
     use std::simd::num::{SimdInt, SimdUint};
     use std::simd::{u8x4, u8x8, LaneCount, Simd, SimdElement, SupportedLaneCount};
@@ -24,6 +26,7 @@ mod simd {
     /// Funnily, the autovectorizer does a better job here
     /// than a handwritten algorithm using std::simd!
     /// We used to have a handwritten one but this is just faster.
+    #[cfg(target_arch = "x86_64")]
     fn paeth_predictor<const N: usize>(
         a: Simd<i16, N>,
         b: Simd<i16, N>,
@@ -75,6 +78,7 @@ mod simd {
     ///
     /// `b` is the current pixel in the previous row.  `x` is the current pixel in the current row.
     /// See also https://www.w3.org/TR/png/#filter-byte-positions
+    #[cfg(target_arch = "x86_64")]
     fn paeth_step<const N: usize>(
         state: &mut PaethState<i16, N>,
         b: Simd<u8, N>,
@@ -112,15 +116,18 @@ mod simd {
         state.a = *x;
     }
 
+    #[cfg(target_arch = "x86_64")]
     fn load3(src: &[u8]) -> u8x4 {
         u8x4::from_array([src[0], src[1], src[2], 0])
     }
 
+    #[cfg(target_arch = "x86_64")]
     fn store3(src: u8x4, dest: &mut [u8]) {
         dest[0..3].copy_from_slice(&src.to_array()[0..3])
     }
 
     /// Undoes `FilterType::Paeth` for `BytesPerPixel::Three`.
+    #[cfg(target_arch = "x86_64")]
     pub fn unfilter_paeth3(mut prev_row: &[u8], mut curr_row: &mut [u8]) {
         debug_assert_eq!(prev_row.len(), curr_row.len());
         debug_assert_eq!(prev_row.len() % 3, 0);
@@ -175,15 +182,18 @@ mod simd {
         }
     }
 
+    #[cfg(target_arch = "x86_64")]
     fn load6(src: &[u8]) -> u8x8 {
         u8x8::from_array([src[0], src[1], src[2], src[3], src[4], src[5], 0, 0])
     }
 
+    #[cfg(target_arch = "x86_64")]
     fn store6(src: u8x8, dest: &mut [u8]) {
         dest[0..6].copy_from_slice(&src.to_array()[0..6])
     }
 
     /// Undoes `FilterType::Paeth` for `BytesPerPixel::Six`.
+    #[cfg(target_arch = "x86_64")]
     pub fn unfilter_paeth6(mut prev_row: &[u8], mut curr_row: &mut [u8]) {
         debug_assert_eq!(prev_row.len(), curr_row.len());
         debug_assert_eq!(prev_row.len() % 6, 0);