Remove RGBA codepaths using std::simd types since they regress perfor…

…mance on Windows, see image-rs#567

src/filter.rs

@@ -43,23 +43,6 @@ mod simd {
         out.into()
     }
 
-    /// Functionally equivalent to `simd::paeth_predictor` but does not temporarily convert
-    /// the SIMD elements to `i16`.
-    fn paeth_predictor_u8<const N: usize>(
-        a: Simd<u8, N>,
-        b: Simd<u8, N>,
-        c: Simd<u8, N>,
-    ) -> Simd<u8, N>
-    where
-        LaneCount<N>: SupportedLaneCount,
-    {
-        let mut out = [0; N];
-        for i in 0..N {
-            out[i] = super::filter_paeth_stbi(a[i].into(), b[i].into(), c[i].into());
-        }
-        out.into()
-    }
-
     /// Memory of previous pixels (as needed to unfilter `Filter::Paeth`).
     /// See also https://www.w3.org/TR/png/#filter-byte-positions
     #[derive(Default)]
@@ -98,24 +81,6 @@ mod simd {
         state.a = x.cast::<i16>();
     }
 
-    /// Computes the Paeth predictor without converting `u8` to `i16`.
-    ///
-    /// See `simd::paeth_step`.
-    fn paeth_step_u8<const N: usize>(
-        state: &mut PaethState<u8, N>,
-        b: Simd<u8, N>,
-        x: &mut Simd<u8, N>,
-    ) where
-        LaneCount<N>: SupportedLaneCount,
-    {
-        // Calculating the new value of the current pixel.
-        *x += paeth_predictor_u8(state.a, b, state.c);
-
-        // Preparing for the next step.
-        state.c = b;
-        state.a = *x;
-    }
-
     fn load3(src: &[u8]) -> u8x4 {
         u8x4::from_array([src[0], src[1], src[2], 0])
     }
@@ -155,30 +120,6 @@ mod simd {
         store3(x, curr_row);
     }
 
-    /// Undoes `Filter::Paeth` for `BytesPerPixel::Four` and `BytesPerPixel::Eight`.
-    ///
-    /// This function calculates the Paeth predictor entirely in `Simd<u8, N>`
-    /// without converting to an intermediate `Simd<i16, N>`. Doing so avoids
-    /// paying a small performance penalty converting between types.
-    pub fn unfilter_paeth_u8<const N: usize>(prev_row: &[u8], curr_row: &mut [u8])
-    where
-        LaneCount<N>: SupportedLaneCount,
-    {
-        debug_assert_eq!(prev_row.len(), curr_row.len());
-        debug_assert_eq!(prev_row.len() % N, 0);
-        assert!(matches!(N, 4 | 8));
-
-        let mut state = PaethState::<u8, N>::default();
-        for (prev_row, curr_row) in prev_row.chunks_exact(N).zip(curr_row.chunks_exact_mut(N)) {
-            let b = Simd::from_slice(prev_row);
-            let mut x = Simd::from_slice(curr_row);
-
-            paeth_step_u8(&mut state, b, &mut x);
-
-            curr_row[..N].copy_from_slice(&x.to_array()[..N]);
-        }
-    }
-
     fn load6(src: &[u8]) -> u8x8 {
         u8x8::from_array([src[0], src[1], src[2], src[3], src[4], src[5], 0, 0])
     }
@@ -804,12 +745,6 @@ pub(crate) fn unfilter(
                     }
                 }
                 BytesPerPixel::Four => {
-                    #[cfg(all(feature = "unstable", target_arch = "x86_64"))]
-                    {
-                        simd::unfilter_paeth_u8::<4>(previous, current);
-                        return;
-                    }
-
                     let mut a_bpp = [0; 4];
                     let mut c_bpp = [0; 4];
                     for (chunk, b_bpp) in current.chunks_exact_mut(4).zip(previous.chunks_exact(4))
@@ -860,12 +795,6 @@ pub(crate) fn unfilter(
                     }
                 }
                 BytesPerPixel::Eight => {
-                    #[cfg(all(feature = "unstable", target_arch = "x86_64"))]
-                    {
-                        simd::unfilter_paeth_u8::<8>(previous, current);
-                        return;
-                    }
-
                     let mut a_bpp = [0; 8];
                     let mut c_bpp = [0; 8];
                     for (chunk, b_bpp) in current.chunks_exact_mut(8).zip(previous.chunks_exact(8))